Forem: Twinkle lahariya

Control program flow with if statement in shell scripting

Twinkle lahariya — Sat, 09 Apr 2022 15:10:11 +0000

In the last post, we learned about tests in shell scripting.

Variables and Tests in shell scripting

Twinkle lahariya ・ Apr 6 '22

In this post, let's see how we can use them with if statement to control the flow of the script.

What is `if` statement?

If statement allows us to make decisions in our script. It starts with the keyword if followed by a test expression, then list of commands followed by the closing fi keyword.

A basic if statement says that if the test expression is true, then perform the following commands.

Syntax

if []
then
    command 1
    command 2
    .
    .
    command n
fi

Let's understand if with an example where we check if a given number is greater than 10.

#!/bin/bash

VAR=20
if [ $VAR -gt 10 ]
then
    echo "Variable is greater than 10"
fi

Output:

> ./IfDemo.sh
Variable is greater than 10

Let's see what we have done here:

if [ $VAR -gt 10 ] : if followed by a test condition $VAR -gt 10
$VAR -gt 10 : returns true if the var is greater (-gt) than 10
then : if the condition is true, then execute the following commands
fi : end of if statement

What if you want to log both the scenarios, i.e. if the number is greater than 10 and then the number is less than 10.

If..else statement

If-else statement follows the following form:

Syntax

if []
then
    command 1
else 
    command 2
fi

If-else tells that if the test is true, then run command 1, else run command 2.

Let's extend the same example of a number greater than 10 and print a message if the number is not.

#!/bin/bash

VAR=5
if [ $VAR -gt 10 ]
then
    echo "Variable is greater than 10"
else 
    echo "Variable is less than 10"
fi

Output:

> ./IfDemo.sh

Variable is less than 10

Now, if we want to check the second test condition if the first one fails and handle it differently, then we use if-elif-else ladder.

If..elif..else statement

Let's check if the number is greater than 10, then print 'greater than'. Else, if the number is equals 10, print 'equals to', else print 'less than'.

#!/bin/bash

VAR=5
if [ $VAR -gt 10 ]
then
    echo "Variable is greater than 10"
elif [ $VAR -eq 10]
    echo "Variable is equals to 10"
else 
    echo "Variable is less than 10"
fi

Output:

> ./IfDemo.sh
Variable is equals to 10

There is more to if statements. To list some of the scenarios:

1: Nested if statements:

if [[ Test condition ]]
then
  if [[ Test condition ]]
  then
    command 1
  else
    command 2
  fi
fi

2: Multiple test conditions

if [[ Test condition ]] && [[ Test condition ]]
then
  command 1
elif [[ Test condition ]] || [[ Test condition ]]
then
  command 2
else
  command 3
fi

Cool, I'll go now and try these scenarios out, do let me know if you try them and have any questions about it.

Up next is loops in shell scripting. Stay tuned.

Until next time. Love ❤️

Variables and Tests in shell scripting

Twinkle lahariya — Wed, 06 Apr 2022 10:42:09 +0000

In the earlier post, we learned the basics of shell scripting and wrote our first shell script.

Shell Scripting Basics

Twinkle lahariya ・ Apr 5 '22

#shellscripting #programming #beginners

In this post, let's start with variables in a shell script

What are variables?

Variables are the storage locations that have a name.
Consider variables as a name-value pair.
VARIABLE_NAME="Value"

Variables' names are case sensitive and the general convention is to have them in all uppercase.

Now, how can I use these variables in my script?

Variables can be used by preceding variable names with $.
$<VARIABLE_NAME>
Let's look at an example:

#!/bin/bash
SHELL_NAME="bash"
echo "I am using $SHELL_NAME scripting!"

Output:

>./VariablesDemo.sh
I am using bash scripting!

We can also enclose the name in curly braces
${<VARIABLE_NAME>}
In that case, the above example will look like this:

#!/bin/bash
SHELL_NAME="bash"
echo "I am using ${SHELL_NAME} scripting!"

Output:

>./VariablesDemo.sh
I am using bash scripting!

Is curly braces mandatory?

No, but in some cases, you need to. Let me show you when

In case you want to concatenate the variable with another string you have to use curly braces.

Say you have to write "I am scripting on my machine" and you have a variable "$SCRIPT="script" in that case you will write:
echo "I am ${SCRIPT}ing on my machine"

If you write $SCRIPTing, in that case, the whole string SCRIPTing will become the name and since we do not have a variable with that name, nothing will be put in that place.

Assigning output of a command to a variable

To assign the output of a command to a variable, just use $(command):

#!/bin/bash

SHELL_NAME="bash"
USER_NAME=$(whoami)
echo "$USER_NAME is using ${SHELL_NAME} scripting!"

Output

> ./VariablesExample.sh
twinklekailashlahariya is using bash scripting!

Variable naming conventions

don'ts

Don't start with a number
Don't use special characters. Only _ is allowed and can be at any location

otherwise, go crazy and name that variable of yours as you want.

Tests in a shell script

Scripts are made to replace the effort to sit and scribble a series of commands. What if we want to perform certain actions based on conditional scenarios? If we are typing in the commands manually, we might check the status of the previous command and write the next command accordingly, right?

In shell scripts, we can achieve the same with tests.
To test a condition, simply place the condition within square brackets.
Syntax = [condition-to-test]

If the test passes, the expression exits with exit status 0, i.e. true, else it returns 1, i.e. false.

