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[Last changed: $Date: 91/05/02 18:39:31 $ by $Author: sahayman $]

This article contains the answers to some Frequently Asked Questions
often seen in comp.unix.questions. Please don't ask these questions
again, they've been answered plenty of times already - and please don't
flame someone just because they may not have read this particular
posting. Thank you.


This article includes answers to:

0) Who helped you put this list together?
1) How do I remove a file whose name begins with a "-" ?
2) How do I remove a file with funny characters in the filename ?
3) How do I get a recursive directory listing?
4) How do I get the current directory into my prompt?
5) How do I read characters from a terminal without requiring the user
to hit RETURN?
6) How do I read characters from the terminal in a shell script?
7) How do I check to see if there are characters to be read without
actually reading?
8) How do I find the name of an open file?
9) How do I rename "*.foo" to "*.bar", or change file names
to lowercase?
10) Why do I get [some strange error message] when I
"rsh host command" ?
11) How do I find out the creation time of a file?
12) How do I use "rsh" without having the rsh hang around
until the remote command has completed?
13) How do I truncate a file?
14) How do I {set an environment variable, change directory} inside a
program or shell script and have that change affect my
current shell?
15) Why doesn't find's "{}" symbol do what I want?
16) How do I redirect stdout and stderr separately in csh?
17) How do I set the permissions on a symbolic link?
18) When someone refers to 'rn(1)' or 'ctime(3)', what does
the number in parentheses mean?
19) What does {awk,grep,fgrep,egrep,biff,cat,gecos,nroff,troff,tee,bss}
stand for?
20) How does the gateway between "comp.unix.questions" and the
"info-unix" mailing list work?
21) How do I "undelete" a file?
22) How can a process detect if it's running in the background?
23) How can an executing program determine its own pathname?
24) How do I tell inside .cshrc if I'm a login shell?
25) Why doesn't redirecting a loop work as intended? (Bourne shell)
26) How do I use popen() to open a process for reading AND writing?
27) How do I run 'passwd', 'ftp', 'telnet', 'tip' and other interactive
programs from a shell script or in the background?
28) How do I sleep() in a C program for less than one second?
29) How can I get setuid shell scripts to work?
30) What are some useful Unix or C books?
31) How do I construct a shell glob-pattern that matches all files
except "." and ".." ?
32) How do I find the last argument in a Bourne shell script?
33) How can I find out which user or process has a file open or is using
a particular file system (so that I can unmount it?)
34) How do I keep track of people who are fingering me?
35) How do I find out the process ID of a program with a particular
name from inside a shell script or C program?
36) What happened to the pronunciation list that used to be
part of this document?


If you're looking for the answer to, say, question 14, and want to skip
everything else, you can search ahead for the regular expression "^14)".

While these are all legitimate questions, they seem to crop up in
comp.unix.questions on an annual basis, usually followed by plenty
of replies (only some of which are correct) and then a period of
griping about how the same questions keep coming up. You may also like
to read the monthly article "Answers to Frequently Asked Questions"
in the newsgroup "news.announce.newusers", which will tell you what
"UNIX" stands for.

With the variety of Unix systems in the world, it's hard to guarantee
that these answers will work everywhere. Read your local manual pages
before trying anything suggested here. If you have suggestions or
corrections for any of these answers, please send them to to
[email protected] or iuvax!sahayman.

0) Who helped you put this list together?

I owe a great deal of thanks to dozens of Usenet readers who submitted
questions, answers, corrections and suggestions for this list. I'd
especially like to thank Maarten Litmaath and Guy Harris, who have both
made many especially valuable contributions.

1) How do I remove a file whose name begins with a "-" ?

Figure out some way to name the file so that it doesn't
begin with a dash. The simplest answer is to use

rm ./-filename

(assuming "-filename" is in the current directory, of course.)
This method of avoiding the interpretation of the "-" works
with other commands too.

Many commands, particularly those that have been written to use
the "getopt(3)" argument parsing routine, accept a "--" argument
which means "this is the last option, anything after this is not
an option", so your version of rm might handle "rm -- -filename".
Some versions of rm that don't use getopt() treat a single "-"
in the same way, so you can also try "rm - -filename".

2) How do I remove a file with funny characters in the filename ?

If the 'funny character' is a '/', skip to the last part of
this answer. If the funny character is something else,
such as a ' ' or control character or character with
the 8th bit set, keep reading.

The classic answers are

rm -i some*pattern*that*matches*only*the*file*you*want

which asks you whether you want to remove each file matching
the indicated pattern; depending on your shell, this may
not work if the filename has a character with the 8th bit set
(the shell may strip that off);

and

rm -ri .

which asks you whether to remove each file in the directory.
Answer "y" to the problem file and "n" to everything else.
Unfortunately this doesn't work with many versions of rm.
Also unfortunately, this will walk through every subdirectory
of ".", so you might want to "chmod a-x" those directories
temporarily to make them unsearchable.

Always take a deep breath and think about what you're doing
and double check what you typed when you use rm's "-r" flag
or a wildcard on the command line;

and

find . -type f ... -ok rm '{}' \;

where "..." is a group of predicates that uniquely identify the
file. One possibility is to figure out the inode number
of the problem file (use "ls -i .") and then use

find . -inum 12345 -ok rm '{}' \;

or
find . -inum 12345 -ok mv '{}' new-file-name \;


"-ok" is a safety check - it will prompt you for confirmation of the
command it's about to execute. You can use "-exec" instead to avoid
the prompting, if you want to live dangerously, or if you suspect
that the filename may contain a funny character sequence that will mess
up your screen when printed.


What if the filename has a '/' in it?

These files really are special cases, and can only be created
by buggy kernel code (typically by implementations of NFS
that don't filter out illegal characters in file names from
remote machines.) The first thing to do is to try to
understand exactly why this problem is so strange.

Recall that Unix directories are simply pairs of
filenames and inode numbers. A directory essentially
contains information like this:

filename inode

file1 12345
file2.c 12349
file3 12347

Theoretically, '/' and '\0' are the only two characters that
cannot appear in a filename - '/' because it's used to separate
directories and files, and '\0' because it terminates a filename.

Unfortunately some implementations of NFS will blithely create
filenames with embedded slashes in response to requests from remote
machines. For instance, this could happen when someone on a Mac or
other non-Unix machine decides to create a remote NFS file on
your Unix machine with the date in the filename. Your Unix
directory then has this in it:

filename inode

91/02/07 12357

No amount of messing around with 'find' or 'rm' as described above
will delete this file, since those utilities and all other Unix
programs, are forced to interpret the '/' in the normal way.

Any ordinary program will eventually try to do unlink("91/02/07"),
which as far as the kernel is concerned means "unlink the file 07
in the subdirectory 02 of directory 91", but that's not what we
have - we have a *FILE* named "91/02/07" in the current directory.
This is a subtle but crucial distinction.

