Bash Scripting Examples

Bash scripting

This is a problem oriented bash scripting post. For understanding what's going on and how the code works, it's of great help to discover and type in these code by yourself.

Though the .sh file runs in Linux for most of the time, you can try all of these using git bash or WSL if you are using windows.

Remember

Do not use bash for OOP
Bash variables are UNTYPED and does not support float point calculation itself
Variable declaration is not necessary

A+B Problem

Include shebang on top of your script file (For other problems, this is same)

#!/bin/bash

read from stdin (or you can reference the parameter variables as $1 $2)

read -p "A and B: " a b

calculation or assignment, either of these works

# Using let
let "c=a+b" # let "c = a+b" is right, space does not matter as long as it is wrapped by quotes
let c=a+b # let c = a+b is wrong, DO NOT add spaces around =

# Using $(()) POSIX arithmethic expression

a=$((a+b))

# Just an example of using +=
let c=0
((c+=a+b)) # this is often used as ((a++)) in loops

# Using expr or other preset bash scripts
# DO ADD spaces between operators
c=$(expr a + b)

output

echo $c

Sum problem

read from stdin

read -p "Sum from 1 to n" n

init variable

let a=1
let sum=0

loop

# DO include $ before variables with in conditional statements
# DO ADD spaces in conditional statements like [SPACE expression SPACE]
# DO ADD ; before do, or you can use
#     while...
#     do
#     ...
#     done
while [ $a -le $n ];do

# (()) is for arithmetic expression
# DO NOT use $ for variable reference in (())
sum=$((sum+a))

a=$((a+1)) # see the alternatives below

# ALT ((a++))
# ALT let a="a+1"
# ALT let a=$a+1

done

output

echo $sum

Read text file

Assuming the text file is like (yeah, that's part of services file)

echo                7/tcp
echo                7/udp
discard             9/tcp    sink null
discard             9/udp    sink null
systat             11/tcp    users                  #Active users
systat             11/udp    users                  #Active users

The goal is to get only the <PORT>/<PROTOCOL> part

input="myfile"

while read -r line
do
	echo $line | grep -E -o "[0-9]+/(tcp|udp)"
done < "$input"

Of course a single command could have it done, but you can have more control if you use a while loop to read (for example output only the lines with odd line number)

grep -E -o "[0-9]+/(tcp|udp)" myfile

Refs

https://ryanstutorials.net/bash-scripting-tutorial/

https://unix.stackexchange.com/questions/40786/how-to-do-integer-float-calculations-in-bash-or-other-languages-frameworks