近期遇到一個比較少見的命令command，詳細如下：

command 是一些shell的內建命令

我本機使用的是dash，伺服器使用的是bash，其他shell沒有測試
- dash

user@PC:~/debug/bin/test$ ll /bin/sh
lrwxrwxrwx 1 root root 4 12月  8  2015 /bin/sh -> dash*
user@PC:~/debug/bin/test$ type command
command 是 shell 內建

• bash

[email protected]:~/work/tina$ ll /bin/sh
lrwxrwxrwx 1
 
 root root 9  4月 20 16:15 /bin/sh -> /bin/bash*
[email protected]:~/work/tina$ type command
command 是 shell 內建

command命令的作用

無視shell指令碼中的“同名”函式，執行特定的命令

• dash
  下面摘自man dash

     command [-p] [-v] [-V] command [arg ...]
            Execute the specified command but ignore shell functions when searching for
 
 it.
            (This is useful when you have a shell function with the same name as a builtin com‐
            mand.)

            -p     search for command using a PATH that guarantees to find all the standard util‐
                   ities.

            -V     Do not execute the command but search for
 
 the command and print the resolution
                   of the command search.  This is the same as the type builtin.

            -v     Do not execute the command but search for the command and print the absolute
                   pathname of utilities, the name for builtins or the expansion of aliases.

• bash
  下面摘自man bash

       command [-pVv] command [arg ...]
              Run command with args suppressing the normal shell function  lookup.  Only  builtin
              commands  or  commands  found in the PATH are executed.  If the -p option is given,
              the search for command is performed using a default value for PATH that is  guaran‐
              teed  to find all of the standard utilities.  If either the -V or -v option is sup‐
              plied, a description of command is printed.  The -v option  causes  a  single  word
              indicating  the  command or filename used to invoke command to be displayed; the -V
              option produces a more verbose description.  If the -V or -v  option  is  supplied,
              the exit status is 0 if command was found, and 1 if not.  If neither option is sup‐
              plied and an error occurred or command cannot be found, the  exit  status  is  127.
              Otherwise, the exit status of the command builtin is the exit status of command.

測試

在test.sh中構建ls函式，測試command命令的作用

[email protected]:~/debug/bin/test$ ls
test.sh

[email protected]:~/debug/bin/test$ cat test.sh 
function ls()
{
    echo "haha"
}

echo "--1--"
ls

echo "--2--"
command ls

echo "--3--"
echo command ls

echo "--4--"
$(echo command ls)

[email protected]:~/debug/bin/test$ ./test.sh 
--1--
haha
--2--
test.sh
--3--
command ls
--4--
test.sh