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@ -1,267 +0,0 @@
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[#]: collector: (lujun9972)
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[#]: translator: (MjSeven)
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[#]: reviewer: ( )
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[#]: publisher: ( )
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[#]: url: ( )
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[#]: subject: (Working with variables on Linux)
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[#]: via: (https://www.networkworld.com/article/3387154/working-with-variables-on-linux.html#tk.rss_all)
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[#]: author: (Sandra Henry-Stocker https://www.networkworld.com/author/Sandra-Henry_Stocker/)
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Working with variables on Linux
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======
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Variables often look like $var, but they also look like $1, $*, $? and $$. Let's take a look at what all these $ values can tell you.
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![Mike Lawrence \(CC BY 2.0\)][1]
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A lot of important values are stored on Linux systems in what we call “variables,” but there are actually several types of variables and some interesting commands that can help you work with them. In a previous post, we looked at [environment variables][2] and where they are defined. In this post, we're going to look at variables that are used on the command line and within scripts.
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### User variables
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While it's quite easy to set up a variable on the command line, there are a few interesting tricks. To set up a variable, all you need to do is something like this:
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```
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$ myvar=11
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$ myvar2="eleven"
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```
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To display the values, you simply do this:
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```
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$ echo $myvar
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11
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$ echo $myvar2
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eleven
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```
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You can also work with your variables. For example, to increment a numeric variable, you could use any of these commands:
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```
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$ myvar=$((myvar+1))
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$ echo $myvar
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12
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$ ((myvar=myvar+1))
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$ echo $myvar
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13
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$ ((myvar+=1))
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$ echo $myvar
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14
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$ ((myvar++))
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$ echo $myvar
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15
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$ let "myvar=myvar+1"
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$ echo $myvar
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16
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$ let "myvar+=1"
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$ echo $myvar
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17
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$ let "myvar++"
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$ echo $myvar
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18
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```
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With some of these, you can add more than 1 to a variable's value. For example:
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```
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$ myvar0=0
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$ ((myvar0++))
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$ echo $myvar0
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1
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$ ((myvar0+=10))
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$ echo $myvar0
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11
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```
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With all these choices, you'll probably find at least one that is easy to remember and convenient to use.
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You can also _unset_ a variable — basically undefining it.
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```
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$ unset myvar
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$ echo $myvar
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```
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Another interesting option is that you can set up a variable and make it **read-only**. In other words, once set to read-only, its value cannot be changed (at least not without some very tricky command line wizardry). That means you can't unset it either.
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```
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$ readonly myvar3=1
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$ echo $myvar3
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1
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$ ((myvar3++))
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-bash: myvar3: readonly variable
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$ unset myvar3
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-bash: unset: myvar3: cannot unset: readonly variable
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```
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You can use any of those setting and incrementing options for assigning and manipulating variables within scripts, but there are also some very useful _internal variables_ for working within scripts. Note that you can't reassign their values or increment them.
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### Internal variables
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There are quite a few variables that can be used within scripts to evaluate arguments and display information about the script itself.
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* $1, $2, $3 etc. represent the first, second, third, etc. arguments to the script.
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* $# represents the number of arguments.
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* $* represents the string of arguments.
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* $0 represents the name of the script itself.
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* $? represents the return code of the previously run command (0=success).
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* $$ shows the process ID for the script.
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* $PPID shows the process ID for your shell (the parent process for the script).
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Some of these variables also work on the command line but show related information:
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* $0 shows the name of the shell you're using (e.g., -bash).
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* $$ shows the process ID for your shell.
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* $PPID shows the process ID for your shell's parent process (for me, this is sshd).
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If we throw all of these variables into a script just to see the results, we might do this:
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```
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#!/bin/bash
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echo $0
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echo $1
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echo $2
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echo $#
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echo $*
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echo $?
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echo $$
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echo $PPID
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```
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When we call this script, we'll see something like this:
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```
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$ tryme one two three
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/home/shs/bin/tryme <== script name
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one <== first argument
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two <== second argument
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3 <== number of arguments
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one two three <== all arguments
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0 <== return code from previous echo command
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10410 <== script's process ID
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10109 <== parent process's ID
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```
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If we check the process ID of the shell once the script is done running, we can see that it matches the PPID displayed within the script:
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```
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$ echo $$
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10109 <== shell's process ID
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```
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Of course, we're more likely to use these variables in considerably more useful ways than simply displaying their values. Let's check out some ways we might do this.
