Basic tutorial about bash scripting
Shell is a command language interpreter that is available on many operating systems.
Shell has many variants. One of them is Bourne Again Shell (aka Bash).
To get started make sure you have a shell variant (Bash and Zsh are used in this tutorial), then make a file with .sh extension
For making a comment in Bash you simply add # and type your comment (similar to Python)
# Hello, this is a comment in bash
# This is also a comment
However, there's a special line that starts with # but is not a comment and that is #!
These two symbols together are (Sharp sign and Bang sign) pronounced Shebang
This line tells which Shell variant are you using
For Bash users you would type #!/bin/bash
For ZSH users you would type #!/bin/zsh
You can always find the path to your shell using this command echo $SHELL
Your script should look like this now
#!/bin/bash
# The rest of the script
...
To declare a variable in bash, you simply type its name and assign it a value (Do never add space before or after the equal sign)
VAR="Abdullah"
Variable names can be combination of upper and lower case letter or numbers and underscores but should start with a letter or underscore
Another way if you want to declare the variable without assigning a value to it, you can use declare
declare VAR
You can also assign a value
declare VAR="Hello World"
By default all variables are strings, but you can declare integers by adding the flag -i
declare -i INT_VAR=5
You can specifiy the variable as readonly (similar to constant) to prevent someone from overwriting it
declare -r NAME_CONST="Linux"
To print anything to the console screen you can use the echo command
echo "Anything to the screen"
You can access any predefined variables using the $ symbol inside double quotation and also without them
echo "Variable value = $VAR" # correct
echo $VAR # correct
echo "$VAR" # correct
echo '$VAR' # Will print $VAR not the value in the variable
There are some variables predefined and found in any Bash script. These variables are called environment variables
Some of them are $HOME, $SHELL and $PATH. You can use these variables anywhere from a shell
For more advanced printing you can use the printf command (similar to C).
You can add some format specifiers and they will be replaced by the corresponding variables
#!/bin/bash
declare -i age=21
declare name="Abdullah"
printf "%s is %d years old\n" "$name" "$age"
%s - denotes a string
%d, %i - denotes integer
%f - denotes float
%e - denotes scientific notation
You can also printf specific precision with floats
printf "%.6f" 2.15214
To read an input value from the user you can use the read command.
declare NAME
printf "Please enter your name: "
read NAME
printf "Hello Mr. %s\n" "$NAME"
You can even delete the declare statement as the variable will be automatically created with the read command.
Furthermore, you can add a flag -p for prompting the user with a message. That means we can delete the first printing statement as well
read -p "Please enter your name: " NAME
printf "Hello Mr. %s\n" "$NAME"
You can't do many arithemetic operations directly.
Try these operations VAR++, VAR-=4. They won't work
To make arithemetic operations you can use the let command
let VAR++
let VAR-=2
let VAR=5+7
let VAR=VAR-1
Another way is to put your operation in (())
((VAR++))
((VAR-=2))
((VAR=5+7))
((VAR=VAR-1))
Basic operators
+ - Addition
- - Subtraction
* - Multiplication
/ - Integer division
% - Remainder
++ - Post/Pre increment
-- - Post/Pre decrement
Basic syntax for if statement is
if (( *comparison condition* ))
then
# Your code
fi
And you can put then on the same line and separate it with ;. You can type multiple commands in the same line and separate them by ;
if (( *comparison condition* )); then
# Your code
fi
Some basic relational operators are
== - is equal
> - is greater than
< - is less than
!= - is not equal
>= - is greater than or equal
<= - is less than or equal
For other conditions (non comparison), you should use [ *condition* ] instead of (( condition )). You should add space before and after the condition
For example to test if a file named new_script exists
if [ -e new_script ]; then
echo "Exists"
fi
To test if it's a file you can use the -f flag and if it's a directory use -d.
You can use else if statement as follows
if [ -f script.sh ]; then
echo "File Exists"
elif [ -d directory/ ]; then
echo "Directory Exists"
fi
You can also add an else statement without a condition that will be executed as a last option.
Furthermore, you can combine multiple conditions together
Some important operators
-a - AND for [] operator
-o - OR for [] operator
&& - AND for (()) operator
|| - OR for (()) operator
Consider the following code
if [ -f script.sh -o -d directory/ ]; then
echo "Exists"
fi
This code prints Exists if there's a file called script.sh or there's a directory called directory.
Consider the following code to make a program that creates a new file with the touch command and if that file already exists we create a new file with 1 after the name
#!/bin/bash
read -p "Enter name of the file " name
if [ -f $name ]; then
printf "The file already exists\n"
printf "File %s1 is created instead\n" "$name"
else
touch $name
printf "File %s created\n" "$name"
fi