Timing out in a shell script

Question

I have a shell script that's reading from standard input. In rare circumstances, there will be no one ready to provide input, and the script must time out. In case of timeout, the script must execute some cleanup code. What's the best way to do that?

This script must be very portable, including to 20th century unix systems without a C compiler and to embedded devices running busybox, so Perl, bash, any compiled language, and even the full POSIX.2 can't be relied on. In particular, $PPID, read -t and perfectly POSIX-compliant traps are not available. Writing to a temporary file is also excluded; the script might run even if all filesystems are mounted read-only.

Just to make things more difficult, I also want the script to be reasonably fast when it doesn't time out. In particular, I also use the script in Windows (mainly in Cygwin), where fork and exec are particularly low, so I want to keep their use to a minimum.

In a nutshell, I have

trap cleanup 1 2 3 15
foo=`cat`

and I want to add a timeout. I can't replace cat with the read built-in. In case of timeout, I want to execute the cleanup function.

---

Background: this script is guessing the encoding of the terminal by printing some 8-bit characters and comparing the cursor position before and after. The beginning of the script tests that stdout is connected to a supported terminal, but sometimes the environment is lying (e.g. plink sets TERM=xterm even if it's called with TERM=dumb). The relevant part of the script looks like this:

text='Éé'  # UTF-8; shows up as Ãé on a latin1 terminal
csi='␛['; dsr_cpr="${csi}6n"; dsr_ok="${csi}5n"  # ␛ is an escape character
stty_save=`stty -g`
cleanup () { stty "$stty_save"; }
trap 'cleanup; exit 120' 0 1 2 3 15     # cleanup code
stty eol 0 eof n -echo                # Input will end with `0n`
# echo-n is a function that outputs its argument without a newline
echo-n "$dsr_cpr$dsr_ok"              # Ask the terminal to report the cursor position
initial_report=`tr -dc \;0123456789`  # Expect ␛[42;10R␛[0n for y=42,x=10
echo-n "$text$dsr_cpr$dsr_ok"
final_report=`tr -dc \;0123456789`
cleanup
# Compute and return initial_x - final_x

How can I modify the script so that if tr hasn't read any input after 2 seconds, it is killed and the script executes the cleanup function?

Answer 1

What about this:

foo=`{ { cat 1>&3; kill 0; } | { sleep 2; kill 0; } } 3>&1`

That is: run the output-producing command and sleep in the same process group, a process group just for them. Whichever command returns first kills the whole process group.

Would anyone wonder: Yes, the pipe is not used; it's bypassed using the redirections. The sole purpose of it is to have the shell run the two process in the same process group.

---

As Gilles pointed out in his comment, this won't work in a shell script because the script process would be killed along with the two subprocesses.

One way¹ to force a command to run in a separate process group is to start a new interactive shell:

#!/bin/sh
foo=`sh -ic '{ cat 1>&3; kill 0; } | { sleep 2; kill 0; }' 3>&1 2>/dev/null`
[ -n "$foo" ] && echo got: "$foo" || echo timeouted

But there might be caveats with this (e.g. when stdin is not a tty?). The stderr redirection is there to get rid of the "Terminated" message when the interactive shell is killed.

Tested with zsh,bash and dash. But what about oldies?

B98 suggests the following change, working on Mac OS X, with GNU bash 3.2.57, or Linux with dash:

foo=`sh -ic 'exec 3>&1 2>/dev/null; { cat 1>&3; kill 0; } | { sleep 2; kill 0; }'`

–
^{1. other than setsid which appears to be non-standard.}

Answer 2

me=$$
(sleep 2; kill $me >/dev/null 2>&1) & nuker=$!
# do whatever
kill $nuker >/dev/null 2>&1

You're already trapping 15 (the numeric version of SIGTERM, which is what kill sends unless told otherwise), so you should already be good to go. That said, if you are looking at pre-POSIX, be aware that shell functions may not exist either (they came from System V's shell).

Answer 3

Although coretuils as of version 7.0 includes a timeout command you've mentioned some environments that won't have it. Fortunately pixelbeat.org has a timeout script written sh.

I've used it before on several occasions and it works very well.

http://www.pixelbeat.org/scripts/timeout (Note: The script below has been slightly modified from the one on pixelbeat.org, see the comments below this answer.)

