Logical judgments in shell scripts

Logical judgments in shell scripts

If logic is judged. The basic syntax for if judgment in the shell is:

1) without Else

If  judgment statement; then    COMMANDFI

For example:

[email protected] sbin]# cat if1.sh#! /bin/bashread-p "Please input your score:" AIF (($a <60); Then    echo "You didn ' t pass the exam." Fi

In if1.sh (($a <60)) This form, which is a unique format in the shell script, with a parenthesis or no error, please remember this format or can be written as [$a LT 60]. The result of the execution is:

[[Email protected] sbin]# sh if1.shplease input your score:90[[email protected] sbin]# sh if1.shplease input your score: 33You didn ' t pass the exam.

2) with Else

If  judgment statement  ; then    commandelse    COMMANDFI

For example:

[email protected] sbin]# cat if2.sh#! /bin/bashread-p "Please input your score:" AIF (($a <60); Then     echo "You didn ' t pass the exam." else     echo "good! You passed the exam. " Fi

Execution Result:

[Email protected] sbin]# sh if2.shplease input your score:80good! You passed the exam. [Email protected] sbin]# sh if2.shplease input your score:25you didn ' t pass the exam.

The only difference from the previous example is that if you enter a number greater than or equal to 60, you will be prompted.

3) with Elif

If  judgment statement one  ; then    commandelif  judgment Statement two; then    Commandelse    COMMANDFI

For example:

[email protected] sbin]# cat if3.sh#! /bin/bash read-p "Please input your score:" A If (($a <60)); Then         echo "You didn ' t pass the exam." Elif (($a >=60) && (($a <85)) and then         echo "good! You pass the exam "else         echo" very good! Your Socre is very high! "fi

&& here means "and", of course, you can use | | means "or" to execute the result as:

[Email protected] sbin]# sh if3.shplease input your score:90very good! Your Socre is very high! [Email protected] sbin]# sh if3.shplease input your score:60good! You pass the exam.

The above is simply a description of the structure of the IF statement. In determining the size of a value in addition to (()) can be used in the form of [] but can not use;, <, = such a symbol, to use-LT (less than),-GT (greater than),-le (less than equals),-ge (greater than or equal),-eq (equals),-ne (not equals).

[Email protected] sbin]# a=10; If [$a-lt 5]; then echo OK; Fi[[email protected] sbin]# a=10; If [$a-gt 5]; then echo OK; Fiok[[email protected] sbin]# a=10; If [$a-ge 10]; then echo OK; Fiok[[email protected] sbin]# a=10; If [$a-eq 10]; then echo OK; Fiok[[email protected] sbin]# a=10; If [$a-ne 10]; then echo OK; Fi

And look at the If using && | | The situation:

[Email protected] sbin]# a=10; If [$a-lt 1] | | [$a-gt 5]; then echo OK; Fiok[[email protected] sbin]# a=10; If [$a-gt 1] | | [$a-lt 10]; then echo OK; Fiok

In a shell script, if is also often judged about the file attributes, such as whether it is a normal file or a directory, to determine if the files have read and write execution rights. There are several options that are commonly used:

-E: Determine if a file or directory exists

-D: Determine if the directory is not present and whether it exists

-F: Determine if the file is normal and exists

-R: Determine if the document has Read permissions

-W: Determine if Write permission is available

-X: Determine if executable

When you use the If judgment, the specific format is:

if [ -e filename ] ; then

Example:

[Email protected] sbin]# if [-d/home/]; then echo OK; Fiok[[email protected] sbin]# if [-f/home/]; then echo OK; Fi

Because/home/is a non-file directory, "OK" is not displayed.

[Email protected] sbin]# if [-f/root/test.txt]; then echo OK; Fiok[[email protected] sbin]# if [-r/root/test.txt]; then echo OK; Fiok[[email protected] sbin]# if [-w/root/test.txt]; then echo OK; Fiok[[email protected] sbin]# if [-x/root/test.txt]; then echo OK; Fi[[email protected] sbin]# if [-e/root/test1.txt]; then echo OK; Fi

In shell scripts, there is a common way to use if to judge logic, which is case. The specific format is:

Case  Variable  invalue1)          command          ;; value2)          command          ;; VALUE3)          command          ;; *)          command          ; Esac

The above structure, which does not limit the number of * values, represents a value other than value above. The following Amin writes a script that determines whether the input value is odd or even:

[email protected] sbin]# cat case.sh#! /bin/bashread-p "Input a number:" Na=$[$n%2]case $a in    1)        echoes "the number is odd."        ;    0)        echo "The number is even."        ;;    *)        echo "It ' s not a number!"        ;; Esac

The value of $a is either 1 or 0, and the result is:

[Email protected] sbin]# sh case.shinput a number:100the number is even. [Email protected] sbin]# sh case.shinput A number:101the number is odd.

Case scripts are often used to write startup scripts for system services, such as those used in/etc/init.d/iptables, so you might want to check them out.

Logical judgments in shell scripts