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Overview of Shell script

With the more and more applications of Linux system in enterprises, the automatic management of servers is becoming more and more important. In some complex maintenance work, a proper Shell script can batch process and automate a series of maintenance tasks, thus reducing the burden of administrators.

The concept of Shell script

Save the commands to be executed to a text file in order

If the file is given executable permission, it can be run

Various Shell control statements can be combined to accomplish more complex operations

Shell script application scenario

Repetitive operation

Batch transaction processing

Automatic operation and maintenance

Service running status monitoring

Scheduled task execution

The role of SHell script-command interpreter

Shell is equivalent to a "translator", between the system kernel and the user, and is responsible for interpreting the command line.

Login Shell of the user

The default shell program used after login, usually / bin/bash

The internal instructions and running environment of different shell will be different.

Write script code

Use the vim text editor

One Linux command per line, written in order of execution

[root@localhost ~] # vim first.sh write the first script

#! / bin/bash#this is fiest scriptcd / bootpwdls-lh vml gives executable permission

Make the script executable

[root@localhost ~] # chmod + x first.sh execution script file [root@localhost ~] # / first.sh (must have execution permission and do not change the directory location)

[root@localhost] #. First.sh (no execution permission is required, but change the directory location)

[root@localhost ~] # source first.sh (no execution permission is required, but change the directory location)

[root@localhost ~] # sh first.sh (no execution permission and no change of directory location)

Better script composition

Script declaration

Comment information

Executable statement

#! / bin/bash#this is fiest scriptcd / bootecho "current path"pwdecho" file information beginning with vml "ls-lh vml"

Redirection and pipe operation

Interactive hardware equipment

Standard input: receives data entered by the user from this device

Standard output: output data to the user through this device

Standard error: an execution error message is reported through this device

Redirect operation

* * Pipeline operation symbol "|"

Awk Vertical recognition-F separator (separated by colon,-F followed by colon) Print output $1 call the first variable $7 call the seventh variable shell variable function, type

The role of variables

Providing specific parameters for flexible management of Linux systems has two meanings

Variable name: use a fixed name, preset by the system or user-defined.

Variable value: can change according to user settings and changes in the system environment

Type of variable

Custom variables: defined, modified, and used by the user

Environment variables: maintained by the system and used to set up the work environment

Position variables: passing parameters to the script from the command line

Predefined variables: a class of variables built into Bash that cannot be modified directly

Custom variable

Define a new variable

Variable names begin with a letter or underscore and are case-sensitive. All uppercase is recommended.

Variable name = variable value

View the value of a variable

Echo $variable name

Use quotation marks when assigning values

Double quotation marks: allows other variable values to be referenced by the $symbol

Single quotation marks: reference to other variable values is prohibited, and $is regarded as a normal character

Reverse apostrophe (key under Esc): replace the command and extract the output of the command.

Assign a value to a variable from keyboard input

Read [- p "prompt message"] variable name

Set the scope of the variable

Format 1:export variable name format 1:export variable name = variable value (the two formats can be mixed)

Operation of integer variables

Expr variable 1 operator variable 2 [operator variable 3]

Common operators

(1) addition operation: +

(2) subtraction:-

(3) multiplication operation:

(4) Division operation: /

(5) Modulus (remainder) operation:%

Example demonstration: calculating the sum of any two integers

#! / bin/bashread-p "Please enter the first integer:" num1read-p "Please enter the second integer:" num2# performs the addition operation sum= `expr $num1 + $num2`echo "the sum is: $sum"

Special Shell variable

Environment variable

Created in advance by the system to set up the user's working environment

Configuration file: / etc/profile, ~ / .bashprofile

Common environmental variables

PWD 、 PATH 、 USER 、 HOME 、 SHELL

Location variable

Expressed as a number between $n and n = 1 to 9

#! / bin/bashecho "value of the first location variable: $1" echo "the value of the second location variable: $2" sum= `expr $1 + $2`echo "summed as: $sum"

Predefined variable

$#: the number of location variables on the command line

$*: contents of all location variables

$? The status returned after the last command is executed. A return value of 0 indicates normal execution, while a non-zero value only indicates an error in execution.

$0: currently executed process / program name

#! / bin/bashecho "first location variable value: $1" echo "second location variable value: $2" sum= `expr $1 + $2`echo "summation is: $sum" echo "script name: $0" echo "detailed number: $* number of" echo "parameters: $#"

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