Learn how to automate common tasks using bash scripting.

Learn Bash Scripting

To make bash scripts more useful, we need to be able to access data external to the bash script file itself. The first way to do this is by prompting the user for input. For this, we use the `read` syntax. To ask the user for input and save it to the `number` variable, we would use the following code:
```
echo "Guess a number"
read number
echo "You guessed $number"
```
Another way to access external data is to have the user add input arguments when they run your script. These arguments are entered after the script name and are separated by spaces. For example:
```
saycolors red green blue
```
Within the script, these are accessed using `$1`, `$2`, etc, where `$1` is the first argument (here, "red") and so on. Note that these are 1 indexed.

If your script needs to accept an indefinite number of input arguments, you can iterate over them using the `"$@"` syntax. For our `saycolors` example, we could print each color using:
```
for color in "$@"
do
  echo $color
done
```
Lastly, we can access external files to our script. You can assign a set of files to a variable name using standard bash pattern matching using regular expressions. For example, to get all files in a directory, you can use the `*` character:
```
files=/some/directory/*
```
You can then iterate through each file and do something. Here, lets just print the full path and filename:
```
for file in $files
do
  echo $file
done
``` 


Inputs

There are 3 different ways to loop within a bash script: `for`, `while` and `until`.

A for loop is used to iterate through a list and execute an action at each step. For example, if we had a list of words stored in a variable `paragraph`, we could use the following syntax to print each one:
```
for word in $paragraph
do
  echo $word
done
```
Note that `word` is being "defined" at the top of the for loop so there is no `$` prepended. Remember that we prepend the `$` when accessing the value of the variable. So, when accessing the variable within the `do` block, we use `$word` as usual.

Within bash scripting `until` and `while` are very similar. `while` loops keep looping while the provided condition is true whereas `until` loops loop until the condition is true. Conditions are established the same way as they are within an `if` block, between square brackets. If we want to print the `index` variable as long as it is less than 5, we would use the following `while` loop:
```
while [ $index -lt 5 ]
do
  echo $index
  index=$((index + 1))
done
``` 
Note that arithmetic in bash scripting uses the `$((...))` syntax and within the brackets the variable name is not prepended with a `$`.

The same loop could also be written as an `until` loop as follows:
```
until [ $index -eq 5 ]
do
  echo $index
  index=$((index + 1))
done
``` 

Loops

Bash (or shell) scripting is a great way to automate repetitive tasks and can save you a ton of time as a developer. Bash scripts execute within a Bash shell interpreter terminal. Any command you can run in your terminal can be run in a Bash script. When you have a command or set of commands that you will be using frequently, consider writing a Bash script to perform it.

There are some conventions to follow to ensure that your computer is able to find and execute your Bash scripts. The beginning of your script file should start with `#!/bin/bash` on its own line. This tells the computer which type of interpreter to use for the script. When saving the script file, it is good practice to place commonly used scripts in the `~/bin/` directory. 

The script files also need to have the "execute" permission to allow them to be run. To add this permission to a file with filename: `script.sh` use:
```
chmod +x script.sh
```

Your terminal runs a file every time it is opened to load its configuration. On Linux style shells, this is `~/.bashrc` and on OSX, this is `~/.bash_profile`. To ensure that scripts in `~/bin/` are available, you must add this directory to your `PATH` within your configuration file:
- `PATH=~/bin:$PATH`

Now any scripts in the `~/bin` directory can be run from anywhere by typing the filename. 

Introduction to Bash Scripting

Take a minute to review what you've learned about bash scripting.
- Any command that can be run in the terminal can be run in a bash script.
- Variables are assigned using an equals sign with no space (`greeting="hello"`).
- Variables are accessed using a dollar sign (`echo $greeting`).
- Conditionals use `if`, `then`, `else`, `fi` syntax.
- Three types of loops can be used: `for`, `while`, and `until`.
- Bash scripts use a unique set of comparison operators:
 - Equal: `-eq`
 - Not equal: `-ne`
 - Less than or equal: `-le`
 - Less than: `-lt`
 - Greater than or equal: `-ge`
 - Greater than: `-gt`
 - Is null: `-z`
- Input arguments can be passed to a bash script after the script name, separated by spaces (myScript.sh "hello" "how are you").
- Input can be requested from the script user with the `read` keyword.
- Aliases can be created in the `.bashrc` or `.bash_profile` using the `alias` keyword.
 

Review

Within bash scripts (or the terminal for that matter), variables are declared by setting the variable name equal to another value. For example, to set the variable `greeting` to "Hello", you would use the following syntax:
```
greeting="Hello"
```
Note that there is no space between the variable name, the equals sign, or "Hello".

To access the value of a variable, we use the variable name prepended with a dollar sign (`$`). In the previous example, if we want to print the variable value to the screen, we use the following syntax:
```
echo $greeting
``` 


Variables

When bash scripting, you can use conditionals to control which set of commands within the script run. Use `if` to start the conditional, followed by the condition in square brackets (`[ ]`). Make sure you leave a space between a bracket and the conditional statement! `then` begins the code that will run if the condition is met. `else` begins the code that will run if the condition is not met. Lastly, the conditional is closed with a backwards `if`, `fi`. 

A complete conditional in a bash script uses the following syntax:
```
if [ $index -lt 5 ]
then
  echo $index
else
  echo 5
fi
```
Bash scripts use a specific list of operators for comparison. Here we used `-lt` which is "less than". The result of this conditional is that if `$index` is less than 5, it will print to the screen. If it is 5 or greater, "5" will be printed to the screen.

Here is the list of comparison operators for numbers you can use within bash scripts:
 - Equal: `-eq`
 - Not equal: `-ne`
 - Less than or equal: `-le`
 - Less than: `-lt`
 - Greater than or equal: `-ge`
 - Greater than: `-gt`
 - Is null: `-z`

When comparing strings, it is best practice to put the variable into quotes (`"`). This prevents errors if the variable is null or contains spaces. The common operators for comparing strings are:
 - Equal: `==`
 - Not equal: `!=`

For example, to compare if the variables `foo` and `bar` contain the same string:
```
if [ "$foo" == "$bar" ]
```
 

Conditionals

You can set up aliases for your bash scripts within your `.bashrc` or `.bash_profile` file to allow calling your scripts without the full filename. For example, if we have our `saycolors.sh` script, we can alias it to the word `saycolors` using the following syntax:
```
alias saycolors='./saycolors.sh'
```
You can even add standard input arguments to your alias. For example, if we always want "green" to be included as the first input to `saycolors`, we could modify our alias to:
```
alias saycolors='./saycolors.sh "green"'
```