1.3.2. Shell built-in commands

Built-in commands are contained within the shell itself. When the name of a built-in command is used as the first word of a simple command, the shell executes the command directly, without creating a new process. Built-in commands are necessary to implement functionality impossible or inconvenient to obtain with separate utilities.

Bash supports 3 types of built-in commands:

• Bourne Shell built-ins:

  :, ., break, cd, continue, eval, exec, exit, export, getopts, hash, pwd, readonly, return, set, shift, test, [, times, trap, umask and unset.

• Bash built-in commands:

  alias, bind, builtin, command, declare, echo, enable, help, let, local, logout, printf, read, shopt, type, typeset, ulimit and unalias.

• Special built-in commands:

  When Bash is executing in POSIX mode, the special built-ins differ from other built-in commands in three respects:

  1. Special built-ins are found before shell functions during command lookup.

  2. If a special built-in returns an error status, a non-interactive shell exits.

  3. Assignment statements preceding the command stay in effect in the shell environment after the command completes.

  The POSIX special built-ins are :, ., break, continue, eval, exec, exit, export, readonly, return, set, shift, trap and unset.

Most of these built-ins will be discussed in the next chapters. For those commands for which this is not the case, we refer to the Info pages.

Next: Executing programs from a script Previous: General