Shell Built-In Commands#

Intro#

A built-in is a command provided by the shell itself, not a program stored somewhere in the path.

Bash’s man page uses the spelling bultin. Zsh man page seems to use a mix of built-in and builtin. The POSIX spec for the Shell Command Language seems to strictly use built-in. Let’s attempt consistently use POSIX spelling (built-in) throughout this website.

POSIX builtin reserved word#

The POSIX spec does not define a built-in command called builtin, but makes it a “reserved word”. Note, especially, this line:

“If the command name matches the name of a utility listed in the following table, the results are unspecified.”

OpenGroup POSIX Spec, Unspecified Behavior (Shell Reserved Words)

OpenGroup POSIX Spec, “reserved words”.#

Shells implementation of builtin#

Shells have implemented that utility called builtin (builtin here is the actual name of the command) for their (the shell’s) specific purposes. For example, in Bash:

$ help builtin
builtin: builtin [shell-builtin [arg ...]]
    Execute shell builtins.

    Execute SHELL-BUILTIN with arguments ARGs without performing command
    lookup.  This is useful when you wish to re-implement a shell builtin
    as a shell function, but need to execute the builtin within the function.

    Exit Status:
    Returns the exit status of SHELL-BUILTIN, or false if SHELL-BUILTIN is
    not a shell builtin.

One such use case is with cd. We may find it useful to have a function cd than when executed first does some other thing, like checking for the existence and reading an .env file in the cd``ed directory, and **then** actually invoking the builtin ``cd to that directory. Something like:

##
# Read .env.txt (if it exists) when changing to a directory.
#
cd () {
  builtin cd "$@"

  if [[ -f ./.env.txt ]]
  then
    cat .env.txt
  fi
}

We could use this approach to read .nvmrc or any other project related setup file for whatever language, library or framework it makes sense.