What can you do in this case?
The first thing to try is to return to the Mac that created this
crummy entry, and see if you can convince it and your local NFS
daemon to rename the file to something without slashes.

If that doesn't work or isn't possible, you'll need help from your
system manager, who will have to try the one of the following.
Use "ls -i" to find the inode number of this bogus file, then
unmount the file system and use "clri" to clear the inode, and
"fsck" the file system with your fingers crossed. This destroys
the information in the file. If you want to keep it, you can try:

create a new directory in the same parent directory as the one
containing the bad file name;

move everything you can (i.e. everything but the file with
the bad name) from the old directory to the new one;

do "ls -id" on the directory containing the file with the
bad name to get its inumber;

umount the file system;

"clri" the directory containing the file with the bad name;

"fsck" the file system.

Then, to find the file,

remount the file system;

rename the directory you created to have the name of
the old directory (since the old directory should have
been blown away by "fsck")

move the file out of "lost+found" into the directory
with a better name.

Alternatively, you can patch the directory the hard way
by crawling around in the raw file system.
Use "fsdb", if you have it.


3) How do I get a recursive directory listing?

One of the following may do what you want:

ls -R (not all versions of "ls" have -R)
find . -print(should work everywhere)
du -a .(shows you both the name and size)

If you're looking for a wildcard pattern that will match
all ".c" files in this directory and below, you won't find one,
but you can use

% some-command `find . -name '*.c' -print`

"find" is a powerful program. Learn about it.

4) How do I get the current directory into my prompt?

It depends which shell you are using. It's easy with some shells,
hard or impossible with others.

C Shell (csh):
Put this in your .cshrc - customize the prompt variable
the way you want.

alias setprompt 'set prompt="${cwd}% "'
setprompt# to set the initial prompt
alias cd 'chdir \!* && setprompt'

If you use pushd and popd, you'll also need

alias pushd 'pushd \!* && setprompt'
alias popd 'popd \!* && setprompt'

Some C shells don't keep a $cwd variable - you can use
`pwd` instead.

If you just want the last component of the current directory
in your prompt ("mail% " instead of "/usr/spool/mail% ")
you can use

alias setprompt 'set prompt="$cwd:t% "'


Some older csh's get the meaning of && and || reversed.
Try doing:

false && echo bug

If it prints "bug", you need to switch && and || (and get
a better version of csh.)


Bourne Shell (sh):

If you have a newer version of the Bourne Shell (SVR2 or newer)
you can use a shell function to make your own command, "xcd" say:

xcd() { cd $* ; PS1="`pwd` $ "; }

If you have an older Bourne shell, it's complicated but not impossible.
Here's one way. Add this to your .profile file:

LOGIN_SHELL=$$ export LOGIN_SHELL
CMDFILE=/tmp/cd.$$ export CMDFILE
# 16 is SIGURG, pick some signal that isn't likely to be used
PROMPTSIG=16 export PROMPTSIG
trap '. $CMDFILE' $PROMPTSIG

and then put this executable script (without the indentation!),
let's call it "xcd", somewhere in your PATH

: xcd directory - change directory and set prompt
: by signalling the login shell to read a command file
cat >${CMDFILE?"not set"} <cd $1
PS1="\`pwd\`$ "
EOF
kill -${PROMPTSIG?"not set"} ${LOGIN_SHELL?"not set"}

Now change directories with "xcd /some/dir".


Korn Shell (ksh):

Put this in your .profile file:
PS1='$PWD $ '

If you just want the last component of the directory, use
PS1='${PWD##*/} $ '

T C shell (tcsh)

Tcsh is a popular enhanced version of csh with some extra
builtin variables (and many other features):

%~the current directory, using ~ for $HOME
%d or %/the full pathname of the current directory
%c or %.the trailing component of the current directory

so you can do

set prompt='%~ '

BASH (FSF's "Bourne Again SHell")

\w in $PS1 gives the full pathname of the current directory,
with ~ expansion for $HOME; \W gives the basename of
the current directory. So, in addition to the above sh and
ksh solutions, you could use

PS1='\w $ '
or
PS1='\W $ '

5) How do I read characters from a terminal without requiring the user
to hit RETURN?

Check out cbreak mode in BSD, ~ICANON mode in SysV.

If you don't want to tackle setting the terminal parameters
yourself (using the "ioctl(2)" system call) you can let the stty
program do the work - but this is slow and inefficient, and you
should change the code to do it right some time:

#include
main()
{
int c;

printf("Hit any character to continue\n");
/*
* ioctl() would be better here; only lazy
* programmers do it this way:
*/
system("/bin/stty cbreak"); /* or "stty raw" */
c = getchar();
system("/bin/stty -cbreak");
printf("Thank you for typing %c.\n", c);

exit(0);
}

You might like to check out the documentation for the "curses"
library of portable screen functions. Often if you're interested
in single-character I/O like this, you're also interested in doing
some sort of screen display control, and the curses library
provides various portable routines for both functions.



6) How do I read characters from the terminal in a shell script?

In sh, use read. It is most common to use a loop like

while read line
do
...
done

In csh, use $< like this:

while ( 1 )
set line = "$<"
if ( "$line" == "" ) break
...
end

Unfortunately csh has no way of distinguishing between
a blank line and an end-of-file.

If you're using sh and want to read a *single* character from
the terminal, you can try something like

echo -n "Enter a character: "
stty cbreak# or stty raw
readchar=`dd if=/dev/tty bs=1 count=1 2>/dev/null`
stty -cbreak

echo "Thank you for typing a $readchar ."

7) How do I check to see if there are characters to be read without
actually reading?

Certain versions of UNIX provide ways to check whether
characters are currently available to be read from a file
descriptor. In BSD, you can use select(2). You can also use
the FIONREAD ioctl (see tty(4)), which returns the number of
characters waiting to be read, but only works on terminals,
pipes and sockets. In System V Release 3, you can use poll(2),
but that only works on streams. In Xenix - and therefore
Unix SysV r3.2 and later - the rdchk() system call reports
whether a read() call on a given file descriptor will block.

There is no way to check whether characters are available to be
read from a FILE pointer. (You could poke around inside stdio data
structures to see if the input buffer is nonempty, but that wouldn't
work since you'd have no way of knowing what will happen the next
time you try to fill the buffer.)

Sometimes people ask this question with the intention of writing
if (characters available from fd)
read(fd, buf, sizeof buf);
in order to get the effect of a nonblocking read. This is not the
best way to do this, because it is possible that characters will
be available when you test for availability, but will no longer
be available when you call read. Instead, set the O_NDELAY flag
(which is also called FNDELAY under BSD) using the F_SETFL option
of fcntl(2). Older systems (Version 7, 4.1 BSD) don't have O_NDELAY;
on these systems the closest you can get to a nonblocking read is
to use alarm(2) to time out the read.


8) How do I find the name of an open file?

