Bash Shell Scripting: Variables

July 21, 2020

Bash Path(Shabang or Hashbang)

#!/bin/bash also know as shabang or hashbang, is the neccessary command to run the bash script. It is basically the path to bash.

#!/bin/bash
echo "Hello World!"

# Output
# Hello World!

Sometimes when the number of commands are large enough, it becomes difficult to understand which command is doing what. So by adding -x after #!/bin/bash, the commands will be visible along with the result in the output

#!/bin/bash -x
echo "Hello World!"
# Output
# + echo 'Hello World!'
# Hello World!

Variables

num1=5 # give no space before and after of '='
num2=6
declare -r MIN=5 # declare constant variable
echo "$num1" # print variable
echo $num1**2 # shell takes everything as string, so it will output: 5**2
echo $((num1+num2)) # $(()) is used for mathematical calculation
echo $((num1**MIN))
echo $((MIN+num1))

# Output
# 5
# 5**2
# 11
# 25
# 10

Shorthand property

rand=5
let rand+=4
echo $rand

echo "rand++ = $((rand++))"
echo "++rand = $((++rand))"
echo "rand-- = $((rand--))"
echo "--rand = $((--rand))"

# Output
# 9
# rand++ = 9
# ++rand = 11
# rand-- = 11
# --rand = 9

# use python to add floats
num3=1.2
num4=4.3
num5=$(python -c "print($num3 + $num4)")
echo $num5

# Output
# 5.5

# cat prints a file or any string pass to it
cat<< END
This text prints many line
etc lorem ipsum END

# Output
# This text prints many line
# etc lorem ipsum