146 lines
8.5 KiB
Markdown
146 lines
8.5 KiB
Markdown
## Marking a script as executable
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Sometimes, you may need to mark a script as an executable to run it. On Linux, this can be accomplished by `chmod +x [FILE]`, where `[FILE]` is the file you want to mark as executable. To unmark a file as executable, use `chmod -x`
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## The Shebang
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There are various shells with their own language syntax (`sh`, `bash`, `fish`, etc). Therefore, more complicated scripts will indicate a particular shell by specifying the absolute path to the interpreter as the first line, prefixed by `#!` like this:
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```sh
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#!/bin/sh
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echo "Hello, World!"
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```
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`#!/bin/sh` means that this script can be executed by the binary located at `/bin/sh`, and so the reader knows this is a `sh`(ell) script.
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## Variables
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In bash, variable assignment is done with the `=` operator. Variables are conventionally named with `SCREAMING_SNAKE_CASE`, and can be accessed by prefixing the variable name with a dollar sign (`$`):
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```bash
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#!/bin/bash
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# There may not be any spaces used during assignment, `A = B` means something different than `A=B`
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# Assignment is done with the equal sign (=) operator:
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MY_VARIABLE="Hi Mom!"
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echo $MY_VARIABLE
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```
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## Quotes
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In Bash, different styles of quotes (or a backtick) mean different things:
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| Quote | Description |
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| ----------------- | -------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------- |
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| Single Quotes (') | Enclosing characters in single quotes (`'`) preserves the literal value of each character within the quotes. A single quote may not occur between single quotes, even when preceded by a backslash. Commands in single quotes will not be evaluated. |
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| Double Quotes (") | Enclosing characters in double quotes (`"`) preserves the literal value of all characters within the quotes, with the exception of `$`, \`, `\`, and, when history expansion is enabled, `!`. The characters `$` and \‘ retain their special meaning within double quotes. A double quote may be quoted within double quotes by preceding it with a backslash. When the shell is in POSIX mode (see Bash POSIX Mode), the ‘!’ has no special meaning within double quotes, even when history expansion is enabled.<br><br>Within double quotes, substitution can be done using `$()`.<br><br>```<br>"Output of ls: $(ls)"<br>``` |
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| Backtick (\`) | While a backtick is not technically a quotation mark, it's included here. Backticks are used to substitute the output a command in a location:<br>```<br>sudo chown `id -u` /some/directory<br>``` |
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## Conditionals
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A basic if statement in bash looks like this:
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```bash
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if somecommand; then
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# The code here will be run if somecommand has an exit code of 0
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fi
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```
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Note that the if statement is terminated by `fi`. This is fairly standard throughout bash scripting, where the blocks are closed with the reverse text used to open them.
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You can also make use of `else` or `elif` for more complex conditional logic:
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```bash
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if somecommand; then
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# If the command succeeds, run this code
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else
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# If the command fails, run this code.
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fi
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```
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`elif`:
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```bash
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if [ "$1" = "hello" ]; then
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echo "hello yourself"
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elif [ "$1" = "goodbye" ]; then
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echo "nice to have met you"
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echo "I hope to see you again"
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else
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echo "I didn't understand that"
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fi
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```
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To test one value against a variety of conditions, you can use `case`:
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```bash
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case "$1" in
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hello|hi)
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echo "hello yourself"
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;;
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goodbye)
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echo "nice to have met you"
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echo "I hope to see you again"
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;;
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*)
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echo "I didn't understand that"
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esac
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```
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If using `test` or the shorthand of test (`[]`), there are a variety of binary comparison operators you can make use of:
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| Operator | Description |
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| -------- | --------------------------- |
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| `-eq` | Is equal to |
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| `-ne` | Is not equal to |
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| `-gt` | Is greater than |
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| `-ge` | Is greater than or equal to |
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| `-lt` | Is less than |
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| `-le` | Is less than or equal to |
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The above list is far from complete but it contains some of the more commonly used operators
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## Loops
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### For loops
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`for` loops are used when you have a finite collection over which you want to iterate, such as a list of files, or a list of server names:
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```bash
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THINGS="thingone thingtwo thingthree"
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for THING in $THINGS; do
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echo "Doing something to $THING"
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done
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```
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The above example iterates over a space separated list of items, but if you wanted to iterate over a range of numbers, you could do something like this:
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```bash
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for i in {1..10} ; do ... ; done
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```
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### While loops
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`while` loops operate on lists of unknown size. It will keep running until the `test` it evaluates returns false:
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```bash
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i=0
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while [ $i -lt 10 ]; do
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echo $i
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i=$(( $i + 1))
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done
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echo “Done counting”
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```
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## Functions
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While there are a few ways to declare a function in bash, the most common syntax looks like this:
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```bash
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function_name () {
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echo "I was called by a function"
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}
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```
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To execute a function, simply include the name of the function. To execute the above function:
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```bash
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function_name
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```
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By default, all variables in bash are global. To declare a local variable in a function, prefix the declaration with the `local` keyword.
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```bash
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function_name () {
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local A='A'
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echo `A`
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}
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```
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To return a value from a function, use the `return` keyword. After calling, it'll be assigned to the global variable `$?`.
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```bash
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my_function () {
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return 55
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}
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my_function
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# Will echo 55
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echo $?
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```
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## Commands
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| Command | Description |
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| ------------------ | --------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------- |
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| `exit [CODE]` | Make the script process exit, where `[CODE]` is the exit code the process will terminate with. |
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| `read [VARIABLE]` | Read user input into the provided variable. |
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| `test [CONDITION]` | Used for conditionals, `test` will return an exit code of 0 if the test evaluates to true. Refer to the [manual](https://man7.org/linux/man-pages/man1/test.1.html) for more info. As a shorthand, you can enclose the condition in square brackets: `[]` |
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