In general, this is too difficult. The file descriptor may
be attached to a pipe or pty, in which case it has no name.
It may be attached to a file that has been removed. It may
have multiple names, due to either hard or symbolic links.

If you really need to do this, and be sure you think long
and hard about it and have decided that you have no choice,
you can use find with the -inum and possibly -xdev option,
or you can use ncheck, or you can recreate the functionality
of one of these within your program. Just realize that
searching a 600 megabyte filesystem for a file that may not
even exist is going to take some time.


9) How do I rename "*.foo" to "*.bar", or change file names to lowercase?

Why doesn't "mv *.foo *.bar" work? Think about how the shell
expands wildcards. "*.foo" and "*.bar" are expanded before the mv
command ever sees the arguments. Depending on your shell, this
can fail in a couple of ways. CSH prints "No match." because
it can't match "*.bar". SH executes "mv a.foo b.foo c.foo *.bar",
which will only succeed if you happen to have a single
directory named "*.bar", which is very unlikely and almost
certainly not what you had in mind.

Depending on your shell, you can do it with a loop to "mv" each
file individually. If your system has "basename", you can use:

C Shell:
foreach f ( *.foo )
set base=`basename $f .foo`
mv $f $base.bar
end

Bourne Shell:
for f in *.foo; do
base=`basename $f .foo`
mv $f $base.bar
done

Some shells have their own variable substitution features, so instead
of using "basename", you can use simpler loops like:

C Shell:

foreach f ( *.foo )
mv $f $f:r.bar
end

Korn Shell:

for f in *.foo; do
mv $f ${f%foo}bar
done

If you don't have "basename" or want to do something like
renaming foo.* to bar.*, you can use something like "sed" to
strip apart the original file name in other ways, but
the general looping idea is the same. You can also convert
file names into "mv" commands with 'sed', and hand the commands
off to "sh" for execution. Try

ls -d *.foo | sed -e 's/.*/mv & &/' -e 's/foo$/bar/' | sh

A program by Vladimir Lanin called "mmv" that does this job nicely
was posted to comp.sources.unix (Volume 21, issues 87 and 88) in
April 1990. It lets you use

mmv '*.foo' '=1.bar'

Shell loops like the above can also be used to translate
file names from upper to lower case or vice versa. You could use
something like this to rename uppercase files to lowercase:

C Shell:
foreach f ( * )
mv $f `echo $f | tr '[A-Z]' '[a-z]'`
end
Bourne Shell:
for f in *; do
mv $f `echo $f | tr '[A-Z]' '[a-z]'`
done
Korn Shell:
typeset -l l
for f in *; do
l="$f"
mv $f $l
done


If you wanted to be really thorough and handle files with
`funny' names (embedded blanks or whatever) you'd need to use

Bourne Shell:

for f in *; do
g=`expr "xxx$f" : 'xxx\(.*\)' | tr '[A-Z]' '[a-z]'`
mv "$f" "$g"
done

The `expr' command will always print the filename, even if it equals
`-n' or if it contains a System V escape sequence like `\c'.

Some versions of "tr" require the [ and ], some don't. It happens
to be harmless to include them in this particular example; versions of
tr that don't want the [] will conveniently think they are supposed
to translate '[' to '[' and ']' to ']'.

If you have the "perl" language installed, you may find this rename
script by Larry Wall very useful. It can be used to accomplish a
wide variety of filename changes.

#!/usr/bin/perl
#
# rename script examples from lwall:
# rename 's/\.orig$//' *.orig
# rename 'y/A-Z/a-z/ unless /^Make/' *
# rename '$_ .= ".bad"' *.f
# rename 'print "$_: "; s/foo/bar/ if =~ /^y/i' *

$op = shift;
for (@ARGV) {
$was = $_;
eval $op;
die [email protected] if [email protected];
rename($was,$_) unless $was eq $_;
}


10) Why do I get [some strange error message] when I "rsh host command" ?

(We're talking about the remote shell program "rsh" or sometimes "remsh";
on some machines, there is a restricted shell called "rsh", which
is a different thing.)

If your remote account uses the C shell, the remote host will
fire up a C shell to execute 'command' for you, and that shell
will read your remote .cshrc file. Perhaps your .cshrc contains
a "stty", "biff" or some other command that isn't appropriate
for a non-interactive shell. The unexpected output or error
message from these commands can screw up your rsh in odd ways.

Fortunately, the fix is simple. There are, quite possibly, a whole
*bunch* of operations in your ".cshrc" (e.g., "set history=N") that are
simply not worth doing except in interactive shells. What you do is
surround them in your ".cshrc" with:

if ( $?prompt ) then
operations....
endif

and, since in a non-interactive shell "prompt" won't be set, the
operations in question will only be done in interactive shells.

You may also wish to move some commands to your .login file; if
those commands only need to be done when a login session starts up
(checking for new mail, unread news and so on) it's better
to have them in the .login file.

11) How do I find out the creation time of a file?

You can't - it isn't stored anywhere. Files have a last-modified
time (shown by "ls -l"), a last-accessed time (shown by "ls -lu")
and an inode change time (shown by "ls -lc"). The latter is often
referred to as the "creation time" - even in some man pages - but
that's wrong; it's also set by such operations as mv, ln,
chmod, chown and chgrp.

The man page for "stat(2)" discusses this.

12) How do I use "rsh" without having the rsh hang around until the
remote command has completed?

(See note in question 10 about what "rsh" we're talking about.)

The obvious answers fail:
rsh machine command &
or rsh machine 'command &'

For instance, try doing rsh machine 'sleep 60 &'
and you'll see that the 'rsh' won't exit right away.
It will wait 60 seconds until the remote 'sleep' command
finishes, even though that command was started in the
background on the remote machine. So how do you get
the 'rsh' to exit immediately after the 'sleep' is started?

The solution - if you use csh on the remote machine:

rsh machine -n 'command >&/dev/null
If you use sh on the remote machine:

rsh machine -n 'command >/dev/null 2>&1
Why? "-n" attaches rsh's stdin to /dev/null so you could run the
complete rsh command in the background on the LOCAL machine.
Thus "-n" is equivalent to another specific "< /dev/null".
Furthermore, the input/output redirections on the REMOTE machine
(inside the single quotes) ensure that rsh thinks the session can
be terminated (there's no data flow any more.)

Note: The file that you redirect to/from on the remote machine
doesn't have to be /dev/null; any ordinary file will do.

In many cases, various parts of these complicated commands
aren't necessary.

13) How do I truncate a file?

The BSD function ftruncate() sets the length of a file. Xenix -
and therefore SysV r3.2 and later - has the chsize() system call.
For other systems, the only kind of truncation you can do is
truncation to length zero with creat() or open(..., O_TRUNC).

14) How do I {set an environment variable, change directory} inside a
program or shell script and have that change affect my
current shell?

In general, you can't, at least not without making special
arrangements. When a child process is created, it inherits a copy
of its parent's variables (and current directory). The child can
change these values all it wants but the changes won't affect the
parent shell, since the child is changing a copy of the
original data.

