Check if there is an stdout redirection in bash script

Question

I need to check if my program's output is being redirected; if yes I need to keep and send its by mail.

example:

$ myprogram -param1 -param2 -param3 > /home/polly/log.txt

myprogram.sh:

if 'redirection is not empty'; then 
    cat <redirection name> | mailx -s "This is a test email." [email protected] 
fi

I'm not sure I understand. If called as myprogram > something, you want to mail the content of something? — gniourf_gniourf
– gniourf_gniourf, Commented Nov 5, 2014 at 16:08

hek2mgl · Accepted Answer · 2014-11-05 16:06:41Z

33

You can check if stdout is a terminal. When stdout is redirected or piped it will not be a terminal. You can use the test command with the -t option to get this information:

if [ -t 1 ] ; then
    # stdout is a terminal
else
    # stdout isn't a terminal
fi

From man test:

  -t FD  file descriptor FD is opened on a terminal

answered Nov 5, 2014 at 16:06

hek2mgl

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9 Comments

kojiro Over a year ago

reference

twalberg Over a year ago

@hek2mgl Well, on Linux, you might actually be able to, at least for redirections (not for pipes, though as there is no name), by looking at e.g. readlink /proc/$$/fd/1 or such...

hek2mgl Over a year ago

@twalberg Yeah, you are right! Cool! Didn't had this in mind.. Thx! :)

JackFrost65 Over a year ago

Thanks twalberg, could you explain me how can manage this file (I tryed to copy/cat/type but everytime I receive an error)?

prof Over a year ago

@hek2mgl, cannot agree regarding "good program design", it depends. Say such fundamental utils as /bin/ls /usr/bin/diff. (1) ls gives tabular output to terminal, but gives one per line when output is redirected. (2) diff may use color output with ANSI escape codes, but does pure black-n-white output by default when redirected. So there are cases when good program design demands to know whether stdout is a terminal

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