Shell conditional expression

1. The common syntax for conditional testing is as follows

1, test expression

2. [Test expression] #两边需要有空格

3, [[Test expression]]

4, (test expression))

Description

The first and second are equivalent, the third is the extended Test command, and syntax 4 is often used to calculate

You can use wildcard characters in [[]] double brackets to make pattern matching,&& | | > < operators can be applied directly in double brackets, but not in single brackets

2, test A simple example of the following, you can view the man test for detailed help

[[email protected] ~]# test-f/etc/passwd && echo 1 | | Echo 0
1
[[email protected] ~]# test-f/etc/passwd11 && echo 1 | | Echo 0
0

3, [] but the bracket instance

[[Email protected] ~]# [-f/etc/passwd]&& echo 1 | | Echo 0
1
[[Email protected] ~]# [-f/etc/passwd11]&& echo 1 | | Echo 0
0

4, [[]] double bracket instance

[[Email protected] ~]# [[3 > 2]]&& echo 1 | | Echo 0
1
[[Email protected] ~]# [[3 > 4]]&& echo 1 | | Echo 0
0

5. File Test expression

[[Email protected] ~]# [-d/etc/]&& echo 1 | | Echo 0
1
[Email protected] ~]#
[[Email protected] ~]# [-D/ETC/11]&& echo 1 | | Echo 0
0

6. String Test expression

[[Email protected] ~]# [-N "abc"]&& echo 1 | | Echo 0
1

[[Email protected] ~]# [-z] "]&& echo 1 | | Echo 0
0
[[Email protected] ~]# ["AB" = "AB"]&& echo 1 | | Echo 0
1
[[Email protected] ~]# ["AB"! = "AB"]&& echo 1 | | Echo 0
0

Observing how the system scripts are written

7. Integer comparison

[[Email protected] ~]# [2-lt 3]&&echo 1| | Echo 0
1
[[Email Protected]ckup ~]# [2-gt 3]&&echo 1| | Echo 0
0

Compare the size of two integers [[email protected]~]#Cat 1.SH #!/bin/bash#no.1a=$1b=$2[ $# -ne2]&&{  Echo "USAGE:"$0 "NUM1 num2"Exit1} #no.2Expr$a + ten &>/dev/dullRETVAL1=$?Expr$b + Ten &>/dev/dullRETVAL2=$?[$RETVAL 1-eq0-A $RETVAL 2-eq0]||{  Echo "pls-input-int nums:"exit 2} #no.3[$a-GT $b] &&Echo "$a > $b"||{[$a-eq $b] &&Echo "$a = $b"||Echo "$a < $b"}[[email protected]~]#SH 1.SH 1 21<2[[Email protected]~]#SH 1.SH 3 23>2[[Email protected]~]#SH 1.SH 2 22=2


Use read mode instead

[email protected] ~]# cat 1.sh
#!/bin/bash
#no. 1
Read-p "Pls input" num: "a B
[-Z "$a"]| | [-Z "$b"]&&{
echo "pls input" num again "
Exit 1
}
#no. 2
Expr $a + &>/dev/dull
Retval1=$?
Expr $b + &>/dev/dull
Retval2=$?
Test $RETVAL 1-eq 0-a $RETVAL 2-eq 0 | | {
echo "pls input" int nums: "
Exit 2
}

#no. 3
[$a-gt $b] && echo "$a > $b"
[$a-lt $b] && echo "$a < $b"
[$a-eq $b] && echo "$a = $b"

[[Email protected] ~]# sh 1.sh
Pls Input 1 Num:1
1=1
[[Email protected] ~]# sh 1.sh
Pls Input 2 Num:1
1<2
[[Email protected] ~]# sh 1.sh
Pls Input 1 Num:2
2>1

Shell conditional expression