This article includes answers to: 3.1) How do I find out 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 out 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. 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 "^5)". 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 tmatimar@nff.ncl.omron.co.jp. 1) 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. 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 2>&1 &1 ( exec 4/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" 10) 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. (A package named "kstuff" to help with kernel grovelling 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.) 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 January, 1991. 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. 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. 13) How do I get rid of zombie processes that persevere? From: jik@pit-manager.MIT.Edu (Jonathan I. Kamens) Date: Fri, 17 Jan 92 14:40:09 -0500 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. If you can't use SIG_IGN 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() 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(), your signal handler should call wait3() over and over again with the WNOHANG option until there are no children left to clean up. 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, signal handlers are 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 first thing in the signal handler, so that it gets reset. Unfortunately, there is a race condition which may cause you to get a SIGCHLD signal and have it ignored between the time your handler gets called and the time you reset the signal. Fortunately, newer implementations of signal() don't reset the handler to SIG_DFL when the handler function is called. To get around this problem, on systems that do not have wait3() 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. The summary of all this is that on systems that have 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. 14) How do I get lines from a pipe as they are written instead of only in larger blocks. From: jik@pit-manager.MIT.Edu (Jonathan I. Kamens) Date: Sun, 16 Feb 92 20:59:28 -0500 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. .