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Checking to see if arguments have been provided:
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```
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if [ $# == 0 ]; then
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echo "$0 filename"
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exit 1
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fi
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```
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Checking to see if a particular process is running:
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```
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ps -ef | grep apache2 > /dev/null
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if [ $? != 0 ]; then
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echo Apache is not running
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exit
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fi
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```
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Verifying that a file exists before trying to access it:
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```
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if [ $# -lt 2 ]; then
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echo "Usage: $0 lines filename"
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exit 1
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fi
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if [ ! -f $2 ]; then
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echo "Error: File $2 not found"
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exit 2
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else
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head -$1 $2
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fi
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```
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And in this little script, we check if the correct number of arguments have been provided, if the first argument is numeric, and if the second argument is an existing file.
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```
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#!/bin/bash
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if [ $# -lt 2 ]; then
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echo "Usage: $0 lines filename"
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exit 1
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fi
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if [[ $1 != [0-9]* ]]; then
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echo "Error: $1 is not numeric"
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exit 2
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fi
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if [ ! -f $2 ]; then
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echo "Error: File $2 not found"
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exit 3
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else
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echo top of file
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head -$1 $2
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fi
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```
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### Renaming variables
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When writing a complicated script, it's often useful to assign names to the script's arguments rather than continuing to refer to them as $1, $2, and so on. By the 35th line, someone reading your script might have forgotten what $2 represents. It will be a lot easier on that person if you assign an important parameter's value to $filename or $numlines.
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```
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#!/bin/bash
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if [ $# -lt 2 ]; then
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echo "Usage: $0 lines filename"
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exit 1
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else
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numlines=$1
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filename=$2
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fi
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if [[ $numlines != [0-9]* ]]; then
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echo "Error: $numlines is not numeric"
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exit 2
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fi
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if [ ! -f $ filename]; then
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echo "Error: File $filename not found"
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exit 3
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else
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echo top of file
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head -$numlines $filename
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fi
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```
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Of course, this example script does nothing more than run the head command to show the top X lines in a file, but it is meant to show how internal parameters can be used within scripts to help ensure the script runs well or fails with at least some clarity.
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**[ Watch Sandra Henry-Stocker's Two-Minute Linux Tips[to learn how to master a host of Linux commands][3] ]**
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Join the Network World communities on [Facebook][4] and [LinkedIn][5] to comment on topics that are top of mind.
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--------------------------------------------------------------------------------
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via: https://www.networkworld.com/article/3387154/working-with-variables-on-linux.html#tk.rss_all
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作者：[Sandra Henry-Stocker][a]
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选题：[lujun9972][b]
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译者：[译者ID](https://github.com/译者ID)
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||||
校对：[校对者ID](https://github.com/校对者ID)
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本文由 [LCTT](https://github.com/LCTT/TranslateProject) 原创编译，[Linux中国](https://linux.cn/) 荣誉推出
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[a]: https://www.networkworld.com/author/Sandra-Henry_Stocker/
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[b]: https://github.com/lujun9972
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[1]: https://images.idgesg.net/images/article/2019/04/variable-key-keyboard-100793080-large.jpg