Some special arrangements are possible. Your child process could
write out the changed variables, if the parent was prepared to read
the output and interpret it as commands to set its own variables.

Also, shells can arrange to run other shell scripts in the context
of the current shell, rather than in a child process, so that
changes will affect the original shell.

For instance, if you have a C shell script named "myscript":

cd /very/long/path
setenv PATH /something:/something-else

or the equivalent Bourne or Korn shell script

cd /very/long/path
PATH=/something:/something-else export PATH

and try to run "myscript" from your shell, your shell will fork and run
the shell script in a subprocess. The subprocess is also
running the shell; when it sees the "cd" command it changes
*its* current directory, and when it sees the "setenv" command
it changes *its* environment, but neither has any effect on the current
directory of the shell at which you're typing (your login shell,
let's say).

In order to get your login shell to execute the script (without forking)
you have to use the "." command (for the Bourne or Korn shells)
or the "source" command (for the C shell). I.e. you type

. myscript

to the Bourne or Korn shells, or

source myscript

to the C shell.

If all you are trying to do is change directory or set an
environment variable, it will probably be simpler to use a
C shell alias or Bourne/Korn shell function. See the "how do
I get the current directory into my prompt" section
of this article for some examples.

15) Why doesn't find's "{}" symbol do what I want?

"find" has a -exec option that will execute a particular
command on all the selected files. Find will replace any "{}"
it sees with the name of the file currently under consideration.

So, some day you might try to use "find" to run a command on every
file, one directory at a time. You might try this:

find /path -type d -exec command {}/\* \;

hoping that find will execute, in turn

command directory1/*
command directory2/*
...

Unfortunately, find only expands the "{}" token when it appears
by itself. Find will leave anything else like "{}/*" alone, so
instead of doing what you want, it will do

command {}/*
command {}/*
...

once for each directory. This might be a bug, it might be a feature,
but we're stuck with the current behaviour.

So how do you get around this? One way would be to write a
trivial little shell script, let's say "./doit", that
consists of

command "$1"/*

You could then use

find /path -type d -exec ./doit {} \;

Or if you want to avoid the "./doit" shell script, you can use

find /path -type d -exec sh -c 'command $0/*' {} \;

(This works because within the 'command' of "sh -c 'command' A B C ...",
$0 expands to A, $1 to B, and so on.)

or you can use the construct-a-command-with-sed trick

find /path -type d -print | sed 's:.*:command &/*:' | sh



If all you're trying to do is cut down on the number of times
that "command" is executed, you should see if your system
has the "xargs" command. Xargs reads arguments one line at a time
from the standard input and assembles as many of them as will fit into
one command line. You could use

find /path -print | xargs command

which would result in one or more executions of

command file1 file2 file3 file4 dir1/file1 dir1/file2


Unfortunately this is not a perfectly robust or secure solution.
Xargs expects its input lines to be terminated with newlines, so it
will be confused by files with odd characters such as newlines
in their names.


16) How do I redirect stdout and stderr separately in csh?

In csh, you can redirect stdout with ">", or stdout and stderr
together with ">&" but there is no direct way to redirect
stderr only. The best you can do is

( command >stdout_file ) >&stderr_file

which runs "command" in a subshell; stdout is redirected inside
the subshell to stdout_file, and both stdout and stderr from the
subshell are redirected to stderr_file, but by this point stdout
has already been redirected so only stderr actually winds up in
stderr_file.

Sometimes it's easier to let sh do the work for you.

sh -c 'command >stdout_file 2>stderr_file'

17) How do I set the permissions on a symbolic link?

Permissions on a symbolic link don't really mean anything. The
only permissions that count are the permissions on the file that
the link points to.

18) When someone refers to 'rn(1)' or 'ctime(3)', what does
the number in parentheses mean?

It looks like some sort of function call, but it isn't.
These numbers refer to the section of the "Unix manual" where
the appropriate documentation can be found. You could type
"man 3 ctime" to look up the manual page for "ctime" in section 3
of the manual.

The traditional manual sections are:

1User-level commands
2System calls
3Library functions
4Devices and device drivers
5File formats
6Games
7Various miscellaneous stuff - macro packages etc.
8System maintenance and operation commands


Some Unix versions use non-numeric section names. For instance,
Xenix uses "C" for commands and "S" for functions.

Each section has an introduction, which you can read with "man # intro"
where # is the section number.

Sometimes the number is necessary to differentiate between a
command and a library routine or system call of the same name. For
instance, your system may have "time(1)", a manual page about the
'time' command for timing programs, and also "time(3)", a manual
page about the 'time' subroutine for determining the current time.
You can use "man 1 time" or "man 3 time" to specify which "time"
man page you're interested in.

You'll often find other sections for local programs or
even subsections of the sections above - Ultrix has
sections 3m, 3n, 3x and 3yp among others.


19) What does {awk,grep,fgrep,egrep,biff,cat,gecos,nroff,troff,tee,bss,rc}
stand for?

awk = "Aho Weinberger and Kernighan"

This language was named by its authors, Al Aho, Peter Weinberger and
Brian Kernighan.

grep = "Global Regular Expression Print"

grep comes from the ed command to print all lines matching a
certain pattern

g/re/p

where "re" is a "regular expression".

fgrep = "Fixed GREP".

fgrep searches for fixed strings only. The "f" does not
stand for "fast" - in fact, "fgrep foobar *.c" is usually slower
than "egrep foobar *.c" (Yes, this is kind of surprising. Try it.)

Fgrep still has its uses though, and may be useful when searching
a file for a larger number of strings than egrep can handle.

egrep = "Extended GREP"

egrep uses fancier regular expressions than grep.
Many people use egrep all the time, since it has some more
sophisticated internal algorithms than grep or fgrep,
and is usually the fastest of the three programs.

cat = "CATenate"

catenate is an obscure word meaning "to connect in a series",
which is what the "cat" command does to one or more files.
Not to be confused with C/A/T, the Computer Aided Typesetter.

gecos = "General Electric Comprehensive Operating System"

When GE's large systems division was sold to Honeywell,
Honeywell dropped the "E" from "GECOS".

Unix's password file has a "pw_gecos" field. The name is
a real holdover from the early days. Dennis Ritchie
has reported:

"Sometimes we sent printer output or batch jobs
to the GCOS machine. The gcos field in the
password file was a place to stash the information
for the $IDENT card. Not elegant."

nroff = "New ROFF"
troff = "Typesetter new ROFF"

These are descendants of "roff", which was a re-implementation
of the Multics "runoff" program (a program that you'd use to
"run off" a good copy of a document).

tee= T

From plumbing terminology for a T-shaped pipe splitter.

bss = "Block Started by Symbol"

Dennis Ritchie says:

Actually the acronym (in the sense we took it up; it may
have other credible etymologies) is "Block Started by Symbol."
It was a pseudo-op in FAP (Fortran Assembly [-er?] Program), an
assembler for the IBM 704-709-7090-7094 machines. It defined
its label and set aside space for a given number of words.
There was another pseudo-op, BES, "Block Ended by Symbol"
that did the same except that the label was defined by
the last assigned word + 1. (On these machines Fortran
arrays were stored backwards in storage and were 1-origin.)

