If-condition based on standard output

Question

I have few commands linked through pipes, and at the end is a conditional awk: example below

command1 | command 2 | awk '$1 > 800'

Now sometimes it will output few lines, and sometimes no lines. I want a condition that will prove true only if there's some output (1 or more lines) Is there a way to make it work?

Like,

if command1 | command 2 | awk '$1 > 800' (some output); then
do command3
else; (blank output)
Do nothing

Answer 1

I normally just use command substitution, then put it in a test, e.g.

if [ ! -z "$(command1 | command 2 | awk '$1 > 800')" ]; then command3; fi

Explanation

• This runs the command as per your question: command1 | command 2 | awk '$1 > 800'
• The output of this is passed to the test [ ! -z "$(…)" ], which will be true if it is not ! a string of zero length -z.

Hence, if there is output to the command pipe, the then commands will run.

Answer 2

Make the awk script exit with the correct return code for your if statement:

if command1 | command2 | awk '$1 > 800 { c++; print } END { exit (c == 0) }'
then
   command3
fi

Or, if you don't actually need the output of the awk program:

if command1 | command2 | awk '$1 > 800 { c++; exit } END { exit (c == 0) }'
then
   command3
fi

Answer 3

You can let awk print the desired exit code, like this:

echo 900 | awk '{ print !($1 > 800) }'

Then this can be wrapped in a subshell using ( and exit, for returning the value from awk:

echo 900 | (exit $(awk '{ print !($1 > 800) }'))

Which can then be used as part of a pipeline:

echo 900 | (exit $(awk '{ print !($1 > 800) }')) && echo yes || echo no