The use of parentheses in the shell Linux shell

Overview of the use of parentheses in the Linux shell

    本文主要介绍Linux中括号的用法，主要包含判断文件和变量。**注意事项**：中括号两边必须有空格。

The knowledge involved in using brackets &&, | | |

            在本文对文件或者变量做判断的时候，可能会多次使用到&&和||这两个符号，所以在我们进入中括号的学习之前首先了解一下这两个符号的作用。            &&：并 --> 两真才为真，如果有两个条件，第一个为真，那么肯定会判断第二个条件；如果第一个条件为假，那么第二个条件将不会判断。（第一个为真，那么需要判断第二个条件才知道整体结果是否为真，如果第一个条件为假，那么无论第二个条件为真还是假，最终的结果都是假）

Example 1:
[FALSE] && echo 1 final result no output, because the first condition is false, the back of the echo 0 will not execute

Example 2:
[true] && echo 0 The final result is 0, because one condition is true and the second condition is definitely executed.

            ||：且 -->一个为真即是真。如果第一个条件为真，那么不会判断第二个条件（因为一真即真），如果第一个条件为假，那么判断第二个条件（第二个条件如果为真还有机会使得整体的结果为真）

Example 3:
[True] | | Echo 0 # The final result has no output because the first condition is true and echo 0 will not execute.

Example 4:
[FALSE] | | echo 0 #最终的结果输出0

Little Practice
[2-eq 3] && echo 0
[2-eq 3] | | Echo 1
What is the result of the final output?

The result is: 1

&& | | Integrated use of

Let's look at a small example:
[2-eq 3] && echo 0 | | Echo 1
Final Result: 1
Explanation: If the [] condition is true, then echo 0 is executed, and if the condition of [] is false, then echo 1 is executed.

The judgment file used in the parentheses of Linux

Common parameters:

-Z: Determine if the file exists

Example:

[[email protected] ~]# [ -z adsfsfsdfs.txt ] && echo 0 || echo 11    # 最终结果为1 ，因为系统中没有存在ads...txt文件

-D: Determine if the directory exists

Example:

[[email protected] ~]# [ -d /var/www ] && echo 0 || echo 10    #输出结果为0，因为系统中有/var/www这个目录

-X to determine if the file has execute (x) permission

Example:

[[email protected] ~]# [ -x a.txt ] && echo 0 || echo 11

The judging variables of the use of parentheses in Linux

-N to determine if the variable value is empty

[[email protected] ~]# [ -n $kfjksdfjl ] && echo 0 || echo 10    #变量为空，即为9

-eq Judging whether variables or parameters are equal

Example:

[ 1 -eq 1 ] && echo 0 || echo 10  # 1=1 so...

-ne Not equal to

Example:

[ 1 -ne 2 ] && echo 0 || echo 10  # 1!=2

-GT greater than,-ge greater than or equal to

Example:

[[email protected] scripts]# [ 1 -gt 2 ] && echo 0 || echo 11

-lt less than,-le less than or equal to

[[email protected] scripts]# [ 1 -lt 2 ] && echo 0 || echo 10

Use of double brackets

Common parameters

=: equals

Example:

[[email protected] scripts]# [[ 1 = 2 ]] && echo 0 || echo 11

! = does not equal

Example:

[[email protected] scripts]# [[ 1 != 2 ]] && echo 0 || echo 10

And so on, there are >,<,<=,>= and other parameters, here is no longer one by one write out.

Copyright: arppinging

The use of parentheses in the shell Linux shell