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[2]: https://www.networkworld.com/article/3385516/how-to-manage-your-linux-environment.html
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||||
[3]: https://www.youtube.com/playlist?list=PL7D2RMSmRO9J8OTpjFECi8DJiTQdd4hua
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[4]: https://www.facebook.com/NetworkWorld/
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||||
[5]: https://www.linkedin.com/company/network-world
|
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262
translated/tech/20190409 Working with variables on Linux.md
Normal file
262
translated/tech/20190409 Working with variables on Linux.md
Normal file
@ -0,0 +1,262 @@
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[#]: collector: (lujun9972)
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[#]: translator: (MjSeven)
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[#]: reviewer: ( )
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[#]: publisher: ( )
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[#]: url: ( )
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[#]: subject: (Working with variables on Linux)
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[#]: via: (https://www.networkworld.com/article/3387154/working-with-variables-on-linux.html#tk.rss_all)
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[#]: author: (Sandra Henry-Stocker https://www.networkworld.com/author/Sandra-Henry_Stocker/)
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在 Linux 中使用变量
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======
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变量通常看起来像 $var，但它们也有 $1、$*、$? 和 $$ 这种形式。让我们来看看所有这些 $ 值可以告诉你什么。
|
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![Mike Lawrence \(CC BY 2.0\)][1]
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|
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我们称为“变量”的许多重要的值都存储在 Linux 系统中，但实际上有几种类型的变量和一些有趣的命令可以帮助你使用它们。在上一篇文章中，我们研究了[环境变量][2]以及它们在哪定义。在本文中，我们来看一看在命令行和脚本中使用的变量。
|
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|
||||
### 用户变量
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|
||||
虽然在命令行中设置变量非常容易，但是有一些有趣的技巧。要设置变量，你只需这样做：
|
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|
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```
|
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$ myvar=11
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$ myvar2="eleven"
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```
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||||
|
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要显示这些值，只需这样做：
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|
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```
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$ echo $myvar
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11
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$ echo $myvar2
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eleven
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```
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你也可以使用这些变量。例如，要递增一个数字变量，使用以下任意一个命令：
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```
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$ myvar=$((myvar+1))
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$ echo $myvar
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12
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$ ((myvar=myvar+1))
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$ echo $myvar
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13
|
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$ ((myvar+=1))
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$ echo $myvar
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14
|
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$ ((myvar++))
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$ echo $myvar
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15
|
||||
$ let "myvar=myvar+1"
|
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$ echo $myvar
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16
|
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$ let "myvar+=1"
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$ echo $myvar
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17
|
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$ let "myvar++"
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$ echo $myvar
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18
|
||||
```
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|
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使用其中的一些，你可以增加一个变量的值。例如：
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|
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```
|
||||
$ myvar0=0
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$ ((myvar0++))
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$ echo $myvar0
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1
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||||
$ ((myvar0+=10))
|
||||
$ echo $myvar0
|
||||
11
|
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```
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||||
|
||||
通过这些选项，你可能会发现至少有一个是容易记忆且使用方便的。
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||||
|
||||
你也可以 _删除_ 一个变量 -- 这意味着没有定义它。
|
||||
|
||||
```
|
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$ unset myvar
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$ echo $myvar
|
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```
|
||||
|
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另一个有趣的选项是，你可以设置一个变量并将其设为**只读**。换句话说，变量一旦设置为只读，它的值就不能改变（除非一些非常复杂的命令行魔法才可以）。这意味着你也不能删除它。
|
||||
|
||||
```
|
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$ readonly myvar3=1
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$ echo $myvar3
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1
|
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$ ((myvar3++))
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-bash: myvar3: readonly variable
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||||
$ unset myvar3
|
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-bash: unset: myvar3: cannot unset: readonly variable
|
||||
```
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||||
|
||||
你可以使用这些设置和递增选项中的任何一个来赋值和操作脚本中的变量，但也有一些非常有用的 _内部变量_ 用于在脚本中工作。注意，你无法重新赋值或增加它们的值。
|
||||
|
||||
### 内部变量
|
||||
|
||||
在脚本中可以使用很多变量来计算参数并显示有关脚本本身的信息。
|
||||
|
||||
* $1、$2、$3 等表示脚本的第一个、第二个、第三个等参数。
|
||||
* $# 表示参数的数量。
|
||||
* $* 表示所有参数。
|
||||
* $0 表示脚本的名称。
|
||||
* $? 表示先前运行的命令的返回码（0 代表成功）。
|
||||
* $$ 显示脚本的进程 ID。
|
||||
* $PPID 显示 shell 的进程 ID（脚本的父进程）。
|
||||
|
||||
其中一些变量也适用于命令行，但显示相关信息：
|
||||
|
||||
* $0 显示你正在使用的 shell 的名称（例如，-bash）。
|
||||
* $$ 显示 shell 的进程 ID。
|
||||
* $PPID 显示 shell 的父进程的进程 ID（对我来说，是 sshd）。
|
||||
|
||||
为了查看它们的结果，如果我们将所有这些变量都放入一个脚本中，比如：
|
||||
|
||||
```
|
||||
#!/bin/bash
|
||||
|
||||
echo $0
|
||||
echo $1
|
||||
echo $2
|
||||
echo $#
|
||||
echo $*
|
||||
echo $?
