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 4.1.2(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 ; }