Judge by document type

Test options

Test options	effect
-b	Judge whether the file exists and whether it is a block device file (yes, the block device file is true)
-c	Judge whether the file exists and whether it is a character device file.
-d	Judge whether the file exists and whether it is used as a directory file.
-e	Determine whether the file exists.
-f	Judge whether the file exists and whether it is an ordinary file.
-L	Judge whether the file exists and whether it is a symbolic link file.
-p	Judge whether the file exists and whether it is a pipeline file.
-s	Judge whether the file exists and is not empty.
-S	Determine whether the file exists and whether it is a socket file.

example

• Determine whether the file exists

[root@localhost ~]# test -e test.txt 
[root@localhost ~]# echo $?
0
#The return value is 0, indicating that there is a problem

• Use "[]" instead of the test command. Generally, brackets are used in use, because the if command does not recognize the test command

[root@localhost ~]# [ -e test.txt ]
[root@localhost ~]# echo $?
0

• Use echo to output true and false directly

[root@localhost ~]# [ -e test.txt ] && echo yes || echo no
yes
[root@localhost ~]# [ -e test.tx ] && echo yes || echo no
no

Judge according to file permissions

Test options

Test options	effect
-r	Judge whether the file exists and has read permission.
-w	Judge whether the file exists and has write permission.
-x	Judge whether the file exists and has execution permission.
-u	Judge whether the file exists and has SUID permission.
-g	Judge whether the file exists and has SGID permission.
-k	Judge whether the file exists and whether it has SBit permission.

example

• Judge whether the file has permission to read

[root@localhost ~]# [ -r mail ] && echo yes || echo no
yes

• Judge whether the file has SUID permission

[root@localhost ~]# [ -u mail ] && echo yes || echo no
no

Compare between two files

Test options

Test options	effect
File 1 -nt file 2	Judge whether the modification time of file 1 is newer than that of file 2.
File 1 -ot file 2	Judge whether the modification time of file 1 is older than that of file 2.
File 1 -ef file 2	Judge whether the Inode number of file 1 is consistent with that of file 2, which can be used to judge the hard link.

example

• Judge the difference between two files

[root@localhost ~]# [ mail -nt test.txt ] && echo yes || echo no
no
[root@localhost ~]# [ mail -ot test.txt ] && echo yes || echo no
yes

• Determine whether the two files are hard links

[root@localhost ~]# ln mail mail2
[root@localhost ~]# ls
anaconda-ks.cfg  mail  mail2  photo  sh  test.txt
[root@localhost ~]# [ mail -ef mail2 ] && echo yes || echo no
yes

Comparison between two integers

Test options

Test options	effect
Integer 1 -eq integer 2	Judge equality as true
Integer 1 -ne integer 2	Unequal judgment is true
Integer 1 -gt integer 2	Judge greater than as true
Integer 1 -lt integer 2	Judge less than as true
Integer 1 -ge integer 2	It is true to judge whether it is greater than or equal to
Integer 1 -le integer 2	Judge whether less than or equal to true

example

• Judge whether two numbers are equal

[root@localhost ~]# [ 20 -eq 21 ] && echo yes || echo no
no

• Judge whether integer 1 is less than integer 2

[root@localhost ~]# [ 20 -lt 21 ] && echo yes || echo no
yes

String judgment

Test options

Test options	effect
-z string	Judge whether the string is empty.
-n string	Judge whether the string is non empty.
String 1 = = string 2	Judge whether the two strings are equal. Equal is true.
String 1= String 2	Judge whether two strings are not equal to true.

Note: you can only judge whether the string is equal in the shell.

example

• Judge whether it is empty.

[root@localhost ~]# wjl=""
[root@localhost ~]# [ -z "$wjl" ] && echo yes || echo no
yes

Multiple conditional judgment

Test options

Test options	effect
Judgment 1 -a judgment 2	When logic and, judgment 1 and judgment 2 are true, the result is true.
Judgment 1 -o judgment 2	Logical or, when one of judgment 1 and judgment 2 is true, the final result is true.
! judge	Logical negation makes the judgment result negative.

example

• True when the value of the variable is not empty and greater than 18

[root@localhost ~]# [ -n "$wjl" -a "$wjl" -gt 18 ] && echo yes || echo no
yes

ps: Shang Silicon Valley linux video course Study notes