Bash

GNU Bourne-Again SHell, an POSIX(file system standard) compliant shell thingy. sh is a earlier verison

The language with literally multiple ways to do everything xO

Alternatives include zsh shell which is default interactive shell after Catalina(you can still run bash scripts with bash with a shebang)

Defining a Bash Script

#!/bin/bash
echo Hello World, arg $1

Call with ./scriptname.sh test as that uses the shebang

Can just run in bash without permission with bash scriptname.sh or . scriptname.sh

Variables

var1='soap' #no spaces
var1=soap

echo $var1 		# => soap
echo ${var1}	# => soap
echo var1 		# => var1

Make var avaliable to all subprocesses

export name=value 

String Interpolation

Single quotes won't interpolate anything, but double quotes will.

"$var1".txt #basically $(echo $var1).txt

Print

printf has more standard behavior doing stuff like newlines

echo "Hello"
printf "Hello\n"

Evaluate

$( echo $1 ) 
echo $(echo hi) # => hi

^generally better(newer and ez nesting without esacpes)

echo `echo 1`  # => 1

Find Length

test=pets.com
echo ${#test} 	# => 8 (b/c its length 8)

Functions

We supply the arguments directly after the function name. Within the function they are accessible as $1, $2, etc.

print_something () { #create ft
    echo Hello $1 #access arguments
    return 5
}
#OR
function print_something() {
    echo Hello $1 #access arguments
    5
    return #empty return will return output of last command 
}

print_something Mars # call ft
print_something Jupiter
echo The previous function has a return value of $? #$? is last return value 

Export with export -f myfun so usable by subshells

Special Parameters

Symbol	Equivalent	Example
$@	"$1 " "$2"	`for INPUT in "$@"
$*	"$1c$2c..." where c is the first value of the IFS variable

Conditionals

man test for info about conditionals

FILE=/etc/resolv.conf
if [ -f "$FILE" ]; then
    echo "$FILE exists."
else 
    echo "$FILE does not exist."
fi

if [ ! -f "$FILE" ]; then
    echo "$FILE does not exist."
fi

man test for a shit ton more about [ ], p.s never forget the spaces

Controls

for i in $( ls ); do
	echo item: $i
done

while [  $COUNTER -lt 10 ]; do
    echo The counter is $COUNTER
    let COUNTER=COUNTER+1 
done

COUNTER=20
until [  $COUNTER -lt 10 ]; do
    echo COUNTER $COUNTER
    let COUNTER-=1
done

Exit

exit [n] will close shell with exit status of [n]