Creating Bash Scripts if then else – Part 3

In the previous pages we looked at some very basic Bash scripting, which involved outputting text to the screen, getting a user’s input, storing it and outputting that to the screen; as well as including a system variable using the Date command. Now let’s combine what you’ve achieved so far and introduce Loops.

With most programming structures there will come a time where you need to loop through the commands you’ve entered to create better functionality, and ultimately a better program.

Step 1

Let’s look at the If, Then and Else statements now, which when executed correctly, compare a set of

instructions and simply work out that IF something is present, THEN do something, ELSE do something different. Create a new script called greeting2.sh and enter the text in the screenshot below

into it.

Step 2

Greeting2.sh is a copy of greeting.sh but with a slight difference. Here we’ve added a loop starting

at the if statement. This means, IF the variable entered is equal to David the next line, THEN, is the reaction to what happens, in this case it will output to the screen ‘Awesome name,’ followed by the variable (which is David).

Step 3

The next line, ELSE, is what happens if the variable doesn’t equal ‘David’. In this case it simply outputs to the screen the now familiar ‘Hello…’. The last line, the FI statement, is the command that will end the loop. If you have an If command without a Fi command, then you get an error.

Step 4

You can obviously play around with the script a little, changing the name variable that triggers a response; or maybe even issuing a response where the first name and surname variables match a specific variable.

More Looping

You can loop over data using the FOR, WHILE and UNTIL statements. These can be handy if you’re batch naming, copying or running a script where a counter is needed.

Step 1

Create a new script called count.sh. Enter the text in the screenshot below, save it and make it executable. This creates the variable ‘count’ which at the beginning of the script equals zero. Then start the WHILE loop, which WHILE count is less than (the LT part) 100 will print the current value of count in the echo command.

Step 2

Executing the count.sh script will result in the numbers 0 to 99 listing down the Terminal screen;

when it reaches 100 the script will end. Modifying the script with the FOR statement, makes it work in much the same way. To use it in our script, enter the text from the screenshot into the count.sh script.

Step 3

The addition we have here is: for count in {0..100}; do. Which means: FOR the variable ‘count’ IN the numbers from zero to one hundred, then start the loop. The rest of the script is the same. Run this script, and the same output should appear in the Terminal.

Step 4

The UNTIL loop works much the same way as the WHILE loop only, more often than not, in reverse.

So our counting to a hundred, using UNTIL, would be: until [ $count -gt 100 ]; do. The difference being, UNTIL count is greater than (the gt part) one hundred, keep on looping.

Step 5

You’re not limited to numbers zero to one hundred. You can, within the loop, have whatever set of commands you like and execute them as many times as you want the loop to run for. Renaming a million files, creating fifty folders etc. For example, this script will create ten folders named folder1

through to folder10 using the FOR loop.

Step 6

Using the FOR statement once more, we can execute the counting sequence by manipulating the {0..100} part. This section of the code actually means {START..END.. INCREMENT}, if there’s no increment then it’s just a single digit up to the END. For example, we could get the loops to count up to 1000 in two’s with: for count in {0..1000..2}; do.

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