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This article mainly introduces the example analysis of Shell variables, has a certain reference value, interested friends can refer to, I hope you can learn a lot after reading this article, the following let the editor take you to understand it.

Regardless of the version of Shell, let's take a look at the Shell variable. There are three kinds of variables in Shell: system variable, environment variable, and user variable. Among them, the user variable is most used in the programming process, the system variable is used in the parameter judgment and command return value judgment, and the environment variable mainly needs to be set when the program is running.

1 system variabl

There are not many system variables commonly used in Shell, but they are very useful, especially when doing some parameter testing. Here are the system variables commonly used in Shell

Description of representation method

$n $1 represents the first parameter, $2 represents the second parameter.

$# number of command line arguments

$0 name of the current program

$? The return code of the previous command or function

* to save all parameters in the form of "Parameter 1, Parameter 2."

$@ with Parameter 1 and Parameter 2. Save all parameters in form

$(process ID number) PID of this program

$! PID of the previous command

The most commonly used is $n $# $0 $?. Take a look at the following example:

The code is as follows:

#! / bin/sh

# This file is used to explain the shell system variable.

Echo "the number of parameter is $#"

Echo "the return code of last command is $?"

Echo "the script name is $0"

Echo "the parameters are $*"

Echo "/ $1 = $1; / $2 = $2"

The following is the running result:

-bash-2.05b$. / chapter2.1.sh winter stlchina

The number of parameter is 2

The return

Code of last command is 0

The script name is. / chapter2.1.sh

The parameters are winter stlchina

$1 = winter; $2 = stlchina

This example is too simple, not practical at all, here is a practical, if you do not understand, it does not matter, the following content will be explained in detail.

The code is as follows:

#! / bin/sh

If [$#-ne 2]; then

Echo "Usage: $0 string file"

Exit 1

Fi

Grep $1 $2

If [$?-ne 0]; then

Echo "Not Found\" $1\ "in $2"

Exit 1

Fi

Echo "Found\" $1\ "in $2"

The above example uses $0 $1 $2 $# $? And so on, here is the explanation of the program:

1. Determine the number of running parameters, if not equal to 2, display the use of "usage help", where $0 means the script itself.

two。 Use grep to find the $1 string in the $2 file.

3. Judge the return value of the previous command after running (usually 0 for success and non-0 for failure).

4. If the display is not successful and the relevant information is not found, otherwise the display has found it.

5. Where / "indicates escape, and the" sign needs to be displayed in "", then the escape character / "needs to be added.

The following example runs:

. / chapter2.2.sh usage chapter2.2.sh

Not Found "usage" in

Chapter2.2.sh

-bash-2.05b$. / chapter2.2.sh Usage chapter2.2.sh

Echo

"Usage: $0 string file"

Found "Usage" in

Chapter2.2.sh

2 Shell user variabl

2.1 Foundation

No matter how many system variables there are, it is not enough for requirements. User variables are the most commonly used variables and are very easy to use.

User-defined variables must consist of alphanumeric and underscores, and the first character of the variable name cannot be a number. Like other UNIX names, variable names are case-sensitive. For user variables, the user can assign values as follows:

Name= "Winter"

When referencing variables, you need to precede the $symbol, and users can also assign values to each other between variables, such as:

Name= "Winter"

WINTER=$name

Echo

"Hello $WINTER!"

The output should be clear: Hello Winter!

One thing to note here: there are no spaces between variables and'=', and no spaces between'= 'and assignments, otherwise shell will not consider the variable to be defined. Now that you have mastered the basic usage, you can start your programming work completely. But sometimes you need to plan ahead. Here are some tips on user variables.

2.2 use skills

You can also use variables and other characters to form new words, and you may need to enclose the variables in {}, such as:

SAT=Satur

Echo

Today is ${SAT} day

The output is: Today is Saturday

Sometimes in order to avoid confusion between variable names and other characters, you'd better get into the habit of enclosing variable names in {}.

