Merge pull request #29590 from lkxed/20230619-1-Bash-Basics-Series-2-Using-Variables-in-Bash

[手动选题][tech]: 20230619.1 ⭐️⭐️ Bash Basics Series 2 Using Variables in Bash.md

sources/tech/20230619.1 ⭐️⭐️ Bash Basics Series 2 Using Variables in Bash.md

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+[#]: subject: "Bash Basics Series #2: Using Variables in Bash"
+[#]: via: "https://itsfoss.com/bash-use-variables/"
+[#]: author: "Abhishek Prakash https://itsfoss.com/author/abhishek/"
+[#]: collector: "lkxed"
+[#]: translator: " "
+[#]: reviewer: " "
+[#]: publisher: " "
+[#]: url: " "
+
+Bash Basics Series #2: Using Variables in Bash
+======
+
+In the first part of the Bash Basics Series, I briefly mentioned variables. It is time to take a detailed look at them in this chapter.
+
+If you have ever done any kind of coding, you must be familiar with the term 'variable'.
+
+If not, think of a variable as a box that holds up information, and this information can be changed over time.
+
+Let's see about using them.
+
+### Using variables in Bash shell
+
+Open a terminal and use initialize a variable with a random number 4:
+
+```
+var=4
+```
+
+So now you have a variable named `var` and its value is `4`. Want to verify it? **Access the value of a variable by adding $ before the variable name**. It's called parameter expansion.
+
+```
+[email protected]:~$ echo The value of var is $var
+The value of var is 4
+```
+
+> 🚧 There must NOT be a space before or after`=`during variable initialization.
+
+If you want, you can change the value to something else:
+
+![Using variables in shell][1]
+
+In Bash shell, a variable can be a number, character, or string (of characters including spaces).
+
+![Different variable types in Bash shell][2]
+
+> 💡 Like other things in Linux, the variable names are also case-sensitive. They can consist of letters, numbers and the underscore "_".
+
+### Using variables in Bash scripts
+
+Did you notice that I didn't run a shell script to show the variable examples? You can do a lot of things in the shell directly. When you close the terminal, those variables you created will no longer exist.
+
+However, your distro usually adds global variables so that they can be accessed across all of your scripts and shells.
+
+Let's write some scripts again. You should have the script directory created earlier but this command will take care of that in either case:
+
+```
+mkdir -p bash_scripts && cd bash_scripts
+```
+
+Basically, it will create `bash_scripts` directory if it doesn't exist already and then switch to that directory.
+
+Here. let's create a new script named `knock.sh` with the following text.
+
+```
+#!/bin/bash
+
+echo knock, knock
+echo "Who's there?"
+echo "It's me, $USER"
+```
+
+Change the file permission and run the script. You learned it in the previous chapter.
+
+Here's what it produced for me:
+
+![Using global variable in Bahs script][3]
+
+**Did you notice how it added my name to it automatically?** That's the magic of the global variable $USER that contains the username.
+
+You may also notice that I used the " sometimes with echo but not other times. That was deliberate. [Quotes in bash][4] have special meanings. They can be used to handle white spaces and other special characters. Let me show an example.
+
+### Handling spaces in variables
+
+Let's say you have to use a variable called `greetings` that has the value `hello and welcome`.
+
+If you try initializing the variable like this:
+
+```
+greetings=Hello and Welcome
+```
+
+You'll get an error like this:
+
+```
+Command 'and' not found, but can be installed with:
+sudo apt install and
+```
+
+This is why you need to use either single quotes or double quotes:
+
+```
+greetings="Hello and Welcome"
+```
+
+And now you can use this variable as you want.
+
+![Using spaces in variable names in bash][5]
+
+### Assign the command output to a variable
+
+Yes! You can store the output of a command in a variable and use them in your script. It's called command substitution.
+
+```
+var=$(command)
+```
+
+Here's an example:
+
+```
+[email protected]:~$ today=$(date +%D)
+[email protected]:~$ echo "Today's date is $today"
+Today's date is 06/19/23
+[email protected]:~$
+```
+
+![Command substitution in bash][6]
+
+The older syntax used backticks instead of $() for the command substitution. While it may still work, you should use the new, recommended notation.
+
+> 💡 Variables change the value unless you declare a 'constant' variable like this:`readonly pi=3.14`. In this case, the value of variable`pi`cannot be changed because it was declared`readlonly`.
+
+### 🏋️ Exercise time
+
+Time to practice what you learned. Here are some exercise to test your learning.
+
+**Exercise 1**: Write a bash script that prints your username, present working directory, home directory and default shell in the following format.
+
+```
+Hello, there
+My name is XYZ
+My current location is XYZ
+My home directory is XYZ
+My default shell is XYZ
+```
+
+**Hint**: Use global variables $USER, $PWD, $HOME and $SHELL.
+
+**Exercise 2:** Write a bash script that declares a variable named `price`. Use it to get the output in the following format:
+
+```
+Today's price is $X
+Tomorrow's price is $Y
+```
+
+Where X is the initial value of the variable `price` and it is doubled for tomorrow's prices.
+
+**Hint**: Use / to escape the special character $.
+
+The answers to the exercises can be discussed in this dedicated thread in the community.
+
+In the next chapter of the Bash Basics Series, you'll see how to make the bash scripts interactive by passing arguments and accepting user inputs.
+
+--------------------------------------------------------------------------------
+
+via: https://itsfoss.com/bash-use-variables/
+
+作者：[Abhishek Prakash][a]
+选题：[lkxed][b]
+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]: https://itsfoss.com/author/abhishek/
+[b]: https://github.com/lkxed/
+[1]: https://itsfoss.com/content/images/2023/06/Using-variables-in-shell.png
+[2]: https://itsfoss.com/content/images/2023/06/bash-variables-types.png
+[3]: https://itsfoss.com/content/images/2023/06/using-global-variable-bash-script.png
+[4]: https://linuxhandbook.com:443/quotes-in-bash/
+[5]: https://itsfoss.com/content/images/2023/06/using-spaces-in-bash-variable.png
+[6]: https://itsfoss.com/content/images/2023/06/command-substitue-bash-variable.png