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Today, I would like to share with you how to set the Linux variable related knowledge points, detailed content, clear logic, I believe that most people still know too much about this knowledge, so share this article for your reference, I hope you have something to gain after reading this article, let's take a look at it.

User variable

Although it's easy to set variables on the command line, there are some interesting tricks. To set a variable, you just need to do this:

$myvar=11 $myvar2= "eleven"

To display these values, simply do this:

$echo $myvar11 $echo $myvar2eleven

You can also use these variables. For example, to increment a numeric variable, use any of the following commands:

$myvar=$ ((myvar+1)) $echo $myvar12 $((myvar=myvar+1)) $echo $myvar13 $(myvar+=1) $echo $myvar14 $((myvar++)) $echo $myvar15 $let "myvar=myvar+1" $echo $myvar16 $let "myvar+=1" $echo $myvar17 $let "myvar++" $echo $myvar18

With some of them, you can increase the value of a variable. For example:

$myvar0=0 $((myvar0++)) $echo $myvar01 $((myvar0+=10)) $echo $myvar011

Through these options, you may find them easy to remember and easy to use. You can also delete a variable-which means it is not defined.

$unset myvar$ echo $myvar

Another interesting option is that you can set a variable and make it read-only. In other words, once a variable is set to read-only, its value cannot be changed (except for some very complex command line magic). That means you can't delete it either.

$readonly myvar3=1$ echo $myvar31 $(myvar3++)-bash: myvar3: readonly variable$ unset myvar3-bash: unset: myvar3: cannot unset: readonly variable

You can use these settings and incremental options to assign and manipulate variables in the script, but there are also some very useful internal variables that can be used in the script. Note that you cannot reassign or increase their values.

Internal variable

You can use many variables in a script to calculate parameters and display information about the script itself. 2, and so on represent the first, second, third, and other parameters of the script. # indicates the number of parameters. Represents all parameters. 0 represents the name of the script. Represents the return code of a previously run command (represents success). Displays the progress of the script. PPID displays the process ID of shell (the parent process of the script). Some of these variables also apply to the command line, but display relevant information: display the name you are using (for example,). The process shown. PPID shows the process ID (for me, sshd) of the parent process of shell. To see their results, if we put all these variables into one script, such as:

#! / bin/bashecho $0echo $1echo $2echo $# echo $* echo $? echo $$echo $PPID

When we call this script, we will see the following:

The return code of $tryme one two three/home/shs/bin/tryme echo command is 10410

If we check shell's process ID after the script has finished running, we can see that it matches the PPID shown in the script:

$echo $$10109

Of course, a more useful way is to use them than to simply display their values. Let's take a look at their possible uses. Check to see if parameters have been provided:

If [$# = = 0]; then echo "$0 filename" exit 1fi

Check whether a specific process is running:

Ps-ef | grep apache2 > / dev/nullif [$?! = 0]; then echo Apache is not running exitfi

Verify that the file exists before attempting to access the file:

If [$#-lt 2]; then echo "Usage: $0 lines filename" exit 1fiif [!-f $2]; then echo "Error: File $2 not found" exit 2else head-$1 $2fi

In the following small script, we check to see if the correct number of parameters are provided, that the first parameter is a number, and that the file represented by the second parameter exists.

#! / bin/bashif [$#-lt 2]; then echo "Usage: $0 lines filename" exit 1fiif [[$1! = [0-9] *]]; then echo "Error: $1 is not numeric" exit 2fiif [!-f $2]; then echo "Error: File $2 not found" exit 3else echo top of file head-$1 $2fi renaming variable

When writing complex scripts, it is often useful to specify names for the script's parameters, rather than continuing to refer to them as, 2, and so on. By line 35, the person who read your script may have forgotten what to say. If you assign the value of an important parameter to filename or $numlines, it's not easy to forget it.

#! / bin/bashif [$#-lt 2]; then echo "Usage: $0 lines filename" exit 1else numlines=$1 filename=$2fiif [[$numlines! = [0-9] *]]; then echo "Error: $numlines is not numeric" exit 2fiif [!-f $filename]; then echo "Error: File $filename not found" exit 3else echo top of file head-$numlines $filenamefi

Of course, this sample script simply runs the head command to display the first x lines in the file, but its purpose is to show how to use internal parameters in the script to help ensure that the script works well, or to clearly know the reason for the failure if it fails.

These are all the contents of the article "how to set Linux variables". Thank you for reading! I believe you will gain a lot after reading this article. The editor will update different knowledge for you every day. If you want to learn more knowledge, please pay attention to the industry information channel.

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