The usage is reasonably appropriate, because just as with
standard Unix loaders, the space assigned didn't have to
be punched literally into the object deck but was represented
by a count somewhere.

biff = "BIFF"

This command, which turns on asynchronous mail notification,
was actually named after a dog at Berkeley.

I can confirm the origin of biff, if you're interested. Biff
was Heidi Stettner's dog, back when Heidi (and I, and Bill Joy)
were all grad students at U.C. Berkeley and the early versions
of BSD were being developed. Biff was popular among the
residents of Evans Hall, and was known for barking at the
mailman, hence the name of the command.

Confirmation courtesy of Eric Cooper, Carnegie Mellon
University

rc (as in ".cshrc" or "/etc/rc") = "RunCom"

"rc" derives from "runcom", from the MIT CTSS system, ca. 1965.

'There was a facility that would execute a bunch of commands
stored in a file; it was called "runcom" for "run commands",
and the file began to be called "a runcom."

"rc" in Unix is a fossil from that usage.'

Brian Kernighan & Dennis Ritchie, as told to Vicki Brown


"rc" is also the name of the shell from the new Plan 9
operating system.



Don Libes' book "Life with Unix" contains lots more of these
tidbits.


20) How does the gateway between "comp.unix.questions" and the
"info-unix" mailing list work?

"Info-Unix" and "Unix-Wizards" are mailing list versions of
comp.unix.questions and comp.unix.wizards respectively.
There should be no difference in content between the
mailing list and the newsgroup.

[Note: The newsgroup "comp.unix.wizards" was recently deleted,
and even more recently resurrected; the "Unix-Wizards" mailing
list still exists. I'm not really sure how this is all going
to sort itself out.]

To get on or off either of these lists, send mail to
[email protected] or [email protected] .
Be sure to use the '-Request'. Don't expect an immediate response.

Here are the gory details, courtesy of the list's maintainer, Bob Reschly.

==== postings to info-UNIX and UNIX-wizards lists ====

Anything submitted to the list is posted; I do not moderate incoming
traffic -- BRL functions as a reflector. Postings submitted by Internet
subscribers should be addressed to the list address (info-UNIX or UNIX-
wizards); the '-request' addresses are for correspondence with the list
maintainer [me]. Postings submitted by USENET readers should be
addressed to the appropriate news group (comp.unix.questions or
comp.unix.wizards).

For Internet subscribers, received traffic will be of two types;
individual messages, and digests. Traffic which comes to BRL from the
Internet and BITNET (via the BITNET-Internet gateway) is immediately
resent to all addressees on the mailing list. Traffic originating on
USENET is gathered up into digests which are sent to all list members
daily.

BITNET traffic is much like Internet traffic. The main difference is
that I maintain only one address for traffic destined to all BITNET
subscribers. That address points to a list exploder which then sends
copies to individual BITNET subscribers. This way only one copy of a
given message has to cross the BITNET-Internet gateway in either
direction.

USENET subscribers see only individual messages. All messages
originating on the Internet side are forwarded to our USENET machine.
They are then posted to the appropriate newsgroup. Unfortunately,
for gatewayed messages, the sender becomes "[email protected]". This is
currently an unavoidable side-effect of the software which performs the
gateway function.

As for readership, USENET has an extremely large readership - I would
guess several thousand hosts and tens of thousands of readers. The
master list maintained here at BRL runs about two hundred fifty entries
with roughly ten percent of those being local redistribution lists.
I don't have a good feel for the size of the BITNET redistribution, but
I would guess it is roughly the same size and composition as the master
list. Traffic runs 150K to 400K bytes per list per week on average.

21) How do I "undelete" a file?

Someday, you are going to accidentally type something like "rm * .foo",
and find you just deleted "*" instead of "*.foo". Consider it a rite
of passage.

Of course, any decent systems administrator should be doing regular
backups. Check with your sysadmin to see if a recent backup copy
of your file is available. But if it isn't, read on.

For all intents and purposes, when you delete a file with "rm" it is
gone. Once you "rm" a file, the system totally forgets which blocks
scattered around the disk comprised your file. Even worse, the blocks
from the file you just deleted are going to be the first ones taken
and scribbled upon when the system needs more disk space. However,
never say never. It is theoretically possible *if* you shut down
the system immediately after the "rm" to recover portions of the data.
However, you had better have a very wizardly type person at hand with
hours or days to spare to get it all back.

Your first reaction when you "rm" a file by mistake is why not make
a shell alias or procedure which changes "rm" to move files into a
trash bin rather than delete them? That way you can recover them if
you make a mistake, and periodically clean out your trash bin. Two
points: first, this is generally accepted as a *bad* idea. You will
become dependent upon this behaviour of "rm", and you will find
yourself someday on a normal system where "rm" is really "rm", and
you will get yourself in trouble. Second, you will eventually find
that the hassle of dealing with the disk space and time involved in
maintaining the trash bin, it might be easier just to be a bit more
careful with "rm". For starters, you should look up the "-i" option
to "rm" in your manual.

If you are still undaunted, then here is a possible simple answer. You
can create yourself a "can" command which moves files into a
trashcan directory. In csh(1) you can place the following commands
in the ".login" file in your home directory:

alias can'mv \!* ~/.trashcan' # junk file(s) to trashcan
alias mtcan'rm -f ~/.trashcan/*' # irretrievably empty trash
if ( ! -d ~/.trashcan ) mkdir ~/.trashcan # ensure trashcan exists

You might also want to put a:

rm -f ~/.trashcan/*

in the ".logout" file in your home directory to automatically empty
the trash when you log out. (sh and ksh versions are left as an
exercise for the reader.)

MIT's Project Athena has produced a comprehensive
delete/undelete/expunge/purge package, which can serve as a
complete replacement for rm which allows file recovery. This
package was posted to comp.sources.misc (volume 17, issue 023-026)


22) How can a process detect if it's running in the background?

First of all: do you want to know if you're running in the background,
or if you're running interactively? If you're deciding whether or
not you should print prompts and the like, that's probably a better
criterion. Check if standard input is a terminal:

sh: if [ -t 0 ]; then ... fi
C: if(isatty(0)) { ... }

In general, you can't tell if you're running in the background.
The fundamental problem is that different shells and different
versions of UNIX have different notions of what "foreground" and
"background" mean - and on the most common type of system with a
better-defined notion of what they mean, programs can be moved
arbitrarily between foreground and background!

UNIX systems without job control typically put a process into the
background by ignoring SIGINT and SIGQUIT and redirecting the standard
input to "/dev/null"; this is done by the shell.

