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Archive-name: unix-faq/faq/part3
Version: $Id: part3,v 2.9 1996/06/11 13:07:56 tmatimar Exp $
These seven articles contain the answers to some Frequently Asked
Questions often seen in comp.unix.questions and comp.unix.shell.
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 collection of documents is Copyright (c) 1994, Ted Timar, except
Part 6, which is Copyright (c) 1994, Pierre Lewis and Ted Timar.
All rights reserved. Permission to distribute the collection is
hereby granted providing that distribution is electronic, no money
is involved, reasonable attempts are made to use the latest version
and all credits and this copyright notice are maintained.
Other requests for distribution will be considered. All reasonable
requests will be granted.
All information here has been contributed with good intentions, but
none of it is guaranteed either by the contributors or myself to be
accurate. The users of this information take all responsibility for
any damage that may occur.
Many FAQs, including this one, are available on the archive site
rtfm.mit.edu in the directory pub/usenet/news.answers.
The name under which a FAQ is archived appears in the "Archive-Name:"
line at the top of the article. This FAQ is archived as
"unix-faq/faq/part[1-7]".
These articles are divided approximately as follows:
1.*) General questions.
2.*) Relatively basic questions, likely to be asked by beginners.
3.*) Intermediate questions.
4.*) Advanced questions, likely to be asked by people who thought
they already knew all of the answers.
5.*) Questions pertaining to the various shells, and the differences.
6.*) An overview of Unix variants.
7.*) An comparison of configuration management systems (RCS, SCCS).
This article includes answers to:
3.1) How do I find the creation time of a file?
3.2) How do I use "rsh" without having the rsh hang around
until the remote command has completed?
3.3) How do I truncate a file?
3.4) Why doesn't find's "{}" symbol do what I want?
3.5) How do I set the permissions on a symbolic link?
3.6) How do I "undelete" a file?
3.7) How can a process detect if it's running in the background?
3.8) Why doesn't redirecting a loop work as intended? (Bourne shell)
3.9) How do I run 'passwd', 'ftp', 'telnet', 'tip' and other interactive
programs from a shell script or in the background?
3.10) How do I find the process ID of a program with a particular
name from inside a shell script or C program?
3.11) How do I check the exit status of a remote command
executed via "rsh" ?
3.12) Is it possible to pass shell variable settings into an awk program?
3.13) How do I get rid of zombie processes that persevere?
3.14) How do I get lines from a pipe as they are written instead of
only in larger blocks?
3.15) How do I get the date into a filename?
3.16) Why do some scripts start with #! ... ?
If you're looking for the answer to, say, question 3.5, and want to skip
everything else, you can search ahead for the regular expression "^3.5)".
While these are all legitimate questions, they seem to crop up in
comp.unix.questions or comp.unix.shell 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].
----------------------------------------------------------------------
Subject: How do I find the creation time of a file?
Date: Thu Mar 18 17:16:55 EST 1993
3.1) How do I find 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.
------------------------------
Subject: How do I use "rsh" without having the rsh hang around ... ?
Date: Thu Mar 18 17:16:55 EST 1993
3.2) How do I use "rsh" without having the rsh hang around until the
remote command has completed?
(See note in question 2.7 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 </dev/null &'
If you use sh on the remote machine:
rsh machine -n 'command >/dev/null 2>&1 </dev/null &'
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.
------------------------------
Subject: How do I truncate a file?
Date: Mon, 27 Mar 1995 18:09:10 -0500
3.3) How do I truncate a file?
The BSD function ftruncate() sets the length of a file.
(But not all versions behave identically.) Other Unix variants
all seem to support some version of truncation as well.
