[ This is a five part series that covers the very basics of Linux shell scripting. For the very beginning, start from Part I here.]
In the previous part, we had seen how to use system as well as user defined variables in shell scripts. This part will cover something really really cool and useful by discussing loops.You will see how useful they can be when used.
Loops do what they mean. Check for some condition and execute some stuff as defined. The simplest and pretty useful loop used in shell scripts is the if-then-else loop. There are various other loops too like while, for, until but for simplicity the most common loops of if-then and for are described here.
If then loop :
Here is how it works :
If some command / condition
then
do some commands as mentioned here
else
do these commands / conditions here
fi
An example of script where if loop is used will make it more clear.
The above script checks if a command (uptime in this case) is valid or not, if it is valid it will display it’s contents else will output an error.
The output of this script is :
As uptime is indeed a valid command, the script displayed it’s output along with the correct message.
Let us change it to a fake command and see what message do we get :
A fake command or any random text that is not a command is entered in above script, output for this script is :
As you can see, since the if statement couldn’t find the above as a real command, the else part was executed displaying some error message.
Here is a cool script based on the loop discussed above :
The purpose of the above script is to check for a specific user name, if present it will list all the files of that specific user folder. This is done by using grep on /etc/passwd file.Here, the above script will list all contents of Pictures folder belonging to user named avp if the user exists.
The output for this script is :
The user named avp did exist based on /etc/passwd file info so it displayed all the files present in the /home/avp/Pictures folder.
Awesome isn’t it, few lines that can accomplish so much ?
For loop:
for something in someone
do
some stuff
done
An example will make it clear, plus you will see how handy can for be when used with previously described if-then loop :
Let’s make a script that will scan a specific directory path, then list the contents found within as being a file or a directory.
Over here, first the directory contents are read using the user variable : content in a for loop. Within the for loop, the if loop is used to test conditions -d or -f as value of variable content. -d will print the contents as directory and -f as files.
Output as below :
As you can see, how simple it is to make useful scripts when loops are used, this is the power of Linux shell in general. Each command does it’s job very very well and can be integrated within a script to achieve the desired result.
The last part will deal with how shell scripts can interact with users, followed by some cool scripts that can be used on daily basis.
Cheers.