Shell/Q/In POSIX shells is colon a comment or a no-op builtin?

• In POSIX shells, is : a comment or a no-op builtin?

  • Answer

    • AI/Response
      • : is a special builtin (POSIX utility name :) that always succeeds and does nothing with the arguments it is given—it is not a comment token. A line like the following still executes; the words after : are operands to that no-op command, not comment text to the parser:
        sh

        : this is ignored

      • People sometimes use that pattern as a pseudo-comment because the command has no side effects when the operands are simple words (avoid redirections, command substitutions, or other expansions you did not intend to run).
      • The same idea applies across common /bin/sh implementations and user shells such as Bash and zsh, and to lean sh clones like dash—all treat : as this no-op builtin.
      • Actual comments in these languages still start with # (to end of line, per shell grammar).
      • Reading other people’s scripts: lines beginning with : are a weak signal you are in a POSIX-style shell script and the author chose a no-op command instead of # (sometimes to keep a line syntactically valid where a # line would not parse the way they want).
      • POSIX.1-2024, Shell Command Language — special built-in utilities (includes :)