In the above example, input to for loop is passed from the list of arguments provided to the script so that for each argument it will process all the commands and displays the output until all arguments iterated.įinally, it’s all about for loop in shell scripting. Let us shell script name as sample.sh and its content as below: Now, we will write for loop without input list in the for loop statement instead it will take input from positional arguments to the script and example as below: In shell scripting, every variable needs to be represented in double-quotes but there are few exceptions such as if we use double quotes in $Month in above for loop line then it will treat entire words as a single line. In the above example we do the same execution of for loop as above example but instead of input list reading will be done using “in” keyword. Now let us see another example where we use in with for before the input list as below: In the above example, we should not pass input values with, as delimiter if we pass it will consider delimiter is also a value like “Fri,” and we should not list of values using double quotes “if we pass them with double quotes, shell script will treat all values as a single value.Īn example o/p: Today day is: Fri Thu Wed Tue Mon ( if we use double quotes) Let us a list of static values as input to for loop and how it will execute will see as below: Now, let’s see a few examples and going through them and explain each and every example of what it does Example #1 So far, we have discussed how for loop works in shell script and different syntaxes. txt file in the filename and then the same process will repeat until all the. txt file will store and then process the filename by listing the. txt files as input to the for loop and the variable name is a filename in which. In the above program, we are passing a list of. We will access the value in the var using $ before the variable so that when shell script processes it the value in the var substitutes in place of it.

Each time when iteration runs an item in the list is assigned to the variable var and the var is processed through the list of commands present in the loop between doing and done are executed before moving to the next item in the list or next iteration. So, this is the flow of for loop which iterates over the list of input filenames.įor loop in shell script works the following way whenever shell sees the keyword for it knows that it needs to repeat a set of commands once for every item in a list. In variable value, iteration by iteration we will get a filename in it such as textfile.dat and during the process, it executes echo statement where it displays copy statement with an old filename and new filename on stdout. In the process flow, it is explaining when the for loop starts, iterator value, checking the condition if satisfies then runs the process otherwise end the process. In the above flow diagram, we are explaining the flow of for loop which is iterating over a list of filenames and there are four steps in the flow such as process flow, variable value, process, and stdout. Let us consider an example of for loop and discuss the flow of it using the flow diagram. Flow Diagram – For loop in Shell Scripting

The execution in above for loop will start like this, before starting off first iteration expr1 is executed which means initialization will be done after than operations/commands between do and done will be executed iteratively until expr2 evaluates to true and after every iteration expr3 updates the iterator value (counter) so that operations will be performed on next element or value in the list or array or string. In for loop, there are three expressions where the first expression is for initialization, second is for conditional check and the third is for updating iterator. In the above syntax, if we observe this syntax is similar to the syntax of for loop in the c programming language. For example, if the size of the list is 5 having numbers from 1 to 5 then during the first iteration 1 will be store in var and operations on var will be performed by the body of the for loop where commands are there.

In the above for loop, it will execute all the commands which are there between do and done for n number of times where n is the size of the list. If a list is not provided then bash will take a positional parameter which we passed in the shell. whereas list is a list of variables or a list of words or a list of numbers and var is a variable name during that iteration. First Syntax Methodįrom the above example, we have pre-defined keywords or built-in keywords such as for, do, done, and in. Hadoop, Data Science, Statistics & others 1.

For loop in batch script example