Test if a command's output is an empty string and print its results if its not empty [duplicate]

Question

I am using solutions from Test if a command outputs an empty string. In addition to those solutions, I want to print command's output if its not empty.

I was wondering would it be possible to print command's output if its not empty by calling the command only one time.

I can do it by re-calling the same command inside the if condition, but now second call may consume additional CPU usage, since we already obtained its result.

---

Possible approach could be, where the same command is called two times:

if [[ $(ls -A) ]]; then
    ls -A
else
    echo "no files found"
fi

---

output=$(git diff --ignore-blank-lines --color-words HEAD | tr -d ' \n\r\t ' | wc -c)
if [ "$output" -gt 0 ]; then
    git diff --ignore-blank-lines --color-words HEAD 
else
    echo "diff is empty"
fi

Answer 1

Note: The question was originally tagged bash, but OP has since stated that the shell used is zsh. This will invalidate the answer somewhat.

You can store the output of a command in a variable and test if that variable is empty:

output="$(your_command)"

if [[ -n $output ]]
then
    printf -- "%s\n" "$output"
else
    printf -- "No output\n"
fi

The -n test will check if $output is a non-empty string. If it is, the content of $output will be printed. Otherwise, a message that no output was produced will be printed. Notice that the quotes around the "$output" are necessary.

Notice also that (as mentioned by @ilkkachu), trailing newlines will be removed by the command substitution, so if you care about these, a different approach is necessary.

Answer 2

If you want the output in any case, you can pipe it to tee and use that to duplicate the output without running the command again: once to standard output, once to a command which can act on empty input.

For example:

ls | tee >([ $(wc -m) -gt 0 ] || echo No output)

Note that this only takes care of standard output, so you can end up with “No output” after an error message. You’ll also have to adjust commands accordingly when their output varies depending on whether it’s going to a terminal or not (e.g. ls -C, or output with colours).

Answer 3

Taking ls -A as command example, the following will do:

if ls -A | grep ^; then
   : the output was already written
else
   : no output
fi

if ! ls -A | grep ^; then
   : handle the no-output condition
fi

This will also ensure that trailing newlines are preserved in the output.

For the actual case mentioned in this comment (do not run less with an empty output), there's the -F ("quit if one screen") option:

command ... | less -FXR

(you can omit the X with newer versions of less).