Timeout a command efficiently and capture output to a variable in bash script

Question

I need to run a command and kill it when running too long, in a bash script. I also need to capture all output to a variable. If the command finishes first, I need to release/kill the watchdog process (e.g. sleep) because I may run a list of such commands.

Unfortunately the "timeout" command is not available to me, othervise I could do something like this:

output=`timeout -s 9 $TIMEOUT my-command`

and check for the exit code 124 to see if there was a timeout.

Therefore my solution of choice is by @Dmitry to a similar question:

( my_command ) & pid=$!
( sleep $TIMEOUT && kill -HUP $pid ) 2>/dev/null & watcher=$!
wait $pid 2>/dev/null && pkill -HUP -P $watcher

Unfortunately the following does not capture anything to the $output:

( output=`my_command` ) & pid=$!

I could dump the output to a file and then load it in the variable like this, but I'd rather do without files:

( `my_command >$outfile` ) & pid=$!
...
output=`cat $outfile`
rm -f $outfile

My question: is there a better way? Ideally capturing the stderr as well to another variable without using files?

Vaughn Cato · Accepted Answer · 2013-06-29 17:16:19Z

1

Fortunately, the $() notation allows for multiple commands, so you can do this:

output=$(
    ( my_command ) & pid=$!
    ( sleep $TIMEOUT && kill -HUP $pid ) 2>/dev/null & watcher=$!
    wait $pid 2>/dev/null && pkill -HUP -P $watcher
)

You can also use regular () to group commands and then redirect all their output. Redirecting stderr to stdout can be done using 2>&1, so you end up with this:

output=$(
    (
        ( my_command ) & pid=$!
        ( sleep $TIMEOUT && kill -HUP $pid ) 2>/dev/null & watcher=$!
        wait $pid 2>/dev/null && pkill -HUP -P $watcher
    ) 2>&1
)

edited Jun 29, 2013 at 17:16

answered Jun 29, 2013 at 17:11

Vaughn Cato

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11 Comments

gniourf_gniourf Over a year ago

Grouping is done with curly brackets {...}, not parentheses.

Vaughn Cato Over a year ago

@gniourf_gniourf: curly braces may work as well, but parentheses do work.

gniourf_gniourf Over a year ago

Parentheses are for subshells. Please see the Grouping Commands section in the bash reference manual

jaypal singh Over a year ago

@gniourf_gniourf Cool, I didn't know that. I thought they were the same with the only difference that we had to place a ; before end squiggly bracket. Good stuff.

Vaughn Cato Over a year ago

@gniourf_gniourf: A subshell seems appropriate in this case since there's no need to keep the variables that were used for implementing the timeout functionality. Some may argue it is less efficient to use a subshell, but that's another issue.

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