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

bash shell pipe
user111422 picture user111422 · May 26, 2009 · Viewed 74.7k times · Source

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.

Answer

dmckee --- ex-moderator kitten picture dmckee --- ex-moderator kitten · May 26, 2009

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 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:

0:     stdin  
1:     stdout  
2:     stderr

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