Shells that support job control, on UNIX systems that support job
control, put a process into the background by giving it a process group
ID different from the process group to which the terminal belongs. They
move it back into the foreground by setting the terminal's process group
ID to that of the process. Shells that do *not* support job control, on
UNIX systems that support job control, typically do what shells do on
systems that don't support job control.

23) How can an executing program determine its own pathname?

Your program can look at argv[0]; if it begins with a "/",
it is probably the absolute pathname to your program, otherwise
your program can look at every directory named in the environment
variable PATH and try to find the first one that contains an
executable file whose name matches your program's argv[0]
(which by convention is the name of the file being executed).
By concatenating that directory and the value of argv[0] you'd
probably have the right name.

You can't really be sure though, since it is quite legal for one
program to exec() another with any value of argv[0] it desires.
It is merely a convention that new programs are exec'd with the
executable file name in argv[0].

For instance, purely a hypothetical example:

#include
main()
{
execl("/usr/games/rogue", "vi Thesis", (char *)NULL);
}

The executed program thinks its name (its argv[0] value) is
"vi Thesis". (Certain other programs might also think that
the name of the program you're currently running is "vi Thesis",
but of course this is just a hypothetical example, don't
try it yourself 🙂

24) How do I tell inside .cshrc if I'm a login shell?

When people ask this, they usually mean either

How can I tell if it's an interactive shell?
or
How can I tell if it's a top-level shell?

You could perhaps determine if your shell truly is a login shell
(i.e. is going to source ".login" after it is done with ".cshrc")
by fooling around with "ps" and "$$"; if you're really interested
in the other two questions, here's one way you can organize
your .cshrc to find out.


if (! $?CSHLEVEL) then
#
# This is a "top-level" shell,
# perhaps a login shell, perhaps a shell started up by
# 'rsh machine some-command'
# This is where we should set PATH and anything else we
# want to apply to every one of our shells.
#
setenv CSHLEVEL 0
set home = ~username # just to be sure
source ~/.env # environment stuff we always want
else
#
# This shell is a child of one of our other shells so
# we don't need to set all the environment variables again.
#
set tmp = $CSHLEVEL
@ tmp++
setenv CSHLEVEL $tmp
endif

# Exit from .cshrc if not interactive, e.g. under rsh
if (! $?prompt) exit

# Here we could set the prompt or aliases that would be useful
# for interactive shells only.

source ~/.aliases

25) Why doesn't redirecting a loop work as intended? (Bourne shell)

Take the following example:

foo=bar

while read line
do
# do something with $line
foo=bletch
done < /etc/passwd

echo "foo is now: $foo"

Despite the assignment ``foo=bletch'' this will print ``foo is now: bar''
in many implementations of the Bourne shell. Why?
Because of the following, often undocumented, feature of historic
Bourne shells: redirecting a control structure (such as a loop, or an
``if'' statement) causes a subshell to be created, in which the structure
is executed; variables set in that subshell (like the ``foo=bletch''
assignment) don't affect the current shell, of course.

The POSIX 1003.2 Shell and Tools Interface standardization committee
forbids the behaviour described above, i.e. in P1003.2 conformant
Bourne shells the example will print ``foo is now: bletch''.

In historic (and P1003.2 conformant) implementations you can use the
following `trick' to get around the redirection problem:

foo=bar

# make file descriptor 9 a duplicate of file descriptor 0 (stdin);
# then connect stdin to /etc/passwd; the original stdin is now
# `remembered' in file descriptor 9; see dup(2) and sh(1)
exec 9<&0 < /etc/passwd

while read line
do
# do something with $line
foo=bletch
done

# make stdin a duplicate of file descriptor 9, i.e. reconnect it to
# the original stdin; then close file descriptor 9
exec 0<&9 9<&-

echo "foo is now: $foo"

This should always print ``foo is now: bletch''.
Right, take the next example:

foo=bar

echo bletch | read foo

echo "foo is now: $foo"

This will print ``foo is now: bar'' in many implementations,
``foo is now: bletch'' in some others. Why?
Generally each part of a pipeline is run in a different subshell;
in some implementations though, the last command in the pipeline is
made an exception: if it is a builtin command like ``read'', the current
shell will execute it, else another subshell is created.

POSIX 1003.2 allows both behaviours so portable scripts cannot depend
on any of them.

26) How do I use popen() to open a process for reading AND writing?

The problem with trying to pipe both input and output to an arbitrary
slave process is that deadlock can occur, if both processes are waiting
for not-yet-generated input at the same time. Deadlock can be avoided
only by having BOTH sides follow a strict deadlock-free protocol, but
since that requires cooperation from the processes it is inappropriate
for a popen()-like library function.

The 'expect' distribution includes a library of functions that a C
programmer can call directly. One of the functions does the
equivalent of a popen for both reading and writing. It uses ptys
rather than pipes, and has no deadlock problem. It's portable to
both BSD and SV. See the next answer for more about 'expect'.

27) How do I run 'passwd', 'ftp', 'telnet', 'tip' and other interactive
programs from a shell script or in the background?

The shell itself cannot interact with interactive tty-based programs
like these. Fortunately some programs have been written to manage
the connection to a pseudo-tty so that you can run these sorts
of programs in a script.

'expect' is a one such program, which you can ftp pub/expect.shar.Z
from durer.cme.nist.gov.

The following expect script is an example of a non-interactive
version of passwd(1).

# username is passed as 1st arg, password as 2nd
set password [index $argv 2]
spawn passwd [index $argv 1]
expect "*password:"
send "$password\r"
expect "*password:"
send "$password\r"
expect eof

Another solution is provided by the 'pty' program, which runs a
program under a pty session and was posted to comp.sources.unix,
volume 23, issue 31. You can also ftp pub/flat/pty-* from
stealth.acf.nyu.edu . A pty-based solution using named pipes
to do the same as the above might look like this:

#!/bin/sh
/etc/mknod out.$$ p; exec 2>&1
( exec 4<&4 waitfor 'password:'
echo "$2"
<&4 waitfor 'password:'
echo "$2"
<&4 cat >/dev/null
) | ( pty passwd "$1" >out.$$ )

Here, 'waitfor' is a simple C program that searches for
its argument in the input, character by character. You can
ftp pub/flat/misc-waitfor.c from stealth.acf.nyu.edu .

A simpler pty solution (which has the drawback of not
synchronizing properly with the passwd program) is

#!/bin/sh
( sleep 5; echo "$2"; sleep 5; echo "$2") | pty passwd "$1"


28) How do I sleep() in a C program for less than one second?

The first thing you need to be aware of is that all you can specify is a
MINIMUM amount of delay; the actual delay will depend on scheduling
issues such as system load, and could be arbitrarily large if you're
unlucky.

