How to enable color output in Bash scripts

Color output lets a Bash script make warnings, success messages, and errors stand apart in a terminal. It works best when the script is writing to an interactive terminal that understands ANSI Select Graphic Rendition sequences, because redirected logs and plain text files may store the escape characters instead of showing colors.

Bash can store terminal control bytes with ANSI-C quoted strings such as $'\033[31m', then print them with printf "%s...%s" around visible text. That keeps escape handling in the variable assignment instead of asking printf to reinterpret every message string.

The sample script enables red, green, and yellow labels only when standard output is an interactive terminal and NO_COLOR is unset. Redirected output and one-off NO_COLOR runs fall back to plain text, and the reset string after each label keeps later output at terminal defaults.

1. Create a script that assigns color variables only for terminal output.

   color-output.sh

   #!/usr/bin/env bash
   set -euo pipefail
    
   if [ -t 1 ] && [ -z "${NO_COLOR:-}" ]; then
       red=$'\033[31m'
       green=$'\033[32m'
       yellow=$'\033[33m'
       reset=$'\033[0m'
   else
       red=""
       green=""
       yellow=""
       reset=""
   fi
    
   printf "%sERROR%s disk space low\n" "$red" "$reset"
   printf "%sOK%s backup completed\n" "$green" "$reset"
   printf "%sWARN%s restart pending\n" "$yellow" "$reset"
   printf "plain text after reset\n"

   ANSI SGR code 31 sets red text, 32 sets green text, 33 sets yellow text, and 0 resets formatting. [ -t 1 ] checks whether standard output is a terminal.

2. Check the script syntax before running it.

   $ bash -n color-output.sh

   No output means Bash parsed the variable assignments and printf calls without a syntax error.

3. Run the script in a terminal and inspect the colored labels.

   $ bash color-output.sh
   ERROR disk space low
   OK backup completed
   WARN restart pending
   plain text after reset

   The words ERROR, OK, and WARN should appear in red, green, and yellow. The final line should use the terminal default color.

4. Run the script with NO_COLOR to force plain text for one command.

   $ NO_COLOR=1 bash color-output.sh
   ERROR disk space low
   OK backup completed
   WARN restart pending
   plain text after reset

   NO_COLOR gives callers a simple opt-out without changing the script file.

5. Redirect the script output to a log file.

   $ bash color-output.sh > color-output.log

6. Inspect the log file to confirm it stayed plain text.

   $ cat color-output.log
   ERROR disk space low
   OK backup completed
   WARN restart pending
   plain text after reset

   Because redirected standard output is not a terminal, the script stores empty color variables before printing.

7. Reuse the reset variable in each colored printf call when adding more labels.

   printf "%sERROR%s disk space low\n" "$red" "$reset"

   Missing the reset code can leave the rest of the terminal session in the last selected color.