For systems which support the ftruncate function, there are
three known behaviours:
BSD 4.2 - Ultrix, SGI, LynxOS
- truncation doesn't grow file
- truncation doesn't move file pointer
BSD 4.3 - SunOS, Solaris, OSF/1, HP/UX, Amiga
- truncation can grow file
- truncation doesn't move file pointer
Cray - UniCOS 7, UniCOS 8
- truncation doesn't grow file
- truncation changes file pointer
Other systems come in four varieties:
F_CHSIZE - Only SCO
- some systems define F_CHSIZE but don't support it
- behaves like BSD 4.3
F_FREESP - Only Interative Unix
- some systems (eg. Interactive Unix) define F_FREESP but
don't support it
- behaves like BSD 4.3
chsize() - QNX and SCO
- some systems (eg. Interactive Unix) have chsize() but
don't support it
- behaves like BSD 4.3
nothing - no known systems
- there will be systems that don't support truncate at all
Moderator's Note: I grabbed the functions below a few years back.
I can no longer identify the original author.
S. Spencer Sun <[email protected]> has also
contributed a version for F_FREESP.
functions for each non-native ftruncate follow
/* ftruncate emulations that work on some System V's.
This file is in the public domain. */
#include
#include
#ifdef F_CHSIZE
int
ftruncate (fd, length)
int fd;
off_t length;
{
return fcntl (fd, F_CHSIZE, length);
}
#else
#ifdef F_FREESP
/* The following function was written by
[email protected] (William Kucharski) */
#include
#include
#include
int
ftruncate (fd, length)
int fd;
off_t length;
{
struct flock fl;
struct stat filebuf;
if (fstat (fd, &filebuf) < 0)
return -1;
if (filebuf.st_size < length)
{
/* Extend file length. */
if (lseek (fd, (length - 1), SEEK_SET) < 0)
return -1;
/* Write a "0" byte. */
if (write (fd, "", 1) != 1)
return -1;
}
else
{
/* Truncate length. */
fl.l_whence = 0;
fl.l_len = 0;
fl.l_start = length;
fl.l_type = F_WRLCK; /* Write lock on file space. */
/* This relies on the UNDOCUMENTED F_FREESP argument to
fcntl, which truncates the file so that it ends at the
position indicated by fl.l_start.
Will minor miracles never cease? */
if (fcntl (fd, F_FREESP, &fl) < 0)
return -1;
}
return 0;
}
#else
int
ftruncate (fd, length)
int fd;
off_t length;
{
return chsize (fd, length);
}
#endif
#endif
------------------------------
Subject: Why doesn't find's "{}" symbol do what I want?
Date: Thu Mar 18 17:16:55 EST 1993
3.4) 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.
------------------------------
Subject: How do I set the permissions on a symbolic link?
Date: Thu Mar 18 17:16:55 EST 1993
3.5) 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.
------------------------------
Subject: How do I "undelete" a file?
Date: Thu Mar 18 17:16:55 EST 1993
3.6) 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 were part of 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)
------------------------------
Subject: How can a process detect if it's running in the background?
Date: Thu Mar 18 17:16:55 EST 1993
3.7) 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.
------------------------------
Subject: Why doesn't redirecting a loop work as intended? (Bourne shell)
Date: Thu Mar 18 17:16:55 EST 1993
3.8) 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.
------------------------------
Subject: How do I run ... interactive programs from a shell script ... ?
Date: Thu Mar 18 17:16:55 EST 1993
3.9) How do I run 'passwd', 'ftp', 'telnet', 'tip' and other interactive
programs from a shell script or in the background?
These programs expect a terminal interface. Shells makes no
special provisions to provide one. Hence, such programs cannot
be automated in shell scripts.
The 'expect' program provides a programmable terminal interface
for automating interaction with such programs. 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
expect can partially automate interaction which is especially
useful for telnet, rlogin, debuggers or other programs that have
no built-in command language. The distribution provides an
example script to rerun rogue until a good starting configuration
appears. Then, control is given back to the user to enjoy the game.
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.
To get expect, email "send pub/expect/expect.shar.Z" to
[email protected] or anonymous ftp same from
ftp.cme.nist.gov.
Another solution is provided by the pty 4.0 program, which runs a
program under a pseudo-tty session and was posted to
comp.sources.unix, volume 25. A pty-based solution using named
pipes to do the same as the above might look like this:
#!/bin/sh
/etc/mknod out.$$ p; exec 2>&1
( exec 4<out.$$; rm -f out.$$
<&4 waitfor 'password:'
echo "$2"
<&4 waitfor 'password:'
echo "$2"
<&4 cat >/dev/null
) | ( pty passwd "$1" >out.$$ )
Here, 'waitfor' is a simple C program that searches for
its argument in the input, character by character.