There is no standard library function that you can count on in all
environments for "napping" (the usual name for short sleeps).
Some environments supply a "usleep(n)" function which suspends
execution for n microseconds. If your environment doesn't support
usleep(), here are a couple of implementations for BSD and
System V environments.

The following code is adapted from Doug Gwyn's System V emulation
support for 4BSD and exploits the 4BSD select() system call.
Doug originally called it 'nap()'; you probably want to call it
"usleep()";

/*
usleep -- support routine for 4.2BSD system call emulations

last edit:29-Oct-1984D A Gwyn
*/

extern intselect();


int
usleep( usec )/* returns 0 if ok, else -1 */
longusec;/* delay in microseconds */
{
static struct/* `timeval' */
{
longtv_sec;/* seconds */
longtv_usec;/* microsecs */
}delay;/* _select() timeout */

delay.tv_sec = usec / 1000000L;
delay.tv_usec = usec % 1000000L;

return select( 0, (long *)0, (long *)0, (long *)0, &delay );
}

On System V you might do it this way:

/*

subseconds sleeps for System V - or anything that has poll()
Don Libes, 4/1/1991

The BSD analog to this function is defined in terms of microseconds
while poll() is defined in terms of milliseconds. For compatibility,
this function provides accuracy "over the long run" by truncating
actual requests to milliseconds and accumulating microseconds across
calls with the idea that you are probably calling it in a tight loop,
and that over the long run, the error will even out.

If you aren't calling it in a tight loop, then you almost certainly
aren't making microsecond-resolution requests anyway, in which case
you don't care about microseconds. And if you did, you wouldn't be
using UNIX anyway because random system indigestion (i.e., scheduling)
can make mincemeat out of any timing code.

Returns 0 if successful timeout, -1 if unsuccessful.

*/

#include

int
usleep(usec)
unsigned int usec;/* microseconds */
{
static subtotal = 0;/* microseconds */
int msec;/* milliseconds */

/* 'foo' is only here because some versions of 5.3 have
* a bug where the first argument to poll() is checked
* for a valid memory address even if the second argument is 0.
*/
struct pollfd foo;

subtotal += usec;
/* if less then 1 msec request, do nothing but remember it */
if (subtotal < 1000) return(0);
msec = subtotal/1000;
subtotal = subtotal%1000;
return poll(&foo,(unsigned long)0,msec);
}


Another possibility for nap()ing on System V, and probably other
non-BSD Unices is Jon Zeeff's s5nap package, posted to
comp.sources.misc, volume 4. It does require a installing
a device driver, but works flawlessly once installed.
(Its resolution is limited to the kernel HZ value, since it
uses the kernel delay() routine.)

29) How can I get setuid shell scripts to work?

[ This is a long answer, but it's a complicated and frequently-asked
question. Thanks to Maarten Litmaath for this answer, and
for the "indir" program mentioned below. ]

Let us first assume you are on a UNIX variant (e.g. 4.3BSD or SunOS)
that knows about so-called `executable shell scripts'. Such a script
must start with a line like:

#!/bin/sh

The script is called `executable' because just like a real (binary)
executable it starts with a so-called `magic number' indicating the
type of the executable. In our case this number is `#!' and the OS
takes the rest of the first line as the interpreter for the script,
possibly followed by 1 initial option like:

#!/bin/sed -f

Suppose this script is called `foo' and is found in /bin,
then if you type:

foo arg1 arg2 arg3

the OS will rearrange things as though you had typed:

/bin/sed -f /bin/foo arg1 arg2 arg3

There is one difference though: if the setuid permission bit for
`foo' is set, it will be honored in the first form of the command;
if you really type the second form, the OS will honor the permission
bits of /bin/sed, which is not setuid, of course.

----------

OK, but what if my shell script does NOT start with such a `#!' line
or my OS does not know about it?

Well, if the shell (or anybody else) tries to execute it, the OS will
return an error indication, as the file does not start with a valid
magic number. Upon receiving this indication the shell ASSUMES the
file to be a shell script and gives it another try:

/bin/sh shell_script arguments

But we have already seen that a setuid bit on `shell_script' will NOT
be honored in this case!

----------

Right, but what about the security risks of setuid shell scripts?

Well, suppose the script is called `/etc/setuid_script', starting
with:

#!/bin/sh

Now let us see what happens if we issue the following commands:

$ cd /tmp
$ ln /etc/setuid_script -i
$ PATH=.
$ -i

We know the last command will be rearranged to:

/bin/sh -i

But this command will give us an interactive shell, setuid to the
owner of the script!
Fortunately this security hole can easily be closed by making the
first line:

#!/bin/sh -

The `-' signals the end of the option list: the next argument `-i'
will be taken as the name of the file to read commands from, just
like it should!

---------

There are more serious problems though:

$ cd /tmp
$ ln /etc/setuid_script temp
$ nice -20 temp &
$ mv my_script temp

The third command will be rearranged to:

nice -20 /bin/sh - temp

As this command runs so slowly, the fourth command might be able to
replace the original `temp' with `my_script' BEFORE `temp' is opened
by the shell!
There are 4 ways to fix this security hole:

1) let the OS start setuid scripts in a different, secure way
- System V R4 and 4.4BSD use the /dev/fd driver to pass the
interpreter a file descriptor for the script

2) let the script be interpreted indirectly, through a frontend
that makes sure everything is all right before starting the
real interpreter - if you use the `indir' program from
comp.sources.unix the setuid script will look like this:

#!/bin/indir -u
#?/bin/sh /etc/setuid_script

3) make a `binary wrapper': a real executable that is setuid and
whose only task is to execute the interpreter with the name of
the script as an argument

4) make a general `setuid script server' that tries to locate the
requested `service' in a database of valid scripts and upon
success will start the right interpreter with the right
arguments.

---------

Now that we have made sure the right file gets interpreted, are there
any risks left?

Certainly! For shell scripts you must not forget to set the PATH
variable to a safe path explicitly. Can you figure out why?
Also there is the IFS variable that might cause trouble if not set
properly. Other environment variables might turn out to compromise
security as well, e.g. SHELL...
Furthermore you must make sure the commands in the script do not
allow interactive shell escapes!
Then there is the umask which may have been set to something
strange...

Etcetera. You should realise that a setuid script `inherits' all the
bugs and security risks of the commands that it calls!

All in all we get the impression setuid shell scripts are quite a
risky business! You may be better off writing a C program instead!

30) What are some useful Unix or C books?

Mitch Wright ([email protected]) maintains a useful list of Unix and
C books, with descriptions and some mini-reviews. There are currently
77 titles on his list.

You can obtain a copy of this list by anonymous ftp from
iuvax.cs.indiana.edu (129.79.254.192), where it's
"pub/Unix-C-Booklist". If you can't use anonymous ftp, email the
line "help" to "[email protected]" for instructions on
retrieving things via email.

Send additions or suggestions to [email protected] .

31) How do I construct a shell glob-pattern that matches all files
except "." and ".." ?

You'd think this would be easy.

