Shell programming and variables-- write your first shell script

First understand the role of shell:

1. Command interpreter: the "translator" is between the system kernel and the user and is responsible for interpreting the command line.

2, the user's login, the default shell program used after login, generally / bin/bash. The internal instructions and running environment of different shell will be different.

[root@localhost] # cat / etc/shells/bin/sh/bin/bash/bin/nologin.

Write the first shell script:

1. Write script code to improve the composition of the script

Use the vi Editor

All scripts have a .sh extension

Every day, linux commands are written in order of execution.

Form a good habit: the first thing to start with is a script statement: #! / bin/bash

[root@localhost ~] # vi first.shroud lh vml*. Bash # this is first scriptcd / bootpwd1s-lh vml*

2. Endow executable authority

Root@localhost ~] # chmod + x first.sh

3. Execute the script file

Method 1: script file path (absolute path and relative path) [this way the script file must have executable permissions]

[root@localhost ~] #. / first.sh

Method 2: sh script file path (this method does not change the location of the environment)

[root@localhost -] # sh / first.sh

Method 3: source script file path

[root@localhost -] # source / first.sh

Method 4: ". + spaces" script file path

[root@localhost] #. First.sh

Introduction of key symbols-- Redirection and Pipeline

Interactive hardware equipment

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

Standard output outputs data to the user through the device

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

Redirect: saves the output to the specified file

| | Pipe symbol: outputs the result of the command on the left as the processing object of the command on the right |

Cmd1 | cmd2 [. | cmdn]

The function and type of shell variable

1. The function 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

2. 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 letters or underscores and are case-sensitive. Full 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: allow references to other variable values through the $symbol: reference to other variable values is prohibited. $is regarded as an ordinary character reverse apostrophe: command replacement, extracting the output result after the execution 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 2:enport variable name + variable value.

Operation of integer variables

Expr variable 1 operator variable 2 [operator variable 3]. Common operator addition operation: + subtraction operation:-multiplication operation:\ * division operation: / module (remainder) operation:%

Special shell variable

Environment variable

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

Configuration file: / etc/profile (system environment variable), ~ / .bash_profile (user environment variable)

Common environmental variables

PWD 、 PATH

USER 、 SHELL 、 HOME

Location variable

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

Predefined variable

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

$*: contents of all location variables

$?: the status returned after the last command was executed. When the return status is set to 0, the execution is normal, and a non-zero value indicates an exception or error.

$0: currently executed process / program