The control structure if__ programming of shell programming

There are control structures in all programming languages, and shell programming is no exception. The IF structure is the most commonly used branch control structure.

The most common if statements in Linux Shell programming are: If.....then...fi,if....then....else....fi,if....then...elif ...

If....then.....else ... The statement is very simple and the syntax is as follows:

An If expression

then command table

[Else Command table]

Fi

The expression is the judgment condition, the command table is when the conditional execution of the shell command, FI represents the end of the entire if statement, must have.

Let's look at an example:

#!/bin/bash
#filename: if.sh
#author: gyb

read num
($num-lt];then
        echo "$num <10"
else
        echo "$num >10"
fi

Increase executable Permissions

root@ubuntu:~# chmod +x if.sh

Perform

root@ubuntu:~#./if.sh
2
2<10
root@ubuntu:~#./if.sh
24
24>10
The code [$num-lt 10] is another way of writing the test command, which requires a space before ' [' and '] '.

If....then....elif ..... is a elif structure that can be written indefinitely if the program requires it (where elif represents the abbreviation of else if).

Example:

#!/bin/sh
#filename: elif.sh
#author: gyb

echo "Input score:"
read score

if [$score-ge 0-a $score-  Lt];then
        echo "E"
elif [$score-ge 60-a $score-lt];then
        echo "D"
elif [$score-ge 70-a $score -lt];then
        echo "C"
elif [$score-ge 80-a $score-lt];then
        echo "B"
elif [$score-ge 90-a $ Score-le];then
        echo "A"
else
        echo "input error"
fi

The code in the judgement condition in the criterion-a represents the logic and the entire expression is true only if the two conditions are set at the same time.

To add executable permissions:

root@ubuntu:~# chmod +x elif.sh

Perform

root@ubuntu:~#./elif.sh

Input score:
34
E
root@ubuntu:~#./elif.sh
Input score:
67
D
root@ubuntu:~#./elif.sh
Input score:
78
C
root@ubuntu:~#./elif.sh
Input score:
89
B

root@ubuntu:~#./elif.sh
Input score:
99
A

If statements can be nested and can be nested in multiple sets

Example:

#!/bin/bash
#filename: ifnest.sh
#author: gyb

read num
($num-lt];then
        echo "$num <100"
        if [$num-lt];then
                echo "$num <50"
                if [$num-lt];then
                        echo "$num <30"
                fi
        fi
F I

Add executable permissions

root@ubuntu:~# chmod +x ifnest.sh

Perform

root@ubuntu:~#./ifnest.sh
23
23<100
23<50
23<30
root@ubuntu:~#./ifnest.sh
45
45<100
45<50

End.