* Matches all files that don't begin with a ".";

.* Matches all files that do begin with a ".", but
this includes the special entries "." and "..",
which often you don't want;

.[!.]* (Newer shells only; some shells use a "^" instead of
the "!"; POSIX shells must accept the "!", but may
accept a "^" as well; all portable applications shall
not use an unquoted "^" immediately following the "[")

Matches all files that begin with a "." and are
followed by a non-"."; unfortunately this will miss
"..foo";

.??* Matches files that begin with a "." and which are
at least 3 characters long. This neatly avoids
"." and "..", but also misses ".a" .

So to match all files except "." and ".." safely you have to use
3 patterns (if you don't have filenames like ".a" you can leave out
the first):

.[!.] .??* *

Alternatively you could employ an external program or two and use
backquote substitution. This is pretty good:

`ls -a | sed -e '/^\.$/d' -e '/^\.\.$/d'`

(or `ls -A` in some Unix versions)

but even it will mess up on files with newlines, IFS characters
or wildcards in their names.

32) How do I find the last argument in a Bourne shell script?

Answer by:
Martin Weitzel <@mikros.systemware.de:[email protected]>
Maarten Litmaath

If you are sure the number of arguments is at most 9, you can use:

eval last=\${$#}

In POSIX-compatible shells it works for ANY number of arguments.
The following works always too:

for last
do
:
done

This can be generalized as follows:

for i
do
third_last=$second_last
second_last=$last
last=$i
done

Now suppose you want to REMOVE the last argument from the list,
or REVERSE the argument list, or ACCESS the N-th argument directly,
whatever N may be. Here is a basis of how to do it, using only
built-in shell constructs, without creating subprocesses:

t0= u0= rest='1 2 3 4 5 6 7 8 9' argv=

for h in '' $rest
do
for t in "$t0" $rest
do
for u in $u0 $rest
do
case $# in
0)
break 3
esac
eval argv$h$t$u=\$1
argv="$argv \"\$argv$h$t$u\""# (1)
shift
done
u0=0
done
t0=0
done

# now restore the arguments
eval set x "$argv"# (2)
shift

This example works for the first 999 arguments. Enough?
Take a good look at the lines marked (1) and (2) and convince yourself
that the original arguments are restored indeed, no matter what funny
characters they contain!

To find the N-th argument now you can use this:

eval argN=\$argv$N

To reverse the arguments the line marked (1) must be changed to:

argv="\"\$argv$h$t$u\" $argv"

How to remove the last argument is left as an exercise.

If you allow subprocesses as well, possibly executing nonbuilt-in
commands, the `argvN' variables can be set up more easily:

N=1

for i
do
eval argv$N=\$i
N=`expr $N + 1`
done

To reverse the arguments there is still a simpler method, that even does
not create subprocesses. This approach can also be taken if you want
to delete e.g. the last argument, but in that case you cannot refer
directly to the N-th argument any more, because the `argvN' variables are
set up in reverse order:

argv=

for i
do
eval argv$#=\$i
argv="\"\$argv$#\" $argv"
shift
done

eval set x "$argv"
shift

33) How can I find out which user or process has a file open or is using
a particular file system (so that I can unmount it?)

Use fuser (system V), ofiles (public domain) or fstat (BSD).
These programs will tell you various things about processes
using particular files.

A port of the 4.3 BSD fstat to Dynix, SunOS and Ultrix
can be found in archives of comp.sources.unix, volume 18.


34) How do I keep track of people who are fingering me?

Generally, you can't find out the userid of someone who is
fingering you from a remote machine. You may be able to
find out which machine the remote request is coming from.
One possibility, if your system supports it and assuming
the finger daemon doesn't object, is to make your .plan file a
"named pipe" instead of a plain file. (Use 'mknod' to do this.)

You can then start up a program that will open your .plan file
for writing; the open will block until some other process
(namely fingerd) opens the .plan for reading. Now, not only
can you write whatever you want through this pipe (which lets
you show different .plan information every time someone
fingers you), you can use getpeername() to find out the name of the
peer connected to this socket, which will let you find out from
which remote machine the finger request originates.

Of course, this may not work at all if your system doesn't
support named pipes or if your local fingerd insists
on having plain .plan files.

Getting the remote userid would require that the remote site be
running some sort of RFC931-style authorization daemon, which
relatively few sites currently run.

35) How do I find out the process ID of a program with a particular
name from inside a shell script or C program?

In a shell script:

There is no utility specifically designed to map between program names
and process IDs. Furthermore, such mappings are often unreliable,
since it's possible for more than one process to have the same name,
and since it's possible for a process to change its name once it
starts running. However, a pipeline like this can often be used to
get a list of processes (owned by you) with a particular name:

ps ux | awk '/name/ && !/awk/ {print $2}'

You replace "name" with the name of the process for which you are
searching.

The general idea is to parse the output of ps, using awk or grep or
other utilities, to search for the lines with the specified name on
them, and print the PID's for those lines. Note that the "!/awk/"
above prevents the awk process for being listed.

You may have to change the arguments to ps, depending on what kind of
Unix you are using.

In a C program:

Just as there is no utility specifically designed to map between
program names and process IDs, there are no (portable) C library
functions to do it either.

However, some vendors provide functions for reading Kernel memory; for
example, Sun provides the "kvm_" functions, and Data General provides
the "dg_" functions. It may be possible for any user to use these, or
they may only be useable by the super-user (or a user in group "kmem")
if read-access to kernel memory on your system is restricted.
Furthermore, these functions are often not documented or documented
badly, and might change from release to release.

Some vendors provide a "/proc" filesystem, which appears as a
directory with a bunch of filenames in it. Each filename is a number,
corresponding to a process ID, and you can open the file and read it
to get information about the process. Once again, access to this may
be restricted, and the interface to it may change from system to
system.

If you can't use vendor-specific library functions, and you don't have
/proc, and you still want to do this completely in C, you are going to
have to do the grovelling through kernel memory yourself. For a good
example of how to do this on many systems, see the sources to
"ofiles", available in the comp.sources.unix archives.

If all else fails, you can call popen() on "ps" and parse its output.


36) What happened to the pronunciation list that used to be
part of this document?

Since its inception in 1989, this FAQ document included a comprehensive
pronunciation list maintained by Maarten Litmaath (thanks, Maarten!).
(Does anyone know who *created* it?)

I've retired it, since it is not really relevant to the topic of
"Unix questions". You can still find it as part of the
widely-distributed "Jargon" file (maintained by Eric S. Raymond,
[email protected]) which seems like a much more appropriate
forum for the topic of "How do you pronounce /* ?"

If you'd like a copy, you can ftp one from iuvax.cs.indiana.edu,
(129.79.254.192), it's "pub/Pronunciation-Guide".

--
Steve Hayman Workstation Manager Computer Science Department Indiana U.
[email protected] (812) 855-6984
NeXT Mail: [email protected]




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