Bash Scripting Crash Course

Info

THESE NOTES TAKEN FROM THE YOUTUBE VIDEO “SHELL SCRIPTING CRASH COURSE - BEGINNER LEVEL” https://youtu.be/v-F3YLd6oMw

#!/bin/bash
 
# VARIABLES
`NAME="jack"`
 
# COMMAND
`echo "Hello $NAME"`
 
# USER INPUT
`read -p "Enter your name: " NAME`
 
`echo "your name is $NAME"`

SIMPLE IF STATEMENT

if [ "$NAME" == "sam" ] 
then 
	echo "your name is sam"
elif [ "$NAME" == "jack" ]
then
	echo "your name is jack"
else
	echo "your name is not sam"
fi

COMPARISON

NUM1=3
NUM2=5
if [ "$NUM1" -gt "$NUM2" ]
then
	echo "$NUM1 is greater than $NUM2"
else
	echo "$NUM1 is less than $NUM2"
fi

FILE CONDITIONS

FILE="test.txt"
if [ -e "$FILE" ]
then
echo "$FILE exists"
else
echo "$FILE does NOT exists"
fi

CASE STATEMENTS

read -p "Are you 21 or over? Y/N " ANS
case "$ANS" in
	[yY] | [yY][eE][sS])
		echo "you can have a beer :)"
	;;
        [nN] | [nN][oO])
		echo "Sorry, no drinking"
	;;
        *)
		echo "Say either yes/no [y/n]"
	;;
esac

SIMPLE FOR LOOP

NAMES="Brad Kevin Alice Mark"
for NAME in $NAMES
do
	echo "Hello $NAME"
done

FOR LOOP TO RENAME FILES

FILES=$(ls *.txt)
NEW="new"
for FILE in $FILES
do
	echo "Renaming $FILE to new-$FILE"
	mv $FILE $NEW-$FILE
done

WHILE LOOP - READ THROUGH A FILE LINE BY LINE

LINE=1
while read -r CURRENT_LINE
do
	echo "$LINE: $CURRENT_LINE"
	((LINE++))
done < "./new-1.txt"

FUNCTIONS

function sayHello() {
echo "Hello World"
}
 
sayHello

FUNCTIONS WITH PARAMS

function greet() {
	echo "Hello, I am $1 and I am $2"
}
 
greet "Brad" "36"

CREATE FOLDER AND WRITE TO A FILE

mkdir hello
touch "hello/world.txt"
echo "Hello World" >> "hello/world.txt"
echo "Created hello/world.txt"

!/bin/sh
 
#This is a comment it will just ignored by computer so wont be run or executed
 
echo "whats your name?"
read NAME
echo $NAME
 
read -p "Enter your name: " NAME
echo "your name is $NAME"
 
NAME=edward
readonly NAME
NAME=bella
echo $NAME
 
NAME=edward
unset NAME
echo $NAME

- $0 = the file name of the script
- $1.....9 = these value correspond with the arguments with which the scripts was invoke
- $# return the number of argument suplied to a script
- $* returns all the arguments that are double quoted
- $@ returns all the arguments that are individaully double qouted
- $? exit status of the last command that you have executed
- $$ will give you the process number of the current shell for the shell script it is also the process id under which it is executed

echo "File name: $0"
echo "First parameter: $1"
echo "second parameter: $2"
echo "quoted values: $@"
echo "quoted values: $*"
echo "No of parameters: $#"

for TOKEN in $*
do
	echo $TOKEN
done

Shell Loops

for loop

for var in 0 1 2 3 4 5 6 7 8 9
do
	echo $var
done

while loop

a=0
while [ $a -lt 10 ]
do
	echo $a
	a=`expr $a + 1`
done

until loop

a=0
until [ ! $a -lt 10 ]
do
	echo $a
	a=`expr $a + 1`
done

loop inside loop

a=0
while [ "$a" -lt 10 ]
do
	b="$a"
	while [ "$b" -ge 0 ]
	do
		echo -n "$b"
		b=`expr $b - 1`
	done
	echo
	a=`expr $a + 1`
done

infinite loop

a=10
until [ $a -gt 0]
do
	echo $a
	a=`expr $a + 1`
done

&&		executes if first succeded
||		executes if first one failed
;		execute afterwards just like another line even if first failed
&		executes another command simultaneously with first
-z 		means if its equal to nothing

#!/bin/sh
if [ "$EDITOR" = "" ]; then
	EDITOR=nano
echo $EDITOR

vs

[ -z "$EDITOR" ] && EDITOR=nano && echo $EDITOR || echo "EDITOR is $EDITOR"

exit codes

$? check if cmd successful or not 0 if command successful 1 if cmd unsuccessful

Functions

function unit_test () {
	echo "running unit test"
	return 0
}
 
unit_test