#!/bin/sh

# Execute a command with a timeout

# Author:
#    http://www.pixelbeat.org/
# Notes:
#    Note there is a timeout command packaged with coreutils since v7.0
#    If the timeout occurs the exit status is 124.
#    There is an asynchronous (and buggy) equivalent of this
#    script packaged with bash (under /usr/share/doc/ in my distro),
#    which I only noticed after writing this.
#    I noticed later again that there is a C equivalent of this packaged
#    with satan by Wietse Venema, and copied to forensics by Dan Farmer.
# Changes:
#    V1.0, Nov  3 2006, Initial release
#    V1.1, Nov 20 2007, Brad Greenlee <brad@footle.org>
#                       Make more portable by using the 'CHLD'
#                       signal spec rather than 17.
#    V1.3, Oct 29 2009, Ján Sáreník <jasan@x31.com>
#                       Even though this runs under dash,ksh etc.
#                       it doesn't actually timeout. So enforce bash for now.
#                       Also change exit on timeout from 128 to 124
#                       to match coreutils.
#    V2.0, Oct 30 2009, Ján Sáreník <jasan@x31.com>
#                       Rewritten to cover compatibility with other
#                       Bourne shell implementations (pdksh, dash)

if [ "$#" -lt "2" ]; then
    echo "Usage:   `basename $0` timeout_in_seconds command" >&2
    echo "Example: `basename $0` 2 sleep 3 || echo timeout" >&2
    exit 1
fi

cleanup()
{
    trap - ALRM               #reset handler to default
    kill -ALRM $a 2>/dev/null #stop timer subshell if running
    kill $! 2>/dev/null &&    #kill last job
      exit 124                #exit with 124 if it was running
}

watchit()
{
    trap "cleanup" ALRM
    sleep $1& wait
    kill -ALRM $$
}

watchit $1& a=$!         #start the timeout
shift                    #first param was timeout for sleep
trap "cleanup" ALRM INT  #cleanup after timeout
"$@" < /dev/tty & wait $!; RET=$?    #start the job wait for it and save its return value
kill -ALRM $a            #send ALRM signal to watchit
wait $a                  #wait for watchit to finish cleanup
exit $RET                #return the value

Answer 4

What about (ab)use NC for this

Like;

   $ nc -l 0 2345 | cat &  # output come from here
   $ nc -w 5 0 2345   # input come from here and times out after 5 sec

Or rolled up into a single command line;

   $ foo=`nc -l 0 2222 | nc -w 5 0 2222`

The last version, although looking stange actually does work when I test it on a linux system -- my best guess is that it should work on any system, and if not a variation of output redirection may solve for portability. benefit here is that no background processes are involved.

Answer 5

Another way to run the pipeline in its own process group is to run sh -c '....' in a pseudo-terminal using the script command (which implicitly applies the setsid function).

#!/bin/sh
stty -echo -onlcr
# GNU script
foo=`script -q -c 'sh -c "{ cat 1>&3; kill 0; } | { sleep 5; kill 0; }" 3>&1 2>/dev/null' /dev/null`
# FreeBSD script
#foo=`script -q /dev/null sh -c '{ cat 1>&3; kill 0; } | { sleep 5; kill 0; }' 3>&1 2>/dev/null`
stty echo onlcr
echo "foo: $foo"


# alternative without: stty -echo -onlcr
# cr=`printf '\r'`
# foo=`script -q -c 'sh -c "{ { cat 1>&3; kill 0; } | { sleep 5; kill 0; } } 3>&1 2>/dev/null"' /dev/null | sed -e "s/${cr}$//" -ne 'p;N'`  # GNU
# foo=`script -q /dev/null sh -c '{ { cat 1>&3; kill 0; } | { sleep 5; kill 0; } } 3>&1 2>/dev/null' | sed -e "s/${cr}$//" -ne 'p;N'`  # FreeBSD
# echo "foo: $foo"

Answer 6

The answer in https://unix.stackexchange.com/a/18711 is very nice.

I wanted to achieve a similar outcome, but without having to explicitly invoke the shell again because I wanted to invoke existing shell functions.

Using bash, it is possible to do the following:

eval 'set -m ; ( ... ) ; '"$(set +o)"

So suppose I already have a shell function f:

f() { date ; kill 0 ; }

echo Before
eval 'set -m ; ( f ) ; '"$(set +o)"
echo After

Executing this I see:

$ sh /tmp/foo.sh
Before
Mon 14 Mar 2016 17:22:41 PDT
/tmp/foo.sh: line 4: 17763 Terminated: 15          ( f )
After

Answer 7

timeout_handler () { echo Timeout. goodbye.;  exit 1; }
trap timeout_handler ALRM
( sleep 60; kill -ALRM $$ ) &