101 lines
4.3 KiB
Markdown
101 lines
4.3 KiB
Markdown
---
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title: Writing a Bash Script
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---
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# Writing a Bash Script
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By typing commands on the Linux command line, you can give the server instructions to get some simple tasks done. A shell
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script is a way to put together a series of instructions to make this easier. Shell scripts become even more powerful when
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you add logic like `if` and `while` to automatically control how they behave as circumstances change.
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## What’s Bash?
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Bash is the name of a command line interpreter, a program that makes sense of the Linux commands you enter at the command
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prompt, or in your script. There are other interpreters as well, such us TC-Shell, Z Shell, Fish-Shell, and many more.
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## What’s in a Script?
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A script is just a file. A basic script is made up of an introductory line, called a "shebang", that tells the server what to make of it, and one
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or more instructions to execute. Here’s an example:
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```
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#!/bin/bash
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echo "Hi. I’m your new favorite bash script."
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```
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The first line has special meaning, which we'll discuss below. The second line is just a Linux command, one you could type
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out on the command line.
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## What’s a Comment?
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Comments are text you add to your script that you intend bash to ignore. Comments start with a pound sign, and are useful for
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annotating your code so you and other users can understand it. To add a comment, type the `#` character, followed by any text
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that's helpful you. Bash will ignore the `#` and everything after it.
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**Note:** the first line of the script is not a comment. This line is always first, always starts with `#!` and has special
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meaning to bash.
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Here’s the script from before, commented:
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```
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#!/bin/bash # Designates the path to the bash program. Must start with '#!' (but isn't a comment).
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echo "Hi. I’m your new favorite bash script." # 'echo' is a program that sends a string to the screen.
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```
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## Executing a Script
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You can open a text editor, paste that example code and save the file, and you’ve got a script. Scripts are conventionally
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named ending in '.sh,' so you might save that code as myscript.sh.
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The script won’t execute until we do 2 things:
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**First, make it executable.** (We’ll only have to do this once.)
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Linux relies extensively on file permissions. They determine a lot about how your server behaves. There's a lot to know about
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permissions, but for now we only need to know this: you can't run your script until you give yourself execute permissions. To
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do that, type:
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`chmod +x myscript.sh`
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**Second, run it.** We execute the script from the command line just like any other command like `ls` or `date`. The script
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name is the command, and you need to precede it with a './' when you call it:
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`./myscript.sh # Outputs "Hi. I'm your new favorite bash script." (This part is a comment!)`
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## Conditionals
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Sometimes you want your script to do something only if something else is true. For example, print a message only if a value is
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below a certain limit. Here's an example of using `if` to do that:
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```
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#!/bin/bash
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count=1 # Create a variable named count and set it to 1
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if [[ $count -lt 11 ]]; then # This is an if block (or conditional). Test to see if $count is 10 or less. If it is, execute the instructions inside the block.
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echo "$count is 10 or less" # This will print, because count = 1.
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fi # Every if ends with fi
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```
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Similarly, we can arrange the script so it executes an instruction only while something is true. We'll change the code so that
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the value of the count variable changes:
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```
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#!/bin/bash
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count=1 # Create a variable named count and set it to 1
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while [[ $count -lt 11 ]]; do # This is an if block (or conditional). Test to see if $count is 10 or less. If it is, execute the instructions inside the block.
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echo "$count is 10 or less" # This will print as long as count <= 10.
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count=$((count+1)) # Increment count
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done # Every while ends with done
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```
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The output of this version of myscript.sh will look like this:
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```
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"1 is 10 or less"
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"2 is 10 or less"
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"3 is 10 or less"
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"4 is 10 or less"
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"5 is 10 or less"
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"6 is 10 or less"
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"7 is 10 or less"
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"8 is 10 or less"
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"9 is 10 or less"
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"10 is 10 or less"
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```
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## Real World Scripts
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These examples aren't terribly useful, but the principles are. Using `while`, `if`, and any command you might otherwise type
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manually, you can create scripts that do valuable work.
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