|
||||
echo $$
|
||||
echo $PPID
|
||||
```
|
||||
|
||||
当我们调用这个脚本时，我们会看到如下内容：
|
||||
```
|
||||
$ tryme one two three
|
||||
/home/shs/bin/tryme <== 脚本名称
|
||||
one <== 第一个参数
|
||||
two <== 第二个参数
|
||||
3 <== 参数的个数
|
||||
one two three <== 所有的参数
|
||||
0 <== 上一条 echo 命令的返回码
|
||||
10410 <== 脚本的进程 ID
|
||||
10109 <== 父进程 ID
|
||||
```
|
||||
|
||||
如果我们在脚本运行完毕后检查 shell 的进程 ID，我们可以看到它与脚本中显示的 PPID 相匹配：
|
||||
|
||||
```
|
||||
$ echo $$
|
||||
10109 <== shell 的进程 ID
|
||||
```
|
||||
|
||||
当然，比起简单地显示它们的值，我们更多的是在需要它们的时候来使用它们。我们来看一看它们可能的用处。
|
||||
|
||||
检查是否已提供参数：
|
||||
|
||||
```
|
||||
if [ $# == 0 ]; then
|
||||
echo "$0 filename"
|
||||
exit 1
|
||||
fi
|
||||
```
|
||||
|
||||
检查特定进程是否正在运行：
|
||||
|
||||
```
|
||||
ps -ef | grep apache2 > /dev/null
|
||||
if [ $? != 0 ]; then
|
||||
echo Apache is not running
|
||||
exit
|
||||
fi
|
||||
```
|
||||
|
||||
在尝试访问文件之前验证文件是否存在：
|
||||
|
||||
```
|
||||
if [ $# -lt 2 ]; then
|
||||
echo "Usage: $0 lines filename"
|
||||
exit 1
|
||||
fi
|
||||
|
||||
if [ ! -f $2 ]; then
|
||||
echo "Error: File $2 not found"
|
||||
exit 2
|
||||
else
|
||||
head -$1 $2
|
||||
fi
|
||||
```
|
||||
|
||||
在下面的小脚本中，我们检查是否提供了正确数量的参数、第一个参数是否为数字，以及第二个参数代表的文件是否存在。
|
||||
|
||||
```
|
||||
#!/bin/bash
|
||||
|
||||
if [ $# -lt 2 ]; then
|
||||
echo "Usage: $0 lines filename"
|
||||
exit 1
|
||||
fi
|
||||
|
||||
if [[ $1 != [0-9]* ]]; then
|
||||
echo "Error: $1 is not numeric"
|
||||
exit 2
|
||||
fi
|
||||
|
||||
if [ ! -f $2 ]; then
|
||||
echo "Error: File $2 not found"
|
||||
exit 3
|
||||
else
|
||||
echo top of file
|
||||
head -$1 $2
|
||||
fi
|
||||
```
|
||||
|
||||
### 重命名变量
|
||||
|
||||
在编写复杂的脚本时，为脚本的参数指定名称通常很有用，而不是继续将它们称为 $1, $2 等。等到第 35 行，阅读你脚本的人可能已经忘了 $2 表示什么。如果你将一个重要参数的值赋给 $filename 或 $numlines，那么他就不容易忘记。
|
||||
|
||||
```
|
||||
#!/bin/bash
|
||||
|
||||
if [ $# -lt 2 ]; then
|
||||
echo "Usage: $0 lines filename"
|
||||
exit 1
|
||||
else
|
||||
numlines=$1
|
||||
filename=$2
|
||||
fi
|
||||
|
||||
if [[ $numlines != [0-9]* ]]; then
|
||||
echo "Error: $numlines is not numeric"
|
||||
exit 2
|
||||
fi
|
||||
|
||||
if [ ! -f $ filename]; then
|
||||
echo "Error: File $filename not found"
|
||||
exit 3
|
||||
else
|
||||
echo top of file
|
||||
head -$numlines $filename
|
||||
fi
|
||||
```
|
||||
|
||||
当然，这个示例脚本只是运行 head 命令来显示文件中的前 x 行，但它的目的是显示如何在脚本中使用内部参数来帮助确保脚本运行良好，或在失败时清晰地知道失败原因。
|
||||
|
||||
**观看 Sandra Henry-Stocker 的两分钟 Linux 技巧：[学习如何掌握大量 Linux 命令][3]。**
|
||||
|
||||
加入 [Facebook][4] 和 [Linkedln][5] 上的网络社区，评论最热的主题。
|
||||
|
||||
--------------------------------------------------------------------------------
|
||||
|
||||
via: https://www.networkworld.com/article/3387154/working-with-variables-on-linux.html#tk.rss_all
|
||||
|
||||
作者：[Sandra Henry-Stocker][a]
|
||||
选题：[lujun9972][b]
|
||||
译者：[MjSeven](https://github.com/MjSeven)
|
||||
校对：[校对者ID](https://github.com/校对者ID)
|
||||
|
||||
本文由 [LCTT](https://github.com/LCTT/TranslateProject) 原创编译，[Linux中国](https://linux.cn/) 荣誉推出
|
||||
|
||||
[a]: https://www.networkworld.com/author/Sandra-Henry_Stocker/
|
||||
[b]: https://github.com/lujun9972
|
||||
[1]: https://images.idgesg.net/images/article/2019/04/variable-key-keyboard-100793080-large.jpg
|
||||
[2]: https://www.networkworld.com/article/3385516/how-to-manage-your-linux-environment.html
|
||||
[3]: https://www.youtube.com/playlist?list=PL7D2RMSmRO9J8OTpjFECi8DJiTQdd4hua
|
||||
[4]: https://www.facebook.com/NetworkWorld/
|
||||
[5]: https://www.linkedin.com/company/network-world
|
||||
Loading…
Reference in New Issue
Block a user