SC2243.md

Pattern: Missing use of explicit -n to check for output

Issue: -

Description

[ "$(mycommand)" ] is equivalent to [ -n "$(mycommand)" ] and checks whether the command's output on stdout was non-empty.

Users more familiar with other languages are often surprised to learn that it is nothing like e.g. if (myfunction()), since it does not care about what the command/function returns.

Using an explicit -n helps clarify that this is purely a string operation. And of course, if the intention was to check whether the command ran successfully, now would be a good time to fix it as in the alternate example.

Example of incorrect code:

if [ "$(mycommand --myflags)" ]
then
  echo "True"
fi

Example of correct code:

# Check that the command outputs something on stdout
if [ -n "$(mycommand --myflags)" ]
then
  echo "The command had output on stdout"
fi

# Check instead that the command succeeded (exit code = 0)
if mycommand --myflags
then
  echo "The command reported success"
fi

Exceptions

If you are familiar with the semantics of [, you can ignore this suggestion.

Further Reading

• ShellCheck - SC2244