Intro:
So, what’s the deal with ‘if’ statements? Think of them as the script’s way of making decisions. It’s like having a little script buddy that can say, “Hey, if this thing happens, do this. Otherwise, try something else!”
Syntax:
if [ condition ]; then
# Do this if the condition is true
fi
We have a set of keywords: if, then, elif, else and fi.
Let’s see a basic example:
#!/bin/bash
a=5
if [ $a -eq 5 ]
then
echo "The condition is True"
fi
In this example, we assigned the var a with the value 5. Now the if condition checks the value of a and if it matches we do some action with the then keyword (just echo here notifying the condition matched). The fi keyword marks the end of conditional block.
The [] is an alias of Linux command test. The -eq is an operator, which means “equals to”. Similarly, there are many other operators like -ne for not equals, -gt for greater than, -f for existence of a file, -d for directory and etc. To see all use man page.
man test
We can even do the test without using [].
#!/bin/bash
a=5
if test $a -eq 5
then
echo "The condition is True"
fi
Using if..else:
If we have multiple checks, then we can use multiple if statements or better yet use else condition.
#!/bin/bash
a=5
if test $a -eq 8
then
echo "The condition is True"
else
echo "The value of a is not 8"
fi
Let’s see another example to check if a file exists, if it does then simply print the content of the file. If it does not then create the file and then print its content.
#!/bin/bash
file="/home/sagar/random.txt"
data="Hello there, this is testing."
if [ -f "$file" ];
then
echo "File exists. Printing its content: "
cat "$file"
else
echo "File does not exist. Creating the file..."
echo "$data" > "$file"
cat "$file"
echo "File created at $file"
fi
Here, we have the -f to check if a file exists. The first time I ran it, I did not have the random.txt, so it was created and running it again just printed the content as the first condition matched.
Using elif:
Another example to check if a particular command is installed. If already installed then just run it, if not, install it and then run.
In case we want to install, we need to use package managers. As there are multiple Linux distros with different package managers, we shall make the script universal and to check the distro at run time and use proper commands.
Note: We can use either -f to check for the command using its path or use the Linux co**mmand called “command”. Use which command to find location of a program.
$ which nano
/usr/bin/nano
$ command -v nano
#!/bin/bash
cmd=/usr/bin/nano
if [ -f $cmd ]
then
echo "$cmd is present"
else
echo "$cmd is not present"
fi
--------------------------
#!/bin/bash
cmd=nano
if command -v $cmd
then
echo "$cmd is present"
else
echo "$cmd is not present"
fi
Let’s now proceed with the main script.
Note : Use update command to refresh the package cache first and then install.
#!/bin/bash
app=htop
if command -v "$app" &> /dev/null; then
echo "$app is present"
else
echo "$app is not present, installing it"
if command -v apt &> /dev/null; then
echo "Debian/Ubuntu Based OS, using apt"
sudo apt install htop -y
elif command -v yum &> /dev/null; then
echo "Red Hat based OS, using DNF/yum"
sudo yum install htop -y
else
echo "Unsupported package manager. Please install htop manually."
exit 1
fi
fi
echo "Running $app..."
"$app"
Note: Usage of the sudo command will ask to enter the password at the time of running the script. To avoid such scenarios the script should either run from the root user or the user should be configured not to ask password for these particular commands.
Use sudo visudo:
<username> ALL=(ALL) NOPASSWD: /usr/bin/apt, /usr/bin/yum
Wrapping It Up:
And there you have it! ‘if’ statements are the secret sauce that’ll take your shell scripts from meh to magnificent. So go ahead, experiment, get creative, and let the ‘if’ magic flow! Your scripts will thank you later. 😉
Thanks for reading!