Bash scripting

An explained introduction of bash scripting is on:

Basics

Structure

In order to execute/run with shell enterpreter, the first line in the script should be:
#!/bin/bash
This line is called Shebang.

Comments

If you want to comment a line, you could use '#' sign, and for a block:

: << COMMENT
comment line 1
comment line 2
comment line 3
...
comment line n
COMMENT

Alias

This command allows us to create custom bash commands, where to use it in permanent way, we should write it on the shell profile file.
Example:
alias conn="echo 'Total connections on port 80 and 443:' ; netstat -plant | grep '80\|443' | grep -v LISTEN | wc -l" Where the output is:
Total connections on port 80 and 443: 12

Create variables

It is important you can not have spaces after and before the '=' sign. name="Anna"

So, if you want to reference it, you should write:
$name or ${name}

Input

For user input we use read command where you define the name of the variable where the input will be stored.
Example:

echo "What is your name?"
read name
echo "Hi there $name"

Another way to get the input without using echo command is using the -p read's parameter:

read -p "What is your name? " name
echo "Hi there $name"

Arrays

To initialize an array by assigning values, they should be separated by space and enclosed in '()'.
my_array=("value 1" "value 2" "value 3" "value 4")
Then, if you want to acces to a value, you should set the index (0-n).
Example:
echo ${my_array[2]}
Should print value 3.
Example 2:
echo ${my_array[@]}
Should print all values.
Note: For arrays you should use curly brackets ({}).

Structures

Functions

Functions in programming, allow us to reuse lines of code and/or segment our application.
To create a function in bash, we use this structure:

function function_name() {
    your_commands
}

We also can omit function key word.
To send parameters, we just reference it.
Example:

server_name=$(hostname)
function memory_check() {
    echo ""
    echo "Memory usage on ${server_name} is: "
    free -h
    echo ""
}

Conditionals

If Else

if [[ condition ]] ; then
    commands
else
    commands
fi

Switch case

case $some_variable in
    pattern_1)
        commands
        ;;
    pattern_2 | pattern_3)
        commands
        ;;
    *)
        default commands
        ;;
esac

Loops

We can use continue and break commands, which permits stop the loop or start the next iteration.

• continue: stops the current iteration and start the next one.
• break: ends the current loop. Example 1:
  

for i in {1..5}
do
    if [[ $i == 2 ]]
    then
        echo "skipping number 2"
        continue
    fi
    echo "i is equal to $i"
done

The output will be:

i is equal to 1
skipping number 2
i is equal to 3
i is equal to 4
i is equal to 5

Example 2:

num=1
while [[ $num –lt 10 ]]
do
    if [[ $num –eq 5 ]]
    then
        break
    fi
    ((num++))
done
echo "Loop completed"

The output will be:

For

for var in ${list}
do
    your_c ommands
done

Example 1:

users="devdojo bobby tony"
for user in ${users}
do
    echo "${user}"
done

Example 2:

for num in {1..10}
do
    echo ${num}
done

While

while [[ your_condition ]]
do
    your_commands
done

Until

until [[ your_condition ]]
do
    your_commands
done