Question

I’m trying to run some shell command if a string is not present in a text file. If I paste this line into the command line if gives me an error.

if [ $(cat textfile.txt | grep "search string") -eq "" ]; then; echo "some string"; fi;

Error:

-bash: [: -eq: unary operator expected

Answer 1

If you use [] for comparison you need to use = instead of -eq. You also need some quotes.

if [ "$(cat textfile.txt | grep 'search string')" = "" ]; then; echo "some string"; fi;

Note that grep can take a filename as argument so the cat is unnecessary. You can also directly use the return value of grep: grep returns 1 if the search string is not found.

if [ "$(grep 'search string' textfile.txt)" ]; then
  echo "some string";
fi

An even more compact way would be to use logical and &&.

grep "search string" textfile.txt && echo "some string"