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Getting help for Linux shell built-ins
Linux built-ins are commands that are part of a user’s shell. Here's an explanation on how to recognize them and get help on their use.
Linux built-ins are commands that are built into the shell, much like shelves that are built into a wall. You won’t find them as stand-alone files the way standard Linux commands are stored in /usr/bin and you probably use quite a few of them without ever questioning how they’re different from commands such as ls and pwd.
Built-ins are used just like other Linux commands. They are likely to run a bit faster than similar commands that are not part of your shell. Bash built-ins include commands such as alias, export and bg.
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As you might suspect, because built-ins are shell-specific, they won't be supplied with man pages. Ask man to help with bg and you'll see something like this:
$ man bg
No manual entry for bg
Another tip-off that a command is a built-in is when you use the which command to identify the source of the command. Bash's non-response will remind you that there is no file associated with the built-in:
$ which bg
$
If your shell is /bin/zsh, on the other hand, you might get a slightly more illuminating response:
% which bg
bg: shell built-in command
Additional help is available with bash, but it comes through the use of the help command:
$ help bg
bg: bg [job_spec ...]
Move jobs to the background.
Place the jobs identified by each JOB_SPEC in the background, as if they
had been started with `&'. If JOB_SPEC is not present, the shell's notion
of the current job is used.
Exit Status:
Returns success unless job control is not enabled or an error occurs.
If you want to see a list of all of the built-ins that bash provides, use the compgen -b command. Pipe the output to column for a nicely formatted listing.
$ compgen -b | column
. compgen exit let return typeset
: complete export local set ulimit
[ compopt false logout shift umask
alias continue fc mapfile shopt unalias
bg declare fg popd source unset
bind dirs getopts printf suspend wait
break disown hash pushd test
builtin echo help pwd times
caller enable history read trap
cd eval jobs readarray true
command exec kill readonly type
If you use the help command, you’ll see a list of built-ins along with short descriptions. This list is, however, truncated (ending with the help command):
$ help
GNU bash, version 5.0.3(1)-release (x86_64-pc-linux-gnu)
These shell commands are defined internally. Type `help' to see this list.
Type `help name' to find out more about the function `name'.
Use `info bash' to find out more about the shell in general.
Use `man -k' or `info' to find out more about commands not in this list.
A star (*) next to a name means that the command is disabled.
job_spec [&] history [-c] [-d offset] [n] or histo>
(( expression )) if COMMANDS; then COMMANDS; [ elif CO>
. filename [arguments] jobs [-lnprs] [jobspec ...] or jobs ->
: kill [-s sigspec | -n signum | -sigsp>
[ arg... ] let arg [arg ...]
[[ expression ]] local [option] name[=value] ...
alias [-p] [name[=value] ... ] logout [n]
bg [job_spec ...] mapfile [-d delim] [-n count] [-O ori>
bind [-lpsvPSVX] [-m keymap] [-f filen> popd [-n] [+N | -N]
break [n] printf [-v var] format [arguments]
builtin [shell-builtin [arg ...]] pushd [-n] [+N | -N | dir]
caller [expr] pwd [-LP]
case WORD in [PATTERN [| PATTERN]...) > read [-ers] [-a array] [-d delim] [-i>
cd [-L|[-P [-e]] [-@]] [dir] readarray [-d delim] [-n count] [-O o>
command [-pVv] command [arg ...] readonly [-aAf] [name[=value] ...] or>
compgen [-abcdefgjksuv] [-o option] [-> return [n]
complete [-abcdefgjksuv] [-pr] [-DEI] > select NAME [in WORDS ... ;] do COMMA>
compopt [-o|+o option] [-DEI] [name ..> set [-abefhkmnptuvxBCHP] [-o option-n>
continue [n] shift [n]
coproc [NAME] command [redirections] shopt [-pqsu] [-o] [optname ...]
declare [-aAfFgilnrtux] [-p] [name[=va> source filename [arguments]
dirs [-clpv] [+N] [-N] suspend [-f]
disown [-h] [-ar] [jobspec ... | pid . <p>'> test [expr]
echo [-neE] [arg ...] time [-p] pipeline
enable [-a] [-dnps] [-f filename] [nam> times
eval [arg ...] trap [-lp] [[arg] signal_spec ...]
exec [-cl] [-a name] [command [argumen> true
exit [n] type [-afptP] name [name ...]
export [-fn] [name[=value] ...] or exp> typeset [-aAfFgilnrtux] [-p] name[=va>
false ulimit [-SHabcdefiklmnpqrstuvxPT] [li>
fc [-e ename] [-lnr] [first] [last] or> umask [-p] [-S] [mode]
fg [job_spec] unalias [-a] name [name ...]
for NAME [in WORDS ... ] ; do COMMANDS> unset [-f] [-v] [-n] [name ...]
for (( exp1; exp2; exp3 )); do COMMAND> until COMMANDS; do COMMANDS; done
function name { COMMANDS ; } or name (> variables - Names and meanings of som>
getopts optstring name [arg] wait [-fn] [id ...]
hash [-lr] [-p pathname] [-dt] [name .> while COMMANDS; do COMMANDS; done
help [-dms] [pattern ...] { COMMANDS ; }
As you can see from the listings above, the help command is itself a built-in.
You can get more information on any of these built-ins by providing the help command with the name of the built-in you're curious about — as in help dirs.
$ help dirs
dirs: dirs [-clpv] [+N] [-N]
Display directory stack.
Display the list of currently remembered directories. Directories
find their way onto the list with the `pushd' command; you can get
back up through the list with the `popd' command.
Options:
-c clear the directory stack by deleting all of the elements
-l do not print tilde-prefixed versions of directories relative
to your home directory
-p print the directory stack with one entry per line
-v print the directory stack with one entry per line prefixed
with its position in the stack
Arguments:
+N Displays the Nth entry counting from the left of the list
shown by dirs when invoked without options, starting with
zero.
-N Displays the Nth entry counting from the right of the list
shown by dirs when invoked without options, starting with
zero.
Exit Status:
Returns success unless an invalid option is supplied or an error occurs.
Built-ins provide much of the functionality of each shell. Any shell you use will have some built-ins, though how to get information on these built-ins may differ from shell to shell. For zsh, for example, you can get a description of built-in commands by using the man zshbuiltins command.
$ man zshbuiltins
ZSHBUILTINS(1) General Commands Manual ZSHBUILTINS(1)
NAME
zshbuiltins - zsh built-in commands
SHELL BUILTIN COMMANDS
Some shell builtin commands take options as described in individual en‐
tries; these are often referred to in the list below as `flags' to avoid
confusion with shell options, which may also have an effect on the behav‐
iour of builtin commands. In this introductory section, `option' always
has the meaning of an option to a command that should be familiar to most
command line users.
…
Within this lengthy man page, you will find a list of built-ins with useful descriptions as in this excerpt.
bg [ job ... ]
job ... &
Put each specified job in the background, or the current job if
none is specified.
bindkey
See the section `Zle Builtins' in zshzle(1).
break [ n ]
Exit from an enclosing for, while, until, select or repeat loop.
If an arithmetic expression n is specified, then break n levels
instead of just one.
Wrap-up
Linux built-ins are essential to each shell and operate like shell-specific commands. If you use a different shell from time to time and notice that some command you often use doesn’t seem to exist or doesn’t work as you were expecting, it just might be that it's one of your normal shell's built-ins.
[ Also see: Invaluable tips and tricks for troubleshooting Linux ]
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作者：Sandra Henry-Stocker 选题：lujun9972 译者：译者ID 校对：校对者ID