20170120-4 选题

sources/tech/20170119 How To Assign Output of a Linux Command to a Variable.md

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+How To Assign Output of a Linux Command to a Variable
+============================================================
+
+When you run a command, it produces some kind of output: either the result of a program is suppose to produce or status/error messages of the program execution details. Sometimes, you may want to store the output of a command in a variable to be used in a later operation.
+
+In this post, we will review the different ways of assigning the output of a shell command to a variable, specifically useful for shell scripting purpose.
+
+To store the output of a command in a variable, you can use the shell command substitution feature in the forms below:
+
+```
+variable_name=$(command)
+variable_name=$(command [option ...] arg1 arg2 ...)
+OR
+variable_name='command'
+variable_name='command [option ...] arg1 arg2 ...'
+```
+
+Below are a few examples of using command substitution.
+
+In this first example, we will store the value of `who` (which shows who is logged on the system) command in the variable `CURRENT_USERS` user:
+
+```
+$ CURRENT_USERS=$(who)
+```
+
+Then we can use the variable in a sentence displayed using the [echo command][1] like so:
+
+```
+$ echo -e "The following users are logged on the system:\n\n $CURRENT_USERS"
+```
+
+In the command above: the flag `-e` means interpret any escape sequences ( such as `\n` for newline) used. To avoid wasting time as well as memory, simply perform the command substitution within the [echo command][2] as follows:
+
+```
+$ echo -e "The following users are logged on the system:\n\n $(who)"
+```
+[
+ ![Shows Current Logged Users in Linux](http://www.tecmint.com/wp-content/uploads/2017/01/Shows-Current-Logged-Users-in-Linux.png) 
+][3]
+
+Shows Current Logged Users in Linux
+
+Next, to demonstrate the concept using the second form; we can store the total number of files in the current working directory in a variable called `FILES` and echo it later as follows:
+
+```
+$ FILES=`sudo find . -type f -print | wc -l`
+$ echo "There are $FILES in the current working directory."
+```
+[
+ ![Show Number of Files in Directory](http://www.tecmint.com/wp-content/uploads/2017/01/Show-Number-of-Files-in-Directory.png) 
+][4]
+
+Show Number of Files in Directory
+
+That’s it for now, in this article, we explained the methods of assigning the output of a shell command to a variable. You can add your thoughts to this post via the feedback section below.
+
+--------------------------------------------------------------------------------
+
+作者简介：
+
+![](http://1.gravatar.com/avatar/4e444ab611c7b8c7bcb76e58d2e82ae0?s=128&d=blank&r=g)
+
+Aaron Kili is a Linux and F.O.S.S enthusiast, an upcoming Linux SysAdmin, web developer, and currently a content creator for TecMint who loves working with computers and strongly believes in sharing knowledge.
+
+--------------------------------------------------------------------------------
+
+via: http://www.tecmint.com/assign-linux-command-output-to-variable/
+
+作者：[Aaron Kili][a]
+译者：[译者ID](https://github.com/译者ID)
+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]:http://www.tecmint.com/author/aaronkili/
+[1]:http://www.tecmint.com/echo-command-in-linux/
+[2]:http://www.tecmint.com/echo-command-in-linux/
+[3]:http://www.tecmint.com/wp-content/uploads/2017/01/Shows-Current-Logged-Users-in-Linux.png
+[4]:http://www.tecmint.com/wp-content/uploads/2017/01/Show-Number-of-Files-in-Directory.png