Bash Script

Scripts are super useful to automate tasks.

Right now, I just write the command in the .zshrc, but in the future, when I have more complex commands, I would like to write my own scripts.

./install/setup.bash vs . install/setup.bash?

When you execute a script (.bash file) , what it is actually doing is creating a new shell, runs the script as a separate process, and copies back the output. If you want to update your current environment variables, ./install/setup.bash won’t work, since it will apply it in the new terminal created.

When you do . install/setup.bash, it executes the script in the current shell, meaning any changes it makes to the environment (such as changing environment variables) will persist in your current shell session.

Documentation

• https://www.man7.org/linux/man-pages/man1/bash.1.html

When you make a new file, you always have to add #!/bin/bash

#!/bin/bash 
echo "Hello World" 
echo $(which neqn) 
cat $(which neqn)

Also, you need to do chmod +x <name_of_file> so that it can be executed with ./name_of_file. The .sh extension is optional

We put quotations ("") around when we are just doing comparison of strings. Without the quotation, you would be loading values in.

#! /bin/bash
# ECHO COMMAND
echo Hello World!
 
# VARIABLES
# Uppercase by convention
# Letters, numbers, underscores
NAME="Bob"
echo "My name is $NAME"
echo "My name is ${NAME}" # This is a bit more readable
 
# USER INPUT
read -p "Enter your name: " NAME
echo "Hello $NAME, nice to meet you!"
 
# IF STATEMENTS
if [ "$NAME" == "Brad" ]
then
  echo "Your name is Brad"
elif [ "$NAME" == "Jack" ]
then  
  echo "Your name is Jack"
else 
  echo "Your name is NOT Brad or Jack"
fi
 
# COMPARISON
NUM1=31
NUM2=5
if [ "$NUM1" -gt "$NUM2" ]
then
  echo "$NUM1 is greater than $NUM2"
else
  echo "$NUM1 is less than $NUM2"
fi
 
######## COMPARISONS
# val1 -eq val2 Returns true if the values are equal
# val1 -ne val2 Returns true if the values are not equal
# val1 -gt val2 Returns true if val1 is greater than val2
# val1 -ge val2 Returns true if val1 is greater than or equal to val2
# val1 -lt val2 Returns true if val1 is less than val2
# val1 -le val2 Returns true if val1 is less than or equal to val2
########
 
# FILE CONDITIONS
FILE="test.txt"
if [ -e "$FILE" ]
then
  echo "$FILE exists"
else
  echo "$FILE does NOT exist"
fi
 
######## CONDITIONS
# -d file   True if the file is a directory
# -e file   True if the file exists (note that this is not particularly portable, thus -f is generally used)
# -f file   True if the provided string is a file
# -g file   True if the group id is set on a file
# -r file   True if the file is readable
# -s file   True if the file has a non-zero size
# -u    True if the user id is set on a file
# -w    True if the file is writable
# -x    True if the file is an executable
########
 
#CASE STATEMENT
read -p "Are you 21 or over? Y/N " ANSWER
case "$ANSWER" in 
  [yY] | [yY][eE][sS])
    echo "You can have a beer :)"
    ;;
  [nN] | [nN][oO])
    echo "Sorry, no drinking"
    ;;
  *)
    echo "Please enter y/yes or n/no"
    ;;
esac # notice how this is case backwards
 
# 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 # TODO: I am not sure about how this reads the file
  do
    echo "$LINE: $CURRENT_LINE"
    ((LINE++))
done < "./new-1.txt"
 
# FUNCTION
function sayHello() {
  echo "Hello World"
}
sayHello
 
# FUNCTION WITH PARAMS
function greet() { # You don't pass in arguments into the function directly, see below
  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"