执行命令过程

• 输入命令按下回车键，提醒Shell程序找到键入命令所对应的可执行程序或代码，并由其分析后提交给内核分配资源将其运行起来

Shell中可执行的两类命令

• 内部命令：由Shell自带的，并且通过某命令形式提供
• 外部命令：在文件系统路径下有对应的可执行程序文件

如何区别指定的命令是内部命令还是外部命令？

type COMMAND

例子：查看是否存在对应内部命令和外部命令

[15:59:39 root@centos7 ~]#type -a echo
echo is a shell builtin
echo is /usr/bin/echo

内部命令相关

help 内部命令列表
enable 管理内部命令

• enable cmd 启用内部命令
• enable -n cmd 禁用内部命令
• enable -n 查看所有禁用的内部命令

[16:03:49 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 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 ; }

执行外部命令

查看外部命令路径：

[16:08:10 root@centos7 ~]#which java
/usr/bin/java
[16:09:07 root@centos7 ~]#whereis java
java: /usr/bin/java /usr/lib/java /etc/java /usr/share/java /usr/share/man/man1/java.1.gz

Hash缓存表
系统初始hash表为空，当外部命令执行时，默认会从PATH路径下寻找该命令，找到后会将这条命令的路径记录到hash表中，当再次使用该命令时，Shell解释器首先会查看hash表，存在将执行，如果不存在，将会去PATH路径下寻找，利用hash缓存表可大大提高命令的调用速率
hash命令常见用法

• hash，显示hash缓存

[16:09:31 root@centos7 ~]#hash
hits    command
   1    /usr/bin/whereis
   1    /usr/bin/clear

• hash -l，显示hash缓存，可作为输入使用

[16:17:46 root@centos7 ~]#hash -l
builtin hash -p /usr/bin/whereis whereis
builtin hash -p /usr/bin/clear clear

• hash -p path name，将命令全路径path起别名为name

[16:20:42 root@centos7 ~]#hash -p path name

• hash -t name，打印缓存中name的路径

[16:20:52 root@centos7 ~]#hash -t name
path

• hash -d name，清除name缓存
• hash -r，清除缓存

命令别名

对于经常执行的较长的命令，可以将其定义成较短的别名，以方便执行
显示当前Shell进程所有可用的命令别名

alias

定义别名NAME，其相当于执行命令VALUE

alias NAME='VALUE'

例子：扫描新加的磁盘

[16:44:15 root@centos7 ~]#alias scandisk='echo - - - > /sys/class/scsi_host/host0/scan'

例子：持久化保存别名

[17:04:17 root@centos7 ~]#vi ~/.bashrc

# .bashrc
# User specific aliases and functions
alias rm='rm -i'
alias cp='cp -i'
alias mv='mv -i'
# New aliase
alias sayhello='echo hello world'
# Source global definitions
if [ -f /etc/bashrc ]; then
        . /etc/bashrc
fi
[17:06:31 root@centos7 ~]#source ~/.bashrc
[17:06:55 root@centos7 ~]#sayhello
hello world

撤销别名：unalias

unalias [-a] name [name ...]
unalias -a #取消所有别名

注意：在命令行中定义的别名，仅对当前Shell进程有效
如果想永久有效，要定义在配置文件中

• 仅对当前用户：~/.bashrc
• 对所有用户生效：/etc/bashrc

编辑配置给出的新配置不会立即生效，bash进程重新读取配置文件

source /path/to/config_file
. /path/to/config_file

如果别名同原命令同名，如果要执行原命令，可使用

\ALIASNAME
"ALIASNAME"
'ALIASNAME'
command ALIASNAME
/path/command #只适用于外部命令

命令格式

COMMAND [OPTIONS...] [ARGUMENTS...]
COMMAND [COMMAND] [COMMAND] ...

选项：用于启用或关闭命令的某个或某些功能

• 短选项：UNIX风格选项， -c 例如：-l，-h
• 长选项：GNU风格选项， --word 例如：–all，–human
• BSD风格选项：一个字母，例如：a，使用相对较少

参数：命令的作用对象，比如：文件名，用户名等
例子：

[17:20:13 root@centos7 ~]#id -u xxx
id: xxx: no such user
[17:20:27 root@centos7 ~]#ls -a
.                .bash_history  .bashrc  .cshrc                .tcshrc
..               .bash_logout   .cache   .dbus                 .viminfo
anaconda-ks.cfg  .bash_profile  .config  initial-setup-ks.cfg
[17:20:37 root@centos7 ~]#ls --all
.                .bash_history  .bashrc  .cshrc                .tcshrc
..               .bash_logout   .cache   .dbus                 .viminfo
anaconda-ks.cfg  .bash_profile  .config  initial-setup-ks.cfg
[17:20:46 root@centos7 ~]#free -h
              total        used        free      shared  buff/cache   available
Mem:           1.8G        512M        930M         14M        376M        1.1G
Swap:          2.0G          0B        2.0G
[17:20:51 root@centos7 ~]#free --human
              total        used        free      shared  buff/cache   available
Mem:           1.8G        512M        930M         14M        376M        1.1G
Swap:          2.0G          0B        2.0G
[17:21:01 root@centos7 ~]#ps a
   PID TTY      STAT   TIME COMMAND
  1738 tty1     Ssl+   0:00 /usr/bin/X :0 -background none -noreset -audit 4 -ve
  2143 pts/0    Ss     0:00 -bash
  3412 pts/0    R+     0:00 ps a

注意：

• 多个选项以及多参数和命令之间使用空白字符分隔
• 取消和结束命令执行：Ctrl+c，Ctrl+d
• 多个命令可以用";"符号分开
• 一个命令可以用\分成多行