shell的内建命令和外部命令

shell的内建命令和外部命令

Shell执行的命令可以分为内建命令（built-in）和外部命令（external），前者是构建在shell内部；后者是一个独立的文件（可以是二进制文件，也可以是一个脚本）。

内建命令的执行比较简单，由当前shell本身来执行，例如echo, eval, exec, export, readonly, read, shift, wait和点(.)等；

外部命令的执行比较复杂，shell进程会fork一个子进程，父进程随后挂起，然后在子进程中exec加载外部文件，子进程返回后，父进程才继续执行。

从上面的区别可以看出，内建命令的性能要优于外部命令，因为它没有新建进程的开销。

使用type命令可以区别一个命令是内建命令，还是外部命令：

[root@localhost tmp]# type cd
cd is a shell builtin
[root@localhost tmp]# type awk
awk is /bin/awk

注意，Shell会沿着路径$PATH来寻找外部命令，环境变量PATH是一个以冒号分隔的目录列表，

[root@localhost tmp]# echo $PATH
/usr/local/sbin:/usr/local/bin:/sbin:/bin:/usr/sbin:/usr/bin:/root/bin

最后，使用help命令可以查看内建命令列表：

[root@localhost tmp]# help
GNU bash, version 4.1.()-release (x86_64-unknown-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 history -anrw [filename] o>
 (( expression ))                                              if COMMANDS; then COMMANDS; [ elif COMMANDS; then COMMANDS>
 . filename [arguments]                                        jobs [-lnprs] [jobspec ...] or jobs -x command [args]
 :                                                             kill [-s sigspec | -n signum | -sigspec] pid | jobspec ...>
 [ arg... ]                                                    let arg [arg ...]
 [[ expression ]]                                              local [option] name[=value] ...
 alias [-p] [name[=value] ... ]                                logout [n]
 bg [job_spec ...]                                             mapfile [-n count] [-O origin] [-s count] [-t] [-u fd] [-C>
 bind [-lpvsPVS] [-m keymap] [-f filename] [-q name] [-u nam>  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]...) COMMANDS ;;]... esac    read [-ers] [-a array] [-d delim] [-i text] [-n nchars] [->
 cd [-L|-P] [dir]                                              readarray [-n count] [-O origin] [-s count] [-t] [-u fd] [>
 command [-pVv] command [arg ...]                              readonly [-af] [name[=value] ...] or readonly -p
 compgen [-abcdefgjksuv] [-o option]  [-A action] [-G globpa>  return [n]
 complete [-abcdefgjksuv] [-pr] [-DE] [-o option] [-A action>  select NAME [in WORDS ... ;] do COMMANDS; done
 compopt [-o|+o option] [-DE] [name ...]                       set [--abefhkmnptuvxBCHP] [-o option-name] [arg ...]
 continue [n]                                                  shift [n]
 coproc [NAME] command [redirections]                          shopt [-pqsu] [-o] [optname ...]
 declare [-aAfFilrtux] [-p] [name[=value] ...]                 source filename [arguments]
 dirs [-clpv] [+N] [-N]                                        suspend [-f]
 disown [-h] [-ar] [jobspec ...]                               test [expr]
 echo [-neE] [arg ...]                                         time [-p] pipeline
 enable [-a] [-dnps] [-f filename] [name ...]                  times
 eval [arg ...]                                                trap [-lp] [[arg] signal_spec ...]
 exec [-cl] [-a name] [command [arguments ...]] [redirection>  true
 exit [n]                                                      type [-afptP] name [name ...]
 export [-fn] [name[=value] ...] or export -p                  typeset [-aAfFilrtux] [-p] name[=value] ...
 false                                                         ulimit [-SHacdefilmnpqrstuvx] [limit]
 fc [-e ename] [-lnr] [first] [last] or fc -s [pat=rep] [com>  umask [-p] [-S] [mode]
 fg [job_spec]                                                 unalias [-a] name [name ...]
 for NAME [in WORDS ... ] ; do COMMANDS; done                  unset [-f] [-v] [name ...]
 for (( exp1; exp2; exp3 )); do COMMANDS; done                 until COMMANDS; do COMMANDS; done
 function name { COMMANDS ; } or name () { COMMANDS ; }        variables - Names and meanings of some shell variables
 getopts optstring name [arg]                                  wait [id]
 hash [-lr] [-p pathname] [-dt] [name ...]                     while COMMANDS; do COMMANDS; done
 help [-dms] [pattern ...]                                     { COMMANDS ; }