9.13. Shell Built-In Commands
The shell has a number of commands that are built-in to its source code. Because the commands are built-in, the shell doesn't have to locate them on disk, making execution much faster. The help feature provided with bash give you online help for any built-in command. The built-in commands are listed in Table 9.12 on page 481.
| Command | What It Does |
|---|---|
| : | Do-nothing command; returns exit status zero. |
| . file | The dot command reads and executes command from file. |
| break [n] | See the looping commands on page 443. |
| : | Do-nothing command; returns 0 exit status. |
| . | Executes program in context of current process; same as source. |
| alias | Lists and creates "nicknames" for existing commands. |
| bg | Puts a job in the background. |
| bind* | Displays current key and function bindings, or binds keys to a readline function or macro. |
| break | Breaks out of the innermost loop. |
| builtin [ sh-builtin [args]]* | Runs a shell built-in, passing it args, and returning 0 exit status. Useful if a function and built-in have the same name. |
| cd [arg] | Changes the directory to home if no arg or to arg. |
| command command* [arg] | Runs a command even if a function has the same name; i.e., bypasses function lookup. |
| continue [n] | See the looping commands on page 444. |
| declare [var]* | Displays all variables or declares variables with optional attributes. |
| dirs | Displays a list of currently remembered directories resulting from pushd. |
| disown | Removes an active job from the job table. |
| echo [args] | Displays args terminated with a newline. |
| enable* | Enables and disables shell built-in commands. |
| eval [args] | Reads args as input to the shell and executes the resulting command(s). |
| exec command | Runs command in place of this shell. |
| exit [ n ] | Exits the shell with status n. |
| export [ var ] | Makes var known to subshells. |
| fc | History's fix command for editing history commands. |
| fg | Puts background job into foreground. |
| getopts | Parses and processes command line options. |
| hash | Controls the internal hash table for quicker searches for commands. |
| help [command ]* | Displays helpful info about built-in commands and, if command is specified, detailed help about that built-in command. |
| history | Displays the history list with line numbers. |
| jobs | Lists jobs put in the background. |
| kill [-signal process ] | Sends the signal to the pid number or job number of the process. Type kill -l for a list of signals. |
| getopts | Used in shell scripts to parse command line and check for legal options. |
| let | Used for evaluating arithmetic expressions and assigning results of arithmetic calculations to variables. |
| local | Used in functions to restrict the scope of variables to the function. |
| logout | Exits the login shell. |
| popd | Removes entries from the directory stack. |
| pushd | Adds entries to the directory stack. |
| pwd | Prints present working directory. |
| read [ var ] | Reads line from standard input into variable var. |
| readonly [ var ] | Makes variable var read-only. Cannot be reset. |
| return [ n ] | Returns from a function where n is the exit value given to the return. |
| set | Sets options and positional parameters. See Table 9.2 on page 396. |
| shift [n] | Shifts positional parameters to the left n times. |
| stop pid | Halts execution of the process number pid. |
| suspend | Stops execution of the current shell (but not if a login shell). |
| test | Checks file types and evaluates conditional expressions. |
| times | Prints accumulated user and system times for processes run from this shell. |
| trap [ arg ] [ n ] | When shell receives signal n ( 0, 1, 2, or 15), executes arg. |
| type [ command ] | Prints the type of command; e.g., pwd is a built-in shell command. |
| typeset | Same as declare. Sets variables and gives them attributes. |
| ulimit | Diplays and sets process resource limits. |
| umask [ octal digits ] | Sets user file creation mode mask for owner, group, and others. |
| unalias | Unsets aliases. |
| unset [ name ] | Unset value of variable or function. |
| wait [ pid#n ] | Waits for background process with pid number n and reports termination status. |
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