#Writing Bash Shell Scripts
###Objectives
- Understand what bash scripts are
- Declare our first variable
- Write our first bash script
###What is a shell script?
A shell script is a little more than a list of command that are run in sequence. By convention, a shell script should begin with
#! /bin/bash
###Writing our first bash script
#! /bin/bash
# Declare a variable
cool='cool'
echo "Hello $USER. You are $cool"In bash, you can use the environmental variables set inside of your computer (See them using printenv
####Local variables
You can declare local variables by:
cool='cool'
Make sure there are no spaces in between!!
In this example, quotes are optional because there is one word. If there are spaces, then quotes are not optional
To call on it, use $:
echo "You are $cool"
Notice in bash, single quotes and double quotes matter. Variables can be called in double quotes but not single quotes
You can escape a variable using \
###Finishing touches on first shell script
#! /bin/bash
cool='cool'
echo "Hello $USER. I will list some files."
echo "Listing files in current directory"
ls
echo -n "You are $cool"
echo ' bro'
###Double quotes are powerful
LS="ls"
LS_FLAGS="-al"
$LS $LS_FLAGS $HOME # shows everything in home ls -al /home/stanley
###When combining variables with string
x='ABC'
echo "${x}abc"
###Conditionals
It follows in this syntax
if condition1
then
statement1
statement2
..........
elif condition2
then
statement3
statement4
........
elif condition3
then
statement5
statement6
........
fi
###Simple if statement
if [ $X -lt $Y ] # if X is less than
Y
then
echo "\$X=${X} is smaller than \$Y=${Y}"
fi###Simple if else statement
empty_string=""
if [ -n "$empty_string" ] # -n tests to see if the argument is non empty
then
echo "\$empty_string is nonempty"
else
echo "\$empty_string is empty"
fi###Check if a file exists
Next, let's create a .stanley file in the current directory
file="./.stanley"
if [ -e "$file" ]; then
echo "You have a .stanley file"
if [ -L "$file" ]; then # -L checks if its a symlink
echo "It's a symlink"
elif [ -f "$file" ]; then # file checks if its a regular file
echo "It's a regular file"
fi
else
echo ".stanley does not exist"
fi
###Common operators
###Kicks and Giggles
#! /bin/bash
curl https://www.google.com -o output.txtThis will curl all your outputs from google into a file
###Grabbing all the files
echo *
echo ${HOME}/*
###Declaring an array
ARRAY=(one two three four five)
ARRAY[5]=blue
echo ${ARRAY[*]}
#Delete a value
unset ARRAY[1]
echo ${ARRAY[*]}
###For loops
####Basic for loop
for X in red green blue
do
echo $X
done
###With an array
ARRAY=(one two three four)
#echo ${ARRAY[*]}
#ARRAY[5]=blue
#unset ARRAY[1]
#echo ${ARRAY[*]}
for X in ${ARRAY[*]}
do
echo $X
done
This raises a natural question: why doesn't bash allow the C like for loops
for (X=1,X<10; X++) As it happens, this is discouraged for a reason: bash is an interpreted language, and a rather slow one for that matter. For this reason, heavy iteration is discouraged.
###While loops
x=0
while [ $x -le 20 ]
do
echo $x
x=$((x+1))
done###Creating the Stanley Framework!
#! /bin/bash
if [ -z $1 ]; then
APP='STANLEY'
else
APP=$1
fi
# Create my app structure
mkdir $APP $APP/js $APP/css
# echo out results
echo "Created $APP"
echo "Created ${APP}/css"
echo "Created ${APP}/js"
# Constants
TITLE="${USER}'s badass framework"
RIGHT_NOW=$(date +"%x %r %Z")
# Call the functions
cat > $APP/index.html <<- EOM
<html>
<head>
<title>$TITLE</title>
<link href='css/style.css'></link>
</head>
<body>
<h1>$TITLE</h1>
<p>Created $RIGHT_NOW</p>
<script src='js/main.js'></script>
</body>
</html>
EOM
# Alert that index.html was created
echo "Created ${APP}/index.html"
cat > $APP/css/style.css <<- EOM
body{
padding: 0;
margin: 0;
}
EOM
echo "Created ${APP}/css/style.css"
cat > $APP/js/main.js <<- EOM
console.log('activate framework!');
EOM
echo "Created ${APP}/js/main.js"Finally, to use it anywhere
$ sudo mv stan /bin
###Congrats! You have written a useful bash script!