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"
------------------------------
Subject: How do I find the process ID of a program with a particular name ... ?
Date: Thu Mar 18 17:16:55 EST 1993
3.10) How do I find 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 rummaging 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. (A package named "kstuff" to help with kernel
rummaging was posted to alt.sources in May 1991 and is also
available via anonymous ftp as
usenet/alt.sources/articles/{329{6,7,8,9},330{0,1}}.Z from
wuarchive.wustl.edu.)
------------------------------
Subject: How do I check the exit status of a remote command executed via "rsh"?
Date: Thu Mar 18 17:16:55 EST 1993
3.11) How do I check the exit status of a remote command
executed via "rsh" ?
This doesn't work:
rsh some-machine some-crummy-command || echo "Command failed"
The exit status of 'rsh' is 0 (success) if the rsh program
itself completed successfully, which probably isn't what
you wanted.
If you want to check on the exit status of the remote program,
you can try using Maarten Litmaath's 'ersh' script, which was
posted to alt.sources in October 1994. ersh is a shell script
that calls rsh, arranges for the remote machine to echo the
status of the command after it completes, and exits with that
status.
------------------------------
Subject: Is it possible to pass shell variable settings into an awk program?
Date: Thu Mar 18 17:16:55 EST 1993
3.12) Is it possible to pass shell variable settings into an awk program?
There are two different ways to do this. The first involves
simply expanding the variable where it is needed in the program.
For example, to get a list of all ttys you're using:
who | awk '/^'"$USER"'/ { print $2 }' (1)
Single quotes are usually used to enclose awk programs because
the character '$' is often used in them, and '$' will be
interpreted by the shell if enclosed inside double quotes, but
not if enclosed inside single quotes. In this case, we *want*
the '$' in "$USER" to be interpreted by the shell, so we close
the single quotes and then put the "$USER" inside double quotes.
Note that there are no spaces in any of that, so the shell will
see it all as one argument. Note, further, that the double
quotes probably aren't necessary in this particular case (i.e. we
could have done
who | awk '/^'$USER'/ { print $2 }' (2)
), but they should be included nevertheless because they are
necessary when the shell variable in question contains special
characters or spaces.
The second way to pass variable settings into awk is to use an
often undocumented feature of awk which allows variable settings
to be specified as "fake file names" on the command line. For
example:
who | awk '$1 == user { print $2 }' user="$USER" - (3)
Variable settings take effect when they are encountered on the
command line, so, for example, you could instruct awk on how to
behave for different files using this technique. For example:
awk '{ program that depends on s }' s=1 file1 s=0 file2 (4)
Note that some versions of awk will cause variable settings
encountered before any real filenames to take effect before the
BEGIN block is executed, but some won't so neither way should be
relied upon.
Note, further, that when you specify a variable setting, awk
won't automatically read from stdin if no real files are
specified, so you need to add a "-" argument to the end of your
command, as I did at (3) above.
A third option is to use a newer version of awk (nawk), which allows
direct access to environment vairables. Eg.
nawk 'END { print "Your path variable is " ENVIRON["PATH"] }' /dev/null
------------------------------
Subject: How do I get rid of zombie processes that persevere?
>From: Jonathan I. Kamens
>From: [email protected] (Casper Dik)
Date: Thu, 09 Sep 93 16:39:58 +0200
3.13) How do I get rid of zombie processes that persevere?
Unfortunately, it's impossible to generalize how the death of
child processes should behave, because the exact mechanism varies
over the various flavors of Unix.
First of all, by default, you have to do a wait() for child
processes under ALL flavors of Unix. That is, there is no flavor
of Unix that I know of that will automatically flush child
processes that exit, even if you don't do anything to tell it to
do so.
