Detecting the output stream type of a shell script

Question

I'm writing a shell script that uses ANSI color characters on the command line.

Example: example.sh

#!/bin/tcsh
printf "\033[31m Success Color is awesome!\033[0m"

My problem is when doing:

$ ./example.sh > out

or

$./example.sh | grep 

The ASCII codes will be sent out raw along with the text, mucking up the output and just generally causing chaos.

I'm interested to know if there is a way to detect this so I could disable color for this special case.

I've search the tcsh man pages and the web for a while and have not been able to find anything shell specific yet.

I'm not bound to tcsh, it's our group standard... but who cares?

Is it possible to detect, inside a shell script, if your output is being redirected or piped?

Answer 1

See this previous SO question, which covers bash. Tcsh provides the same functionality with filetest -t 1 to see if standard output is a terminal. If it is, then print the color stuff, else leave it out. Here's tcsh:

#!/bin/tcsh
if ( -t 1 ) then
        printf "\033[31m Success Color is awesome!\033[0m"
else
        printf "Plain Text is awesome!"
endif

Answer 2

Inside a bourne shell script (sh, bask, ksh, ...), you can feed the standard output to the tty program (standard in Unix) which tells you whether its input is a tty or not, by using the -s flag.

Put the following into "check-tty":

    #! /bin/sh
    if tty -s <&1; then
      echo "Output is a tty"
    else
      echo "Output is not a tty"
    fi

And try it:

    % ./check-tty
    Output is a tty
    % ./check-tty | cat
    Output is not a tty

I don't use tcsh, but there must be a way to redirect your standard output to tty's standard input. If not, use

    sh -c "tty -s <&1"

as your test command in your tcsh script, check its exit status and you're done.