How to grep and then fail an if-statement on specific output from grep?

Question

Ok I need to find the output that a command gives, notely "gbak: ERROR" and then fail on it. I don't know if I'm going about it the right way, I tried to make if fail if the grep did an output to /dev/null but I couldn't get that working either (probably just poor syntax). I'm sure this is a simple one, please let me know.

The if statement I've got at the moment is:

if [ `sudo -u firebird $GBAK_COMMAND | grep "gbak: ERROR"` == *gbak: ERROR* ]; then
   echo "$DATE Unsucessful $1.gdb Gbak. Incorrect user/password" >> /var/log/messages
   echo "Failed"
   exit 1
else
   echo "pass"
fi

Instead of backticks, which are deprecated, you should always use $(...) if you need. It is better readable, font-agnostic and you can easyly nest it a=$(b $(c $(d)) $(e)) — user unknown
– user unknown, Commented Mar 8, 2011 at 2:55
Related: Grep output of command and use it in “if” statement, bash and grep in IF statement — blong
– blong, Commented May 27, 2014 at 14:08
Also related: How do I use a file grep comparison inside a bash if/else statement? — blong
– blong, Commented May 27, 2014 at 14:21

Jonathan Leffler · Accepted Answer · 2012-04-19 14:11:49Z

14

You don't need the 'test' operator square brackets; just test the exit status of grep:

if sudo -u firebird $GBAK_COMMAND | grep -q "gbak: ERROR"
then
   echo "$DATE Unsuccessful $1.gdb Gbak. Incorrect user/password" >> /var/log/messages
   echo "Failed"
   exit 1
else
   echo "pass"
fi

The -q option to grep suppresses all output (POSIX and GNU variants) so you just get the status, which the if tests. If grep finds the pattern, it returns success, which means that the firebird command failed. The only residual issue is 'does firebird write its error to standard output or to standard error?' If you need to redirect the standard error, write:

if sudo -u firebird $GBAK_COMMAND 2>&1 | grep -q "gbak: ERROR"
then
   echo "$DATE Unsuccessful $1.gdb Gbak. Incorrect user/password" >> /var/log/messages
   echo "Failed"
   exit 1
else
   echo "pass"
fi

edited Apr 19, 2012 at 14:11

answered Mar 8, 2011 at 1:49

Jonathan Leffler

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3 Comments

Jeremiah Willcock Over a year ago

For GNU grep, you might want -q instead of/in addition to -s (GNU -s only suppresses complaints about unreadable files and such, not matched lines).

edumike Over a year ago

Great work, thanks! I've fixed it up now no problems. Yes very interesting, if I don't have 2>&1 there it will pass straight over it as if it's passed. I need to learn about this std error stuff as I don't know too much about it or what it does.

Jonathan Leffler Over a year ago

@Jeremiah: yes - you're right; and it is not just GNU grep but also POSIX. I'll modify the answer - thanks.

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