Second, under some SysV-derived systems, if you do
"signal(SIGCHLD, SIG_IGN)" (well, actually, it may be SIGCLD
instead of SIGCHLD, but most of the newer SysV systems have
"#define SIGCHLD SIGCLD" in the header files), then child
processes will be cleaned up automatically, with no further
effort in your part. The best way to find out if it works at
your site is to try it, although if you are trying to write
portable code, it's a bad idea to rely on this in any case.
Unfortunately, POSIX doesn't allow you to do this; the behavior
of setting the SIGCHLD to SIG_IGN under POSIX is undefined, so
you can't do it if your program is supposed to be
POSIX-compliant.
So, what's the POSIX way? As mentioned earlier, you must
install a signal handler and wait. Under POSIX signal handlers
are installed with sigaction. Since you are not interested in
``stopped'' children, only in terminated children, add SA_NOCLDSTOP
to sa_flags. Waiting without blocking is done with waitpid().
The first argument to waitpid should be -1 (wait for any pid),
the third should be WNOHANG. This is the most portable way
and is likely to become more portable in future.
If your systems doesn't support POSIX, there's a number of ways.
The easiest way is signal(SIGCHLD, SIG_IGN), if it works.
If SIG_IGN cannot be used to force automatic clean-up, then you've
got to write a signal handler to do it. It isn't easy at all to
write a signal handler that does things right on all flavors of
Unix, because of the following inconsistencies:
On some flavors of Unix, the SIGCHLD signal handler is called if
one *or more* children have died. This means that if your signal
handler only does one wait() call, then it won't clean up all of
the children. Fortunately, I believe that all Unix flavors for
which this is the case have available to the programmer the
wait3() or waitpid() call, which allows the WNOHANG option to
check whether or not there are any children waiting to be cleaned
up. Therefore, on any system that has wait3()/waitpid(), your
signal handler should call wait3()/waitpid() over and over again
with the WNOHANG option until there are no children left to clean
up. Waitpid() is the preferred interface, as it is in POSIX.
On SysV-derived systems, SIGCHLD signals are regenerated if there
are child processes still waiting to be cleaned up after you exit
the SIGCHLD signal handler. Therefore, it's safe on most SysV
systems to assume when the signal handler gets called that you
only have to clean up one signal, and assume that the handler
will get called again if there are more to clean up after it
exits.
On older systems, there is no way to prevent signal handlers
from being automatically reset to SIG_DFL when the signal
handler gets called. On such systems, you have to put
"signal(SIGCHILD, catcher_func)" (where "catcher_func" is the
name of the handler function) as the last thing in the signal
handler, so that it gets reset.
Fortunately, newer implementations allow signal handlers to be
installed without being reset to SIG_DFL when the handler
function is called. To get around this problem, on systems that
do not have wait3()/waitpid() but do have SIGCLD, you need to
reset the signal handler with a call to signal() after doing at
least one wait() within the handler, each time it is called. For
backward compatibility reasons, System V will keep the old
semantics (reset handler on call) of signal(). Signal handlers
that stick can be installed with sigaction() or sigset().
The summary of all this is that on systems that have waitpid()
(POSIX) or wait3(), you should use that and your signal handler
should loop, and on systems that don't, you should have one call
to wait() per invocation of the signal handler.
One more thing -- if you don't want to go through all of this
trouble, there is a portable way to avoid this problem, although
it is somewhat less efficient. Your parent process should fork,
and then wait right there and then for the child process to
terminate. The child process then forks again, giving you a
child and a grandchild. The child exits immediately (and hence
the parent waiting for it notices its death and continues to
work), and the grandchild does whatever the child was originally
supposed to. Since its parent died, it is inherited by init,
which will do whatever waiting is needed. This method is
inefficient because it requires an extra fork, but is pretty much
completely portable.
------------------------------
Subject: How do I get lines from a pipe ... instead of only in larger blocks?
>From: Jonathan I. Kamens
Date: Sun, 16 Feb 92 20:59:28 -0500
3.14) How do I get lines from a pipe as they are written instead of only in
larger blocks?
