# Bash scripting cheatsheet

* by Lee2sman
* Updated 2017-09-18

# Bash shebang

* ```#!/usr/bin/env bash``` for portability, or less good, ```#!/bin/bash```

# Commenting

```# this symbol makes everything after it on a line a comment```

# Variables

* No data types. Number, char, string, ok!
* No need to declare. Just assign a value to create it.
* Get the value of a variable (whatever is stored in it) by calling it with ```$``` in front.
* declare a local variable in a function by saying ```local``` before creating it.

### Builtin shell variables

```
$0             #Name of this shell script itself.
$1             #Value of first command line parameter (similarly $2, $3, etc)
$#             #In a shell script, the number of command line parameters.
$*             #All of the command line parameters.
$-             #Options given to the shell.
$?             #Return the exit status of the last command.
$$             #Process id of script (really id of the shell running the script)
```

# File redirection and Piping

* Three default files: 0. standard input (stdin), 1. standard output (stdout) and 2. standard error (stderr)
* ```> filename``` redirects stdout to a file (and creates the file if it didn't already exist)
* ```>> filename``` redirects stdout to a file but it *appends* to the end of the file if it already exists
* ```|``` is the pipe. **Piping** is used to chain commands, scripts, files and programs together.
* Piping allows for robust mini programs to be built together like lego kits of separate parts
* Example ```cat *.txt | sort | uniq > result-file``` Sorts the output of all the .txt files and deletes duplicate lines, saving outcome to *result-file*

# Input

* ```read``` command e.g. ```read num``` asks for input and puts it in $num

# Arithmetic

* uses the ```expr``` command

```
result=`expr $1 + 2`
result2=`expr $2 + $1 / 2`
result=`expr $2 \* 5`               #note the \ on the * symbol
```

# Conditionals

* Usually surround comparisons and conditions in brackets not parens e.g. ```if [ $# -gt 1 ]```
* ```&&``` is logical *and*, ```||``` is logical *or*

## Comparisons

### Booleans

```
!     #not
-a    #and
-o    #or
```

### Numerical comparisons

```
-eq == equal to 
-ne ≠ (not equal to)
-gt > greater than
-ge >= greather than or equal to
-lt < less than
-le <= less than or equal to
< less than
<= less than or equal to ((within double parenthesis))
> greater than ((within double parenthesis))
>= greater than or equal to ((within double parenthesis))
= equal to, including for strings, e.g. ```if [ "$a" = "$b" ]```
```

### STRING COMPARISONS

```
== is equal to
!= is NOT equal to
< less than (ascii-betically)
> greater than (ascii-betically)
-z string is null
-n string is not null
```

### Example comparisons

```
if [ "$VAR1" = "$VAR2" ]; then
	echo "expression evaluated as true"
else
	echo "expression evaluated as false"
fi
```

```
case "$C" in
"1")
	do_this()
	;;
"2" | "3")
	do_what_you_are_supposed_to_do()
	;;
*)    #fallback default case
	do_nothing()
	;;
esac
```

# Looping

* Iterates over a string of values

```
### FOR loop
for i in 1 2 3 4 5    # can also be written for i in {1..5} or {start..end..increment}
do
	echo "Welcome $i times"
done
```

* ```select``` looping works just like ```for```
* with select, use p3 prompt. User options get a number prefix and waits for number response.

## Example - Basic text menu

```
#!/bin/bash
OPTIONS="Hello Quit"
select opt in $OPTIONS; do
		if [ "$opt" = "Quit" ]; then
		echo done
		exit
	elif [ "$opt" = "Hello" ]; then
		echo Hello World
	else
		clear
		echo bad option
	fi
done
```

### WHILE loop

```
while [ condition ]
do
	command
done
```

```
### UNTIL loop
until [ condition ] # executes until condition = true
do
	command
done
```

# Functions

* works same as other languages
* can take parameters

```
function e {
	echo $1
}

e Hello #will echo Hello when called
```

# Debugging

```#!/bin/bash -x``` Adding ```-x``` to the shebang produces output information

# Additional shell features

```
$var           #Value of shell variable var.
${var}abc      #Example: value of shell variable var with string abc appended.
#              #At start of line, indicates a comment.
var=value      #Assign the string value to shell variable var.
cmd1 && cmd2   #Run cmd1, then if cmd1 successful run cmd2, otherwise skip.
cmd1 || cmd2   #Run cmd1, then if cmd1 not successful run cmd2, otherwise skip.
cmd1; cmd2     #Do cmd1 and then cmd2.
cmd1 & cmd2    #Do cmd1, start cmd2 without waiting for cmd1 to finish.
(cmds)         #Run cmds (commands) in a subshell.
```

* Additional shell features from this [Bash cheatsheet](http://johnstowers.co.nz/pages/bash-cheat-sheet.html)