Shell script Advanced (top)

Shell script is our learning operational difficulties, this content simple example shell script advanced, there are any technical guidance, knowledge of the shortcomings are welcome to point out.

Process Control

Programming languages: Sequential execution selection execution loop execution

Conditional Select if statement

? Single Branch

if judgment condition; then condition is true Branch code fi

Double-branch if judging condition; Then condition for true Branch code else condition for false Branch code FI

Multi-Branch if judging condition 1; Then condition is true branch code elif judging condition 2; Then condition is true branch code elif judging Condition 3; Then condition is true branch code else above condition is false Branch code FI

Judge by condition, when the first encounter is a "true" condition, execute its branch, and then end the entire if statement

Example:

Write script/root/bin/filetype.sh, determine user input file path, display its file type (normal, directory, link, other file type)

Execution results are as follows

Example 2

Write script/root/bin/checkint.sh to determine if the user input parameter is a positive integer

Execution results are as follows

Conditional Judgment: Case statement

Case variable reference in

PAT1) branch 1;;

PAT2) branch 2;;

.. *) default branch;;

Esaccase supports GLOB-style wildcard characters: *: Any character of any length?: any single character []: any single character in the specified range a|b:a or b

Example

Create a pin name of useradd.sh, when the-a option is executed, the user Mageuser1~mageuser20 is created in bulk, requiring the Mageuserx uid to be 200x,mageuserxx uid for the 20xx,1~8 user's shell to/ The UID of the sbin/nologin,9~20 user is/bin/bash. The last 20 user passwords are set to CentOS when the-D option is executed, the user MAGEUSER1~MAGEUSER20 is deleted in bulk. Delete together with home directory

Cycle

Loop execution repeats how many times a piece of code is run repeatedly: number of cycles known beforehand that there is an entry condition and exit condition

For loop

For variable name in list;

Loop body

Done

Example

Example

Writing a script print rectangle

The results of the implementation are as follows:

While loop

? While CONDITION; Do

Loop body

Done

? CONDITION: cyclic control conditions; Before entering the cycle, make a judgment; once each loop is judged again; the condition is "true", then a loop is executed until the condition test state is "false" to terminate the loop

? Therefore: Condtion generally should have a cyclic control variable, and the value of this variable will be continuously corrected in the loop body

? Enter condition: Condition is true

? Exit Condition: Condition is false

Example

Loop Control Shift Command

? Shift [N]

? Used to move the list of parameters to the left for a specified number of times, the default is to move left once.

? Parameter list once moved, the left-most argument is removed from the list. While looping through positional parameter lists, use the shift

?./doit.sh a b c d e F g h

?./shfit.sh a b c d e F g h

Example

Execution results are as follows

Create an infinite loop

? While true; Do

Loop body

? Done

? until false; Do

Loop body

? Done

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Shell script Advanced (top)