For unassigned variables, Shell is treated as null, and the user can use the unset command to clear the value assigned to the variable. Look at an example:

The code is as follows:

#! / bin/sh

Echo "astradama"

Axi2

Echo "astradama"

Unset a

Echo "astradama"

Guess what the result is first?

-bash-2.05b$. / test

.sh

A =

Axi2

A =

If you know about clocking, you should know that there is a variable modifier "const" to prevent the program from accidentally modifying variables. In shell, you can use the same modifier "readonly" for user variables if I modify the above example like this:

The code is as follows:

#! / bin/sh

Echo "astradama"

# readonly has been added below

Readonly axi2

Echo "astradama"

Unset a

Echo "astradama"

Of course, the result will be different:

-bash-2.05b$. / test

.sh

A =

Axi2

Axi2

2.3 arrays in shell

Arrays can also be set in shell variables, but different versions of shell have different array assignment methods, which are not supported in bourne shell. Therefore, if you do not really need it, it is recommended that you do not use arrays. If your data structure is very complex and you have to use arrays, then I suggest you choose another language. Shell is not a panacea.

There are two ways to assign values to shell. The first is to assign values directly with subscript:

Name [0] = "Tom"

Name [1] = "Tomy"

Name [2] = "John"

...

The other way is different for different versions of shell. Assignment in bash:

[code]

#! / usr/local/bin/bash

Name= ("Tom", "Tomy", "John")

For i in 0 1 2

Do echo $iVOR ${name [$I]}

Done

[html]

The above two assignment methods achieve the same effect. By the way, do you see the way to access array elements? In the form of ${name [index]}. Notice that the first line uses #! / usr/local/bin/bash, which is a little different from before. The output is as follows:

-bash-2.05b$. / test

.sh

0:Tom

1:Tomy

2:John

3 shell environment variabl

The shell environment variable is a parameter that all shell programs accept. When the shell program runs, it receives a set of variables, which are environment variables. Common environment variables:

The name describes the PATH command search path, with a colon as the delimiter. Note that unlike under DOS, the current directory is not the pathname of the HOME user's home directory in the system path. Is the default parameter of the cd command COLUMNS defines the length of the command line that can be used in command editing mode EDITOR default line editor VISUAL default visual editor FCEDIT command fc editor HISTFILE command history file HISTSIZE command history file the maximum number of lines contained in the HISTFILESIZE command history file IFS defines the delimiter LOGNAME user login name MAIL used by SHELL points to a file that requires SHELL to monitor its modification time. When the file is modified, SHELL will send a message You hava mail to the user MAILCHECKSHELL to check the period of the MAIL file, in seconds MAILPATH function is similar to MAIL. However, you can use a set of files, separated by colons, each file can be followed by a question mark and a message to the user SHELLSHELL path name TERM terminal type TMOUTSHELL automatic exit time, in seconds, if set to 0, prohibit SHELL from automatically exiting PROMPT_COMMAND specifies the command PS1 main command prompt PS2 secondary command prompt that should be executed before the main command prompt During the execution of the command, when you are required to enter data, use the PS3select command prompt PS4 debug command prompt MANPATH to find the path to the man page, separate LD _ LIBRARY_PATH to find the path to the library with a colon, and separate it with a colon.

The most important thing to pay attention to these variables is PATH. I don't want to say the importance of these variables, right?

If you want to make the variables you define available to all other shell programs, that is, define new environment variables. All you have to do is use the export keyword. For example:

Export

MY_NAME=Winter

Export

PATH=/home/winter/bin:$PATH

In the above program, the first line outputs the MY_NAME variable, and the second line adds a path / home/winter/bin to the environment variable PATH. If you want these settings to be valid when you log into unix/linux, you need to add them to your shell startup script, if you are using bash

~ / .bash_profile

You can see the other versions at a glance. In your home directory, use "." The first file is usually hidden, and you need to use the'ls-al' command to display it.

Thank you for reading this article carefully. I hope the article "sample Analysis of Shell variables" shared by the editor will be helpful to you. At the same time, I also hope you will support us and pay attention to the industry information channel. More related knowledge is waiting for you to learn!

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