当在终端中输入命令后，shell程序会找到键入命令所对应的可执行程序或大麻，并由其分析后提交给内核分配资源将其运行起来。
在shell 中的可执行的命令有两类
内部命令：由shell自带的，而且通过某命令形式提供
可以通过使用help列出shell中的所有内部命令。
[root@XX ~]$help GNU bash, version 4.2.46(2)-release (x86_64-redhat-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 hist> (( expression )) if COMMANDS; then COMMANDS; [ elif C> . filename [arguments] jobs [-lnprs] [jobspec ...] or jobs > : kill [-s sigspec | -n signum | -sigs> [ arg... ] let arg [arg ...] [[ expression ]] local [option] name[=value] ... alias [-p] [name[=value] ... ] logout [n] bg [job_spec ...] mapfile [-n count] [-O origin] [-s c> bind [-lpvsPVS] [-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] [-> cd [-L|[-P [-e]]] [dir] readarray [-n count] [-O origin] [-s> command [-pVv] command [arg ...] readonly [-aAf] [name[=value] ...] o> compgen [-abcdefgjksuv] [-o option] > return [n] complete [-abcdefgjksuv] [-pr] [-DE] > select NAME [in WORDS ... ;] do COMM> compopt [-o|+o option] [-DE] [name ..> set [-abefhkmnptuvxBCHP] [-o option-> continue [n] shift [n] coproc [NAME] command [redirections] shopt [-pqsu] [-o] [optname ...] declare [-aAfFgilrtux] [-p] [name[=va> 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] [na> times eval [arg ...] trap [-lp] [[arg] signal_spec ...] exec [-cl] [-a name] [command [argume> true exit [n] type [-afptP] name [name ...] export [-fn] [name[=value] ...] or ex> typeset [-aAfFgilrtux] [-p] name[=va> false ulimit [-SHacdefilmnpqrstuvx] [limit> fc [-e ename] [-lnr] [first] [last] o> umask [-p] [-S] [mode] fg [job_spec] unalias [-a] name [name ...] for NAME [in WORDS ... ] ; do COMMAND> unset [-f] [-v] [name ...] for (( exp1; exp2; exp3 )); do COMMAN> until COMMANDS; do COMMANDS; done function name { COMMANDS ; } or name > variables - Names and meanings of so> getopts optstring name [arg] wait [id] hash [-lr] [-p pathname] [-dt] [name > while COMMANDS; do COMMANDS; done help [-dms] [pattern ...] { COMMANDS ; }
外部命令：在文件系统路径下有对应的可执行程序文件
可以通过命令which -a|--skip-alias; whereis查看
[root@XX ~]#which -a msyql /usr/bin/which: no msyql in (/usr/lib64/qt-3.3/bin:/usr/local/bin:/usr/local/sbin:/usr/bin:/usr/sbin:/bin:/sbin:/root/bin)
内部命令可以被enable命令禁用，以及重新启用
enable命令的用法
enable cmd 启动内部命令
enable -n cmd 禁用内部命令
enable -n 不接内部命令则为查看所有禁用的内部命令
[root@XX ~]#enable -n echo
[root@XX ~]#enable -n enable -n echo
[root@XX ~]#enable echo
[root@XX ~]#enable -n
type命令区别指定的命令是内部或外部命令
[root@XX ~]#type wc wc is /usr/bin/wc [root@XX ~]#type echo echo is a shell builtin