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The bash shell offers a variety of methods to iterate through data, making your tasks – particularly scripting challenges – more efficient.
By looping through a set of data, you can accomplish a lot by simply executing a single script. Additionally, you can create and iterate through various sequences of values. Whether you’re processing an extensive list of numeric values, days of the week, usernames, words, file names, or anything else, bash provides options that ease the process for you.
The looping constructs available in bash consist of for, while, and until loops. This article presents examples of each option, ranging from basic to moderately complex.
One of the easiest loops to work with is the for loop, as demonstrated below. It iterates as many times as there are items specified as arguments (in this case, 1, 2, 3, and 4). We could just as easily loop through the phrase “cats are very smart” as we would with the numbers 1, 2, 3, and 4.
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As mentioned earlier, loops aren’t just limited to numeric variables. You can also iterate through various other types of variables – such as strings representing colors, states, planets, or even the days of the week. Here’s an illustration:
Once you execute this script and share your plans, the “week” file will serve as a reminder of how you plan to spend each day throughout the week.
You might utilize a loop like the ones demonstrated below to traverse the letters of the alphabet or a range of numbers.
The for command presented below would generate calendars for the last three months of the ongoing year.
The following script iterates through each month of the year.
While loops continue to execute as long as the specified condition remains true. Here’s an illustration:
The previously mentioned script increases and shows the value of $n as long as it is less than or equal to 4. You can create a while loop that runs indefinitely (i.e., until the system crashes, you log out, or someone terminates your script) by using “while true” as demonstrated below. The sleep command makes certain that it doesn’t execute numerous times before you get a chance to halt it.
The script provided below will keep running as long as the user it’s monitoring has not yet logged into the system. This can be quite useful when you are waiting for a colleague to resolve an issue before you can proceed with your work. It checks once every minute to see if the user has logged in.
Until loops operate by continuing to run until a specified condition becomes true. To illustrate a similar concept as the previous script, we can implement one using an until command that executes until the count of the user being monitored exceeds 0.
There’s also no restriction against nesting looping commands within one another. In this scenario, we’re demonstrating a for loop situated inside a while loop.
This script iterates through the letters a, b, c, and d for the numbers ranging from 1 to 5.
The commands break and continue are useful for controlling the flow of bash loops. The break command will exit the loop, skipping any subsequent commands, while the continue command will skip the remaining commands within the loop and return to the beginning of the loop. In the script that follows, the break command is executed and the loop is terminated once the user guesses a randomly chosen 1- or 2-digit number.
The for, while, and until loops are powerful tools that allow you to accomplish a significant amount of work with minimal effort. They provide a diverse array of options for configuring the range of values they iterate over.
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