grep - exit with 1 - if match

Question

I have a bash function like so:

run_mongo(){
  mongo foo bar baz 2>&1  # send stderr to stdout
}

unfortunately this mongo command doesn't exit with 1 if there is error, so I have to match on stdout/stderr

is there some way to exit with code > 0 with grep if there is a first match? something like this:

run_mongo | grep -e "if stdout/stderr matches this we exit with 1"

I assume the way to do this would be like so:

run_mongo | grep -e "if stdout/stderr matches" | killer

where killer is a program that dies as soon as it gets its first bit of stdin.

Answer 1

Yes, you can do it with grep -vz which tells grep to find lines that don't match the pattern you give it (-v) and to read the entire input at once (z), so that one match means the whole thing fails:

$ printf 'foo\nbar\n' | grep -zqv foo && echo no || echo yes
yes
$ printf 'foo\nbar\n' | grep -zq foo && echo no || echo yes
no

So, in your case, something like:

run_mongo(){
  mongo foo bar baz 2>&1 | grep -vzq "whatever means failure" 
}

if run_mongo; then
 echo "Worked!"
else
  echo "Failed!"
fi

---

If you want to avoid reading the whole output, just use another tool. Perl, for example:

mongo foo bar baz 2>&1 | perl -ne 'exit 1 if /whatever means failure/' 

Answer 2

If you need to terminate as quickly as possible:

run_mongo () {
    ! mongo foo bar baz 2>&1 | grep -q 'bad-pattern'
}

This would run mongo and terminate the function as soon as possible if the pattern bad-pattern is found in the output. When the pattern is matched, grep exits immediately, and mongo receives a PIPE signal as soon as it tries to write to the dead pipe.

Due to the !, the exit status of the pipeline will be inverted, i.e. it will be zero if the pattern wasn't found or non-zero if it was found.

If you need to let mongo finish writing its output:

run_mongo () {
    ! mongo foo bar baz 2>&1 | grep 'bad-pattern' > /dev/null
}

This is the same as the first piece of code in this answer, but grep now consumes all output from mongo.

Answer 3

@terdon's solution looks good, since I am a Node.js fanboy, I just wrote a program like this:

#!/usr/bin/env node

process.stdin.resume().once('data', function () {
   process.exit(1);
});

then in my bash script have:

run_mongo(){
  mongo foo bar baz 2>&1
}

exit_if_match(){
    grep "$1" | "./node_modules/.bin/imminent_suicide"
}

run_mongo "foo" "bar" | exit_if_match "errmsg"

Answer 4

echo 'if stdout/stderr matches this' |
    awk '$0 ~ "if stdout/stderr matches this" { exit 1; }' ; echo $?