Let's look at this with an example:

#!/bin/bash

FILENAME=/etc/myFile.txt
if [ -f "$FILENAME" ]
then echo "File exist"
else echo "File does not exist"
fi

Output:

> ./TestsExample.sh
File does not exist

Let's see what we have done here:
FILENAME=/etc/myFile.txt : Defined a variable with a file path.
if [ -f "#FILENAME" ] : An if condition that checks if the file exists at the given path.
[ -f "#FILENAME" ] : Our test condition where -f is a flag that returns true if file exists and is a regular file.
then echo "File exist" : if the file exists, the control flow goes into this condition and prints "File exist" message.
else echo "File does not exist" : otherwise the control falls into this block and prints "File does not exist" message.

Like -f, we have more such test conditions, some of the popular ones are listed below:

-n STRING the length of STRING is nonzero
-z STRING the length of STRING is zero
STRING1 = STRING2 the strings are equal
STRING1 != STRING2 the strings are not equal
INTEGER1 -eq INTEGER2 INTEGER1 is equal to INTEGER2
INTEGER1 -ge INTEGER2 INTEGER1 is greater than or equal to INTEGER2
INTEGER1 -gt INTEGER2 INTEGER1 is greater than INTEGER2
INTEGER1 -le INTEGER2 INTEGER1 is less than or equal to INTEGER2
INTEGER1 -lt INTEGER2 INTEGER1 is less than INTEGER2
INTEGER1 -ne INTEGER2 INTEGER1 is not equal to INTEGER2
FILE1 -ef FILE2 FILE1 and FILE2 have the same device and inode numbers
FILE1 -nt FILE2 FILE1 is newer (modification date) than FILE2
FILE1 -ot FILE2 FILE1 is older than FILE2
-b FILE FILE exists and is block special
-c FILE FILE exists and is character special
-d FILE FILE exists and is a directory
-e FILE FILE exists
-g FILE FILE exists and is set-group-ID
-G FILE FILE exists and is owned by the effective group ID
-h FILE FILE exists and is a symbolic link (same as -L)
-k FILE FILE exists and has its sticky bit set
-L FILE FILE exists and is a symbolic link (same as -h)
-O FILE FILE exists and is owned by the effective user ID
-p FILE FILE exists and is a named pipe
-r FILE FILE exists and read permission is granted
-s FILE FILE exists and has a size greater than zero
-S FILE FILE exists and is a socket
-w FILE FILE exists and write permission is granted
-x FILE FILE exists and execute (or search) permission is granted

We will look into more examples of these flags in our next post when we look into conditions and loops.

Control program flow with if statement in shell scripting

Twinkle lahariya ・ Apr 9 '22

#shellscripting #bash #beginners

Until next time... Peace ✌️

Shell Scripting Basics

Twinkle lahariya — Tue, 05 Apr 2022 15:26:30 +0000

What is a Script?

A Script is a command line program that contains a series of instructions.
Anything that you can type in a command line, you can put in a script.

Let's greet the world with a script.

#!/bin/bash echo "Hello World"

we will look into every component of this script a bit, but before that, let's first see how a script is interpreted and executed.

Interpreting a Script.

Any script is interpreted by an interpreter that executes the command one after the other. In the case of shell script, the shell accesses the interpreter and executes the commands. Functions that shell scripts support include loops, variables, if/then/else statements, arrays and shortcuts.

Now the question is, why do we need scripts?

If you find yourself doing a set of tasks over and over again, you might want to automate that task. Scripting is a great way to automate tasks.

Components of a Script

#!/bin/bash 
echo "Hello World"

A script starts with
#! : Shebang, which represents the beginning of the script followed by the path of the interpreter.
/bin/bash : This is the path to the interpreter used to execute commands written in the script.
Here are some more examples of shell scripts, each using a different shell program as the interpreter:
#!/bin/zsh : This script uses zsh as the interpreter. #!/bin/csh : This script uses csh as the interpreter. #!/usr/bin/python : This script uses python as the interpreter.

Is the Shebang mandatory?

No, the shebang is an optional command. If we do not specify shebang, then the script runs in the current shell.

Is this a problem?
No, if you are lucky.
Different shells have slightly different syntax. It's safe to be explicit and specify the exact interpreter to be used with the script.

After, the first line of the script follows the executable instructions. In our case, it is:
echo "Hello World" : Displays hello would to the console.

Now, save the file with the .sh extension in the location that the shell can access.

Run a shell script

Make the script executable with command chmod +X <fileName>
Run the script using ./<fileName>

In order to run our HelloWorld program:

save the file with HelloWorld.sh
run the command chmod +X HelloWorld.sh
execute the script ./HelloWorld.sh

output :

> ./HelloWorld.sh
Hello World

What happens when we execute a script?

When we execute a script that has a shebang, what happens it that the interpreter executes and the path used to call the script is passed as an argument to the interpreter.
And then every command following the Shebang gets executed line by line.

Voila! We just ran our first shell script.

Up next is variables and tests in a shell script..

Variables and Tests in shell scripting

Twinkle lahariya ・ Apr 6 '22

#shellscripting #bash #beginners

Until next time, love ❤️

Forem: Twinkle lahariya

Control program flow with if statement in shell scripting

Variables and Tests in shell scripting

Twinkle lahariya ・ Apr 6 '22

What is if statement?

If..else statement

If..elif..else statement

Variables and Tests in shell scripting

Shell Scripting Basics

Twinkle lahariya ・ Apr 5 '22

What are variables?

Now, how can I use these variables in my script?

Is curly braces mandatory?

Assigning output of a command to a variable

Variable naming conventions

don'ts

Tests in a shell script

Control program flow with if statement in shell scripting

Twinkle lahariya ・ Apr 9 '22

Shell Scripting Basics

What is a Script?

Interpreting a Script.

Now the question is, why do we need scripts?

Components of a Script

Is the Shebang mandatory?

Run a shell script

What happens when we execute a script?

Variables and Tests in shell scripting

Twinkle lahariya ・ Apr 6 '22

What is `if` statement?