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How to check if bash can print colors

Solutons:


The idea is for my application to know not to color the output if the program can’t print, say, logging output from through a cron job to a file, no need to log colored output, but when running manually, i like to view the output colored

What language are you writing your application in?

The normal approach is to check if the output device is a tty, and if it is, check if that type of terminal supports colors.

In bash, that would look like

# check if stdout is a terminal...
if test -t 1; then

    # see if it supports colors...
    ncolors=$(tput colors)

    if test -n "$ncolors" && test $ncolors -ge 8; then
        bold="$(tput bold)"
        underline="$(tput smul)"
        standout="$(tput smso)"
        normal="$(tput sgr0)"
        black="$(tput setaf 0)"
        red="$(tput setaf 1)"
        green="$(tput setaf 2)"
        yellow="$(tput setaf 3)"
        blue="$(tput setaf 4)"
        magenta="$(tput setaf 5)"
        cyan="$(tput setaf 6)"
        white="$(tput setaf 7)"
    fi
fi

echo "${red}error${normal}"
echo "${green}success${normal}"

echo "${green}0.052${normal} ${bold}${green}2,816.00 kb${normal}"
# etc.

In C, you have to do a lot more typing, but can achieve the same result using isatty and the functions listed in man 3 terminfo.

This should be enough:

$ tput colors

tput colors explained:

If you look at the manpage, you’ill notice this:

SYNOPSIS
       tput [-Ttype] capname [parms ... ]

And…

   capname
          indicates the capability from the terminfo database.  When term‐
          cap  support is compiled in, the termcap name for the capability
          is also accepted.

The termcap colors is in the terminfo database, so you can ask for it. If you have a zero exit status, then the termcap is compiled in. But if you have somethin like:

$ tput unknowntermcap
tput: unknown terminfo capability 'unknowntermcap'
$ echo $?
4

This shows that unknowntermcap doesn’t exist. So, this:

$ tput colors
8
$ echo $?
0

Shows that your command was right.

Other useful ways:

  • In C, you can just use isatty and see if it’s a TTY
  • See if it’s a dumb terminal looking $TERM variable

Cheers

The idea is for my application to know not to color the output if the program can’t print, say, logging output from through a cron job to a file, no need to log colored output, but when running manually, i like to view the output colored.

For this use case, what programs typically do (e.g. GNU ls or GNU grep with --color=auto) is to use colors if their output is going to a terminal, and no colors otherwise. Terminals that don’t support ANSI color-changing sequences are rare enough that it’s acceptable to make their users override the default choice. In any case, make sure your application has an option to force colors on or off.

In a shell script, use [ -t 1 ] to test if standard output is a terminal.

# option processing has set $color to yes, no or auto
if [ $color = auto ]; then
  if [ -t 1 ]; then color=yes; else color=no; fi
fi

From a program using the C API, call isatty(1).

# option processing has set use_color to 0 for no, 1 for yes or 2 for auto
if (use_color == 2) use_color = isatty(1);

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