The stdio library does buffering differently depending on whether
it thinks it's running on a tty. If it thinks it's on a tty, it
does buffering on a per-line basis; if not, it uses a larger
buffer than one line.
If you have the source code to the client whose buffering you
want to disable, you can use setbuf() or setvbuf() to change the
buffering.
If not, the best you can do is try to convince the program that
it's running on a tty by running it under a pty, e.g. by using
the "pty" program mentioned in question 3.9.
------------------------------
Subject: How do I get the date into a filename?
>From: melodie neal <[email protected]>
Date: Fri, 7 Oct 1994 09:27:33 -0400
3.15) How do I get the date into a filename?
This isn't hard, but it is a bit cryptic at first sight. Let's
begin with the date command itself: date can take a formatting
string, to modify the way in which the date info is printed. The
formatting string has to be enclosed in quotes, to stop the shell
trying to interpret it before the date command itself gets it.
Try this:
date '+%d%m%y'
you should get back something like 130994. If you want to
punctuate this, just put the characters you would like to use in
the formatting string (NO SLASHES '/'):
date '+%d.%m.%y'
There are lots of token you can use in the formatting string:
have a look at the man page for date to find out about them.
Now, getting this into a file name. Let's say that we want to
create files called report.130994 (or whatever the date is today):
FILENAME=report.`date '+%d%m%y'`
Notice that we are using two sets of quotes here: the inner set
are to protect the formatting string from premature
interpretation; the outer set are to tell the shell to execute
the enclosed command, and substitute the result into the
expression (command substitution).
------------------------------
Subject: Why do some scripts start with #! ... ?
>From: chip@@chinacat.unicom.com (Chip Rosenthal)
Date: Tue, 14 Jul 1992 21:31:54 GMT
3.16) Why do some scripts start with #! ... ?
Chip Rosenthal has answered a closely related question in
comp.unix.xenix in the past.
I think what confuses people is that there exist two different
mechanisms, both spelled with the letter `#'. They both solve the
same problem over a very restricted set of cases -- but they are
none the less different.
Some background. When the UNIX kernel goes to run a program (one
of the exec() family of system calls), it takes a peek at the
first 16 bits of the file. Those 16 bits are called a `magic
number'. First, the magic number prevents the kernel from doing
something silly like trying to execute your customer database
file. If the kernel does not recognize the magic number then it
complains with an ENOEXEC error. It will execute the program only
if the magic number is recognizable.
Second, as time went on and different executable file formats were
introduced, the magic number not only told the kernel *if* it
could execute the file, but also *how* to execute the file. For
example, if you compile a program on an SCO XENIX/386 system and
carry the binary over to a SysV/386 UNIX system, the kernel will
recognize the magic number and say `Aha! This is an x.out
binary!' and configure itself to run with XENIX compatible system
calls.
Note that the kernel can only run binary executable images. So
how, you might ask, do scripts get run? After all, I can type
`my.script' at a shell prompt and I don't get an ENOEXEC error.
Script execution is done not by the kernel, but by the shell. The
code in the shell might look something like:
/* try to run the program */
execl(program, basename(program), (char *)0);
/* the exec failed -- maybe it is a shell script? */
if (errno == ENOEXEC)
execl ("/bin/sh", "sh", "-c", program, (char *)0);
/* oh no mr bill!! */
perror(program);
return -1;
(This example is highly simplified. There is a lot
more involved, but this illustrates the point I'm
trying to make.)
If execl() is successful in starting the program then the code
beyond the execl() is never executed. In this example, if we can
execl() the `program' then none of the stuff beyond it is run.
Instead the system is off running the binary `program'.
If, however, the first execl() failed then this hypothetical shell
looks at why it failed. If the execl() failed because `program'
was not recognized as a binary executable, then the shell tries to
run it as a shell script.
