Builtins, External Commands, Aliases and Functions

Not every command is a program

When you type a command name, Bash checks, in order: aliases (interactive shortcuts, not expanded in scripts by default), keywords (if, for, [[), functions defined in the current shell, builtins (commands implemented inside Bash, such as cd, echo, read, export, test, printf), and finally external programs found by searching the PATH (/usr/bin/grep, /usr/bin/curl). Builtins run inside the shell process, which is why cd must be a builtin: an external program could not change your shell's directory. type name tells you which kind a command is, and type -a lists all matches. For documentation, use help builtin for builtins (help read), man command for external programs, and man bash for the shell itself. Prefer printf over echo in scripts when output must be exact, because echo's handling of options and backslashes varies between shells.

Finding out what a command is

type answers the question for any name.

type cd          # cd is a shell builtin
type ls          # often an alias in interactive shells, e.g. ls='ls --color=auto'
type -a printf   # builtin, and also /usr/bin/printf
type if          # if is a shell keyword
command -v jq || echo "jq is not installed"   # portable existence check

help read | head
man 1 printf

# printf is predictable; echo varies
printf '%s has %d items\n' "cart" 3

Check dependencies up front

If a script needs jq or curl, check with command -v at the start and exit with a clear message. Failing halfway through a deployment because a tool is missing is far worse.

Quick check: Why must cd be a shell builtin rather than an external program?

• External programs cannot read directories
• cd is too small to be a program
• A separate process cannot change the working directory of the parent shell
• POSIX forbids external cd