check if a line is empty using bash

Question

Hello,

I am trying to do a simple comparison to check if a line is empty using bash:

line=$(cat test.txt | grep mum )
if [ "$line" -eq "" ]
        then
        echo "mum is not there"
    fi

But it is not working, it says: [: too many arguments

Thanks a lot for your help!

Answer 1

if [ ${line:-null} = null ]; then
    echo "line is empty"
fi

or

if [ -z "${line}" ]; then
    echo "line is empty"
fi

Answer 2

You could also use the $? variable that is set to the return status of the command. So you'd have:

line=$(grep mum test.txt)
if [ $? -eq 1 ]
    then
    echo "mum is not there"
fi

For the grep command if there are any matches $? is set to 0 (exited cleanly) and if there are no matches $? is 1.

Answer 3

The classical sh answer that will also work in bash is

if [ x"$line" = x ]
then
    echo "empty"
fi

Your problem could also be that you are using '-eq' which is for arithmetic comparison.

Answer 4

grep "mum" file || echo "empty"

Answer 5

if line=$(grep -s -m 1 -e mum file.txt)
then
    echo "Found line $line"
else
    echo 'Nothing found or error occurred'
fi