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

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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:

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