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Text File | 1992-12-27 | 27KB | 615 lines

Archive-name: unix-faq/part3
Version: $Id: part3,v 1.4 92/03/19 14:07:42 tmatimar Exp $

These four 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.

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.

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
[e-mail].

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 &'

    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.

3)  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).

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.

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.

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 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)

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.

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.

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
    [e-mail] or anonymous ftp same from durer.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"

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: [e-mail] (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: [e-mail] (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.
--
Ted Timar - [e-mail]
Omron Corporation, Shimokaiinji, Nagaokakyo-city, Kyoto 617,  Japan