Shell Scripts

Create shell scripts with the .sh file extension.
Bash will run this script in a subprocess.
For example if you run a change directory command in a script, it will not change the directory in the terminal you are running the script from.

Hashbang #!

This says interpret this file with the following program... bash

#! /bin/bash

This also allows it to run without a file extension on it.
Use which to find what interpreter is being used for a file. (e.g. which node)
Then put this path at the top with the hashbang, then you can write a script with that interpreter.

Path

You can add to your path by appending to it in bachrc like so to the bottom of the file.

export PATH=~/<directory or file to add>:$PATH

Then run source ~/.bashrc to update the path.

Variables

You can make variables in shell scripts and it's convention to use all caps for them.

DESTINATION=~/bin
FILE_PREFIX=file

cd ${DESTINATION}
touch ${FILE_PREFIX}{1..10}.txt

Arguments

You can pass arguments to a shell script.
$0 is the name of the script. e.g.bash or zsh $1 is the first argument.
$2 is the second argument.
$# is the number of arguments.
$@ is all the arguments.
$* is all the arguments as a single string.

Inputs

You can use read to get input from the user.

read -p "enter a file prefix: " FILE_PREFIX

Conditionals

if

You can use if then else fi to create conditionals.

if [-z $DESTINATION ]; then
    echo "no path provided, defaulting to ~/temp"
    DESTINATION=temp
fi

Flags in an if statement are syntactic sugar, they are essentially using the built in test command.
Common if statement flags:

• -z checks if the variable is empty.
• -n checks if the variable is not empty.
• -e checks if the file exists.
• -d checks if the directory exists.
• -f checks if the file exists and is a regular file.
• -x checks if the file exists and is executable.
• -eq checks if two numbers are equal.
• -le checks if the first number is less than or equal to the second number.
• -ge checks if the first number is greater than or equal to the second number.
• -w checks if the file exists and is writable.

More complex conditionals

if [ $1 -gt 10 ]; then
    echo "greater than 10"
elif [ $1 -lt 10 ]; then
    echo "less than 10"
else
    echo "equal to 10"
fi

Switch cases:

case $1 in
    "smile")
        echo ":-)"
        ;;
    "sad")
        echo ":-("
        ;;
    "laugh")
        echo ":-D"
        ;;
    *)
        echo ":-|"
        ;;
esac

Loops and Arrays

Arrays

ARRAY=(one two three)
echo ${ARRAY[0]}
echo ${ARRAY[1]}
echo ${ARRAY[2]}
echo ${ARRAY[*]}

While

i=0
while [ $i -lt 10 ]; do
    echo $i
    i=$((i+1))
done

Until

i=0
until [ $i -ge 10 ]; do
    echo $i
    i=$((i+1))
done

For

for i in {1..10}; do
    echo $i
done

While Loop Random Number example:

#!/bin/bash

NUM_TO_GUESS=$(( $RANDOM % 10 + 1 ))
GUESSED_NUM=0

echo "guess a number between 1 and 10"

while [ $NUM_TO_GUESS -ne $GUESSED_NUM ]
do
  read -p "your guess: " GUESSED_NUM
done

echo "you got it!"

NOTE: In the above program we don't want a dollar sign for GUESSED_NUM in the loop because we don't want to evaluate it as a variable (0).
We want to compare it to the number to guess.

Random Takeaways

Random numbers in bash: Below generates a random number between 1 and 10.

NUMBER=$(( $RANDOM % 10 + 1 ))

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