Shulou(Shulou.com)06/02 Report--

Conditional statements of shell programming-- if and case I. File test 1.1test command

Test whether a specific expression is valid, and the return value is 0, otherwise, non-0

Test conditional expression

[conditional expression]-- add a space between the content and parentheses

File testing

[operator file or directory]

Commonly used test operators

●-d: test whether it is a directory (Directory)

●-e: test whether a directory or file exists (Exist)

●-f: test whether it is a file (File)

●-r: test whether the current user has permission to read (Read)

●-w: test whether the current user has permission to write (Write)

●-x: test whether the current user has permission to execute (Excute)

Examples are as follows (beginners must knock on their own to experience and understand):

[root@localhost] # ls-l shell/ total dosage 16m r w r m r m el el. 1 root root 134. November 25 19:59 demo.sh-rwxr-xr-x. 1 root root 274 November 25 18:58 state.sh-rwxr-xr-x. 1 root root 208 November 25 19:17 sujiaju.sh-rwxr-xr-x. 1 root root 345 November 25 19:11 welcome.sh [root@localhost ~] # [- d shell/] / / the first test method is valid for current users, or test-d shell/ [root@localhost ~] # echo $? 0 [root@localhost ~] # [- d shell/state.sh] [root@localhost ~] # echo $? 1 [root@localhost ~] # test-d shell/ the second test method Valid for the current user [root@localhost ~] # echo $? 0 [root@localhost ~] # test-d shell/welcome.sh [root@localhost ~] # echo $? 1 [root@localhost ~] # test-x shell/welcome.sh [root@localhost ~] # echo $? 0 [root@localhost ~] # test-x shell/demo.sh [root@localhost ~] # echo $? 1Univer / the above process can be used & once execute [root@localhost ~] # test -x shell/welcome.sh & & echo "yes" yes [root@localhost ~] # echo $? 0 [root@localhost ~] # [- d shell/state.sh] & & echo "yes" [root@localhost ~] # echo $? 1

The test command can also be used to determine the relationship between files:

-nt-- determine whether A file is newer than B file

-ot-- determines whether file An is older than file B.

-ef-- determines whether two files are the same file (whether the inode is the same or not) 1.2 integer value comparison

[integer 1 operator integer 2]

Test operators commonly used in ■

●-eq: equal to (Equal)

●-ne: not equal to (Not Equal)

●-gt: greater than (Greater Than)

●-lt: less than (Lesser Than)

●-le: less or equal to (Lesser or Equal)

●-ge: greater than or equal to (Greater or Equal)

Reference example:

[root@localhost shell] # [5-gt 3] & & echo "yes" yes [root@localhost shell] # [5-lt 3] & & echo "yes" [root@localhost shell] # 1.3 string comparison

● format 1

[string 1 = string 2]

[string 1! = string 2]

● format 2

[- Z string]

Test operators commonly used in ■

● =: the string content is the same

●! =: string content is different,! The sign means the opposite.

●-Z: the string content is empty

[root@localhost shell] # [hello = hello] & & echo "yes" yes [root@localhost shell] # [hello = hell] & & echo "yes" [root@localhost shell] # [hello! = hell] & & echo "yes" yes [root@localhost shell] # [- z] & & echo "yes" yes [root@localhost shell] # [- z hello] & & echo "yes" [root@localhost shell] # 1.4 Logic Test

​ format 1: [expression 1] operator [expression 2]

​ format 2: command 1 operator command 2.

Commonly used test operators

​ ●-an or & &: logic and, 'and'

​ ●-o or | |: logical OR, meaning "or"

​ ●!: logical No

The instance can refer to the string test instance above.

5. The structure of if sentence 5.1 single branch structure:

If conditional test operation

​ then command sequence

Fi

5.2 double branch structure

If conditional test operation

​ then Command sequence 1

​ else Command sequence 2

Fi

5.3 Multi-branch structure

If conditional Test Action 1

​ then Command sequence 1

Elif conditional Test Action 2

​ then Command sequence 2

Else

Command sequence 3

Fi

6. Case sentence structure format: case variable value in mode 1) command sequence 1; pattern 2) command sequence 2; *) default command sequence esac summary: if condition judgment is generally used for interval judgment, case is generally used for fixed value or string judgment.

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