Redirecting command output to a variable in bash fails

Question

I'm trying to redirect command output to a variable:

OUTPUT=$(sudo apache2ctl configtest)

and then to read it:

echo $OUTPUT

When running it the output is the following:

19:19:12 user@user ~ OUTPUT=$(sudo apache2ctl configtest)
Syntax OK
Syntax OK

But the variable stays blank. I've tried the same for other commands and everything works fine.

OUTPUT=$(ls -l)

This writes file list to variable OUTPUT so that it can be read later. What should i do to make it work?

The usual cause is from having a space between OUTPUT and = this isn't true in your case? — ControlAltDel
– ControlAltDel, Commented Apr 25, 2012 at 16:26

Edward Z. Yang · Accepted Answer · 2023-01-19 17:58:00Z

87

Maybe the output goes to stderr, not stdout? Try this:

OUTPUT="$(sudo apache2ctl configtest 2>&1)"

edited Jan 19, 2023 at 17:58

Edward Z. Yang

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answered Apr 25, 2012 at 16:26

johnshen64

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1 Comment

reallynice Over a year ago

Personally I find a bit more explanatory the answer stackoverflow.com/a/3130425/1504300

beliy · Accepted Answer · 2017-03-07 09:20:24Z

2

For nginx possible situation when configtest can be successful with error in config files. Example:

nginx: [warn] conflicting server name "test.com" on 0.0.0.0:80, ignored
nginx: the configuration file /etc/nginx/nginx.conf syntax is ok
nginx: configuration file /etc/nginx/nginx.conf test is successful

for correct check errors in bash scripts need use:

if [[ $((sudo /sbin/service nginx configtest) 2>&1 | grep "failed\|warn" ) ]]; then
    echo "ERROR!!!"
else
    echo "OK!!!"
fi

answered Mar 7, 2017 at 9:20

beliy

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1 Comment

flotzilla Over a year ago

if [[ $(<cmd>) ]]; then; echo error; else echo ok; fi cound be written as <cmd> || { echo error; } && { echo ok; }, which is a bit less verbose and gives the command a bit more prominence. (The curly braces (along with the semicolons) in my example could be left out as well if they only contain one command and the whole thing is a one-liner.)

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