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 to describe shell built-in commands (“builtin” is mentioned once as a reserved name; see below). Let’s attempt to consistently use POSIX spelling built-in in our text.
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."
.
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 that 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, .rvmrc, or any other project-related setup file for whatever language, library or framework we may be working with.