The Berkeley folks had a neat idea to extend how the kernel starts
up programs. They hacked the kernel to recognize the magic number
`#!'. (Magic numbers are 16-bits and two 8-bit characters makes
16 bits, right?) When the `#!' magic number was recognized, the
kernel would read in the rest of the line and treat it as a
command to run upon the contents of the file. With this hack you
could now do things like:
#! /bin/sh
#! /bin/csh
#! /bin/awk -F:
This hack has existed solely in the Berkeley world, and has
migrated to USG kernels as part of System V Release 4. Prior to
V.4, unless the vendor did some special value added, the kernel
does not have the capability of doing anything other than loading
and starting a binary executable image.
Now, lets rewind a few years, to the time when more and more folks
running USG based unices were saying `/bin/sh sucks as an
interactive user interface! I want csh!'. Several vendors did
some value added magic and put csh in their distribution, even
though csh was not a part of the USG UNIX distribution.
This, however, presented a problem. Let's say you switch your
login shell to /bin/csh. Let's further suppose that you are a
cretin and insist upon programming csh scripts. You'd certainly
want to be able to type `my.script' and get it run, even though it
is a csh script. Instead of pumping it through /bin/sh, you want
the script to be started by running:
execl ("/bin/csh", "csh", "-c", "my.script", (char *)0);
But what about all those existing scripts -- some of which are
part of the system distribution? If they started getting run by
csh then things would break. So you needed a way to run some
scripts through csh, and others through sh.
The solution introduced was to hack csh to take a look at the
first character of the script you are trying to run. If it was a
`#' then csh would try to run the script through /bin/csh,
otherwise it would run the script through /bin/sh. The example
code from the above might now look something like:
/* try to run the program */
execl(program, basename(program), (char *)0);
/* the exec failed -- maybe it is a shell script? */
if (errno == ENOEXEC && (fp = fopen(program, "r")) != NULL) {
i = getc(fp);
(void) fclose(fp);
if (i == '#')
execl ("/bin/csh", "csh", "-c", program, (char *)0);
else
execl ("/bin/sh", "sh", "-c", program, (char *)0);
}
/* oh no mr bill!! */
perror(program);
return -1;
Two important points. First, this is a `csh' hack. Nothing has
been changed in the kernel and nothing has been changed in the
other shells. If you try to execl() a script, whether or not it
begins with `#', you will still get an ENOEXEC failure. If you
try to run a script beginning with `#' from something other than
csh (e.g. /bin/sh), then it will be run by sh and not csh.
Second, the magic is that either the script begins with `#' or it
doesn't begin with `#'. What makes stuff like `:' and `: /bin/sh'
at the front of a script magic is the simple fact that they are
not `#'. Therefore, all of the following are identical at the
start of a script:
:
: /bin/sh
<--- a blank line
: /usr/games/rogue
echo "Gee...I wonder what shell I am running under???"
In all these cases, all shells will try to run the script with /bin/sh.
Similarly, all of the following are identical at the start of a script:
#
# /bin/csh
#! /bin/csh
#! /bin/sh
# Gee...I wonder what shell I am running under???
All of these start with a `#'. This means that the script will be
run by csh *only* if you try to start it from csh, otherwise it
will be run by /bin/sh.
(Note: if you are running ksh, substitute `ksh' for
`sh' in the above. The Korn shell is theoretically
compatible with Bourne shell, so it tries to run these
scripts itself. Your mileage may vary on some of the
other available shells such as zsh, bash, etc.)
Obviously, if you've got support for `#!' in the kernel then the
`#' hack becomes superfluous. In fact, it can be dangerous
because it creates confusion over what should happen with `#! /bin/sh'.
The `#!' handling is becoming more and more prevelant. System V
Release 4 picks up a number of the Berkeley features, including
this. Some System V Release 3.2 vendors are hacking in some of
the more visible V.4 features such as this and trying to convince
you this is sufficient and you don't need things like real,
working streams or dynamically adjustable kernel parameters.
XENIX does not support `#!'. The XENIX /bin/csh does have the `#'
hack. Support for `#!' in XENIX would be nice, but I wouldn't
hold my breath waiting for it.
------------------------------
End of unix/faq Digest part 3 of 7
**********************************
--
Ted Timar - [email protected]
ISG Technologies Inc., 6509 Airport Road, Mississauga, Ontario, Canada L4V 1S7
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