bashshellpipe

How can I detect if my shell script is running through a pipe?

How do I detect from within a shell script if its standard output is being sent to a terminal or if it's piped to another process?

The case in point: I'd like to add escape codes to colorize output, but only when run interactively, but not when piped, similar to what ls --color does.

Solution

  • In a pure POSIX shell,

    if [ -t 1 ] ; then echo terminal; else echo "not a terminal"; fi

    returns "terminal", because the output is sent to your terminal, whereas

    (if [ -t 1 ] ; then echo terminal; else echo "not a terminal"; fi) | cat

    returns "not a terminal", because the output of the parenthetic element is piped to cat.

    The -t flag is described in man pages as

    -t fd True if file descriptor fd is open and refers to a terminal.

    ... where fd can be one of the usual file descriptor assignments: