The judgment command test can be abbreviated as brackets []. It can be divided into string judgment and general judgment.
Note that there must be spaces between the brackets [] and the judgment conditions, and between the judgment symbols of the judgment conditions and the comparison value.
If test is used alone, if the judgment condition is true, enter echo $? 0 will be returned. If the judgment condition is false, any number between 1 and 255 will be returned.
(1) Numerical judgment
| Judgment symbol | explain | example |
|---|---|---|
| -eq | Equal is true | [ $a -eq $b ] |
| -ne | Unequal is true | [ $a -ne $b ] |
| -gt | Greater than is true | [ $a -gt $b ] |
| -ge | Greater than or equal to is true | [ $a -ge $b ] |
| -lt | Less than is true | [ $a -lt $b ] |
| -le | True if less than or equal to | [ $a -le $b ] |
Operation example:
[root@youxi1 ~]# vim a.sh
#!/bin/bash
read -p "Please enter the first number:" num1
read -p "Please enter the second number:" num2
if [ $num1 -eq $num2 ] ; then
echo "The first number is equal to the second number"
elif [ $num1 -gt $num2 ] ; then
echo "The first number is greater than the second"
elif [ $num1 -lt $num2 ] ; then
echo "The first number is less than the second"
else
echo "Judgment failure"
fi
[root@youxi1 ~]# sh a.sh
Please enter the first number:1
Please enter the second number:2
The first number is less than the second
[root@youxi1 ~]# sh a.sh
Please enter the first number:2
Please enter the second number:1
The first number is greater than the second
[root@youxi1 ~]# sh a.sh
Please enter the first number:1
Please enter the second number:1
The first number is equal to the second number
[root@youxi1 ~]# sh a.sh
Please enter the first number:aa
Please enter the second number:bb
a.sh: Line 4:[: aa: Expected integer expression
a.sh: Line 6:[: aa: Expected integer expression
a.sh: Line 8:[: aa: Expected integer expression
Judgment failure
(2) String judgment
| Judgment symbol | explain | example |
|---|---|---|
| = | Equal is true | [ $a = $b ] |
| != | Unequal is true | [ $a != $b ] |
| -z | True if string length is 0 | [ -z $a ] |
| -n | True if string length is not 0 | [ -n $a ] |
| > | Greater than is true | [ str1 > str2 ] |
| < | Less than is true | [ str1 < str2 ] |
Operation example:
[root@youxi1 ~]# vim a.sh
#!/bin/bash
read -p "Please enter the first string:" str1
read -p "Please enter the second string:" str2
if [ $str1 = $str2 ] ; then
echo "The two strings are the same"
elif [ -n $str1 ] && [ -n $str2 ] ; then
echo "Two strings are not empty"
fi
[root@youxi1 ~]# sh a.sh
Please enter the first string:aaa
Please enter the second string:bbb
Two strings are not empty
Operation example 2: note that when using string size comparison: 1 The order of strings, greater than or less than, is different from sort sorting; 2. Greater than and less than must be escaped; 3. In the test comparison test, the ASCII table is used, and the upper case letters are smaller than the lower case letters.
[root@youxi1 ~]# vim a.sh
#!/bin/bash
read -p "Please enter the first string:" str1
read -p "Please enter the second string:" str2
if [ $str1 \> $str2 ] ; then
echo "The first string is larger than the second string"
else
echo "The first string is smaller than the second string"
fi
[root@youxi1 ~]# sh a.sh / / prove that the comparison is not the sum
Please enter the first string:ab
Please enter the second string:ba
The first string is smaller than the second string
[root@youxi1 ~]# sh a.sh / / the following two comparisons prove that they are word for word
Please enter the first string:aa
Please enter the second string:ab
The first string is smaller than the second string
[root@youxi1 ~]# sh a.sh
Please enter the first string:aa
Please enter the second string:ba
The first string is smaller than the second string
(3) Document judgment
| Judgment symbol | explain | example |
|---|---|---|
| -e | True if the file or directory exists | [ -e file ] |
| -r | True if the file exists and is readable | [ -r file ] |
| -w | True if the file exists and can be written | [ -w file ] |
| -x | True if the file exists and is executable | [ -x file ] |
| -s | True if the file exists and at least one character exists | [ -s file] |
| -d | True if the file exists and is a directory | [ -d file ] |
| -f | True if the file exists and is a normal file | [ -f file ] |
| -c | True if the file exists and is a character file | [ -c file ] |
| -b | True if the file exists and is a block special file | [ -b file ] |
| -nt | If file1 is newer than file2, it is true | [ file1 -nt file2 ] |
| -ot | If file1 is older than file2, it is true | [ file1 -ot file2 ] |
| -ef | True if file1 and file2 are the same file | [ file1 -ef file2 ] |
Operation example:
[root@youxi1 ~]# vim a.sh
#!/bin/bash
read -p "Please enter the first address:" -r addr1
read -p "Please enter a second address:" -r addr2
if [ $addr1 -nt $addr2 ] ; then
echo "New document 1"
elif [ $addr1 -ot $addr2 ] ; then
echo "File 1 is older than file 2"
fi
[root@youxi1 ~]# touch file1 file2
[root@youxi1 ~]# stat file1
File:"file1"
Size: 0 Block: 0 IO Block: 4096 normal empty file
Equipment: fd00h/64768d Inode: 33574997 Hard link: 1
jurisdiction:(0644/-rw-r--r--) Uid: ( 0/ root) Gid: ( 0/ root)
Recent visit: 2019-10-12 15:17:18.341481971 +0800
Recent changes: 2019-10-12 15:17:18.341481971 +0800
Recent changes: 2019-10-12 15:17:18.341481971 +0800
Created on:-
[root@youxi1 ~]# stat file2
File:"file2"
Size: 0 Block: 0 IO Block: 4096 normal empty file
Equipment: fd00h/64768d Inode: 33574998 Hard link: 1
jurisdiction:(0644/-rw-r--r--) Uid: ( 0/ root) Gid: ( 0/ root)
Recent visit: 2019-10-12 15:17:18.341481971 +0800
Recent changes: 2019-10-12 15:17:18.341481971 +0800
Recent changes: 2019-10-12 15:17:18.341481971 +0800
Created on:-
[root@youxi1 ~]# cat file2
[root@youxi1 ~]# stat file2
File:"file2"
Size: 0 Block: 0 IO Block: 4096 normal empty file
Equipment: fd00h/64768d Inode: 33574998 Hard link: 1
jurisdiction:(0644/-rw-r--r--) Uid: ( 0/ root) Gid: ( 0/ root)
Recent visit: 2019-10-12 15:32:06.337458140 +0800
Recent changes: 2019-10-12 15:17:18.341481971 +0800
Recent changes: 2019-10-12 15:17:18.341481971 +0800
Created on:-
[root@youxi1 ~]# sh a.sh / / the certificate has nothing to do with the access time atime
Please enter the first address:/root/file1
Please enter a second address:/root/file2
[root@youxi1 ~]# echo bbb >> file2
[root@youxi1 ~]# stat file2
File:"file2"
Size: 4 Block: 8 IO Block: 4096 normal files
Equipment: fd00h/64768d Inode: 33574998 Hard link: 1
jurisdiction:(0644/-rw-r--r--) Uid: ( 0/ root) Gid: ( 0/ root)
Recent visit: 2019-10-12 15:32:06.337458140 +0800
Recent changes: 2019-10-12 15:32:59.498600870 +0800
Recent changes: 2019-10-12 15:32:59.498600870 +0800
Created on:-
[root@youxi1 ~]# sh a.sh / / should be related to one of the change time mtime or ctime
Please enter the first address:/root/file1
Please enter a second address:/root/file2
File 1 is older than file 2