• by Lee2sman
• Updated 2017-09-18

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

• 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.

$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)

• 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

-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" ]```

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

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

• 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 [ condition ] do command done

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


# Resources
* Other better(!) [Bash cheatsheet](http://johnstowers.co.nz/pages/bash-cheat-sheet.html)