How do I run a command, and have it abort (timeout) after N seconds?

FIRST check whether the command you're running can be told to timeout directly. The methods described here are "hacky" workarounds to force a command to terminate after a certain time has elapsed. Configuring your command properly is always preferable to the alternatives below.

If the command has no native support for stopping after a specified time, then you're forced to use an external wrapper. There are a few available now:

Beware: by default, some implementations of timeout issue a SIGKILL (kill -9), which is roughly the same as pulling out the power cord, leaving no chance for the program to commit its work, often resulting in corruption of its data. You should use a signal that allows the program to shut itself down cleanly instead (i.e. SIGTERM). See ProcessManagement for more information on SIGKILL.

Also be aware that some of these wrappers "exec" your program after setting up an alarm, which makes them wonderful to use in wrapper scripts, while others launch your program as a child and then hang around (because they want to send a second signal if the first one is ignored, or whatever). Be sure to read your tool's documentation.

If you don't have or don't want one of the above programs, you can use a perl one-liner to set an ALRM and then exec the program you want to run under a time limit. In any case, you must understand what your program does with SIGALRM; programs with periodic updates usually use ALRM for that purpose and update rather than dying when they receive that signal.

doalarm() { perl -e 'alarm shift; exec @ARGV' -- "$@"; }

doalarm ${NUMBER_OF_SECONDS_BEFORE_ALRMING} program arg arg ...

If you don't even have perl, then the best you can do is an ugly hack like:

command & pid=$!
{ sleep 10; kill "$pid"; } &

This will, as you will soon discover, produce quite a mess regardless of whether the timeout condition kicked in or not, if it's run in an interactive shell. Cleaning it up is not something worth my time. Also, it can't be used with any command that requires a foreground terminal, like top.

It is possible to do something similar, but to keep command in the foreground:

sh -c '(sleep 10; kill "$$") & exec command'

kill $$ would kill the shell, except that exec causes the command to take over the shell's PID. It is necessary to use sh -c so that the calling shell isn't replaced; in bash 4, it is possible to use a subshell instead:

( cmdpid=$BASHPID; (sleep 10; kill "$cmdpid") & exec command )

The shell-script "timeout" (not to be confused with any of the commands named timeout) uses the second approach above. It has the advantage of working immediately (no need for compiling a program), but has problems e.g. with programs reading standard input.

But... just use one of the timeout or doalarm commands instead. Really.

BashFAQ/068 (last edited 2019-07-25 13:38:08 by GreyCat)