1、執行命令

輸入命令后回車

提請shell程序找到鍵入命令所對應的可執行程序或者代碼，并且由其分期后，提交給內核分配資源將其運行起來。

在shell中可執行的命令有倆類：

內部命令：由shell自帶的內部集成命令

help 可以查看內部命令列表

[root@centos7 ~]# 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 history -anrw [filename] or >

(( 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 ... o>

[ 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 c>

bind [-lpvsPVS] [-m keymap] [-f filename] [-q name] [-u name]> 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] [-N >

cd [-L|[-P [-e]]] [dir] readarray [-n count] [-O origin] [-s count] [-t] [-u fd] [-C>

command [-pVv] command [arg ...] readonly [-aAf] [name[=value] ...] or readonly -p

compgen [-abcdefgjksuv] [-o option] [-A action] [-G globpat]> 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 [-aAfFgilrtux] [-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 [-aAfFgilrtux] [-p] name[=value] ...

false ulimit [-SHacdefilmnpqrstuvx] [limit]

fc [-e ename] [-lnr] [first] [last] or fc -s [pat=rep] [comma> 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 ; }

enable 也可以查看命令列表

[root@centos7 ~]# enable

enable .

enable :

enable [

enable alias

enable bg

enable bind

enable break

enable builtin

enable caller

enable cd

enable command

enable compgen

enable complete

enable compopt

enable continue

enable declare

enable dirs

enable disown

enable echo

enable enable

enable eval

enable exec

enable exit

enable export

enable false

enable fc

enable fg

enable getopts

enable hash

enable help

enable history

enable jobs

enable kill

enable let

enable local

enable logout

enable mapfile

enable popd

enable printf

enable pushd

enable pwd

enable read

enable readarray

enable readonly

enable return

enable set

enable shift

外部命令：在文件系統路徑下有對應的可執行的文件

查看路徑命令

一、wheris

[root@centos7 ~]# whereis ls #多了幫助文檔，更加詳細

ls: /usr/bin/ls /usr/share/man/man1/ls.1.gz /usr/share/man/man1p/ls.1p.gz

二、which -a

[root@centos7 ~]# which -a ls

alias ls='ls --color=auto'

/usr/bin/ls

如果判斷命令是內部命令還是外部命令

命令：type

實例：[root@centos7 ~]# type pwd

pwd is a shell builtin #內部命令

[root@centos7 ~]# type hostname

hostname is /usr/bin/hostname #外部命令表示為一個文件

也有可能一個命令既是外部命令也是內部命令，可以這樣查看

[root@centos7 ~]# type -a pwd

pwd is a shell builtin

pwd is /usr/bin/pwd

那么問題來了，輸入命令是執行內部命令，還是外部命令呢？

命令執行是過程是這樣的：

先看下是不是內部命令(內部命令執行速度更快)，內部命令優先級更高，然后再去檢查hash表(記錄外部命令的路徑)，如果找到，找到后再執行。如果找不到這個命令在磁盤下的路徑，就會以下面的方式尋找。

外部命令尋找路徑的過程(首次執行)

[root@centos7 bin]# echo $PATH

/usr/local/sbin:/usr/local/bin:/usr/sbin:/usr/bin:/root/bin

在我們再執行一個命令時，當確認不是內部命令后，就會當成外部命令在以上的目錄文件中尋找，找到了(找到就停下)就執行，比如這個：

[root@centos7 bin]# type hostname

hostname is /usr/bin/hostname #在/usr/bin 下面找到

如果找不到就會報錯。

[root@centos7 bin]# lsls

bash: lsls: command not found... #命令輸入有誤

再找到這個命令后，就會將這個命令的路徑記錄在hash表中，再次使用時，就可以很快執行，提高命令調用速率。

實例：

root@centos7 bin]# hash

hits command

2 /usr/bin/whereis

2 /usr/bin/man

3 /usr/bin/ls

再次查看命令：

[root@centos7 bin]# type whereis

whereis is hashed (/usr/bin/whereis)

[root@centos7 bin]# type ifconfig

ifconfig is hashed (/usr/sbin/ifconfig)

清除hash緩存

[root@centos7 etc]# hash

hits command

6 /usr/sbin/ifconfig

1 /usr/bin/rm

1 /usr/bin/cat

9 /usr/bin/whereis

3 /usr/bin/cp

2 /usr/bin/man

4 /usr/bin/ls

1 /usr/bin/clear

[root@centos7 etc]# hash -r

[root@centos7 etc]# hash

hash: hash table empty

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