This line defines what shell we will use, which is Bash shell in our case. You can type multiple commands on the same line but you must separate them with semicolons, but it is preferable to write commands on separate lines, this will make it simpler to read later. Now, set that file to be executable, otherwise, it will give you permissions denied. You can review how to set permissions using chmod command. Sometimes you need to interact with system variables, you can do this by using environment variables.
Your shell scripts will need conditional statements. Like if the value is smaller than 10 do this else do that. You can imagine any logic you want. You can check our tutorial about the grep command. If the first command runs and returns zero; which means success, it will not hit the commands after the else statement, otherwise, if the if statement returns non-zero; which means the statement condition fails, in this case, the shell will hit the commands after else statement.
Based on the programming requirements, a particular loop is used in the script. The uses of these three types of loops are shown in this section by using simple examples.
Types of Shell
Here, a list of weekday name is used and each weekday name is iterated by using for loop. If you want to know more used of bash for loop then you can check this tutorial. This script will print those numbers from 1 to 20 which are even and divisible by 5. There are only numbers within those are even and divisible by 5. The following output will appear after running the script.
This script will print all odd numbers from 0 to The script will print all even numbers from 20 to 1. When a block of code needs to execute multiple times in a script then the function is used to do the task. You will need to call the function by name only when the script defined in the function need to execute many times rather than adding the same script multiple times. The starting and ending first bracket is used with function name to declare the function in bash script.
The function can be called by just the function name in bash. Bash does not support function argument like another standard programming language. But the value can be passed to bash function in a different way that is shown in this section. The value can be returned from the function with the return statement or without using the return statement. Create a bash file with the following script to know how the functions can be declared and called in bash script. Three functions are declared in the script.
The third function is declared to read an argument value as circle radius that will be provided at the time of calling the function. Here, local is used to read the argument value. If you want to know about returning a string from bash function then you can this tutoria l. You can use many types of commands to call other programs in any bash script, such as source, bash, eval, exec , etc.
Suppose three bash files, add. Here, subtract. The script of all these four files is given below. Create a bash file named, callpro. You must set execution permission for the above four files before running the following script. The last command is exec command that works with absolute path only. For this, the full path name of divide.
Different types of menus can be created by using this command. A particular data list is used to create a menu by this command. In this example, the list of five items will be printed as a menu and prompt the user to choose any language from the list. The script will continuously ask for selecting the language until the user presses 6 to terminate from the script. According to the following output, the user pressed 3 for the first time that printed PHP and pressed 6 for the second time that terminated from the script. Sometimes we need to provide input values when executing the script from the command line.
This task can be done in two ways in bash.
One way is by using argument variables and another way is by using getopts function. Reading command line input data from the terminal by using the argument variable is shown in this section.
Learn Shell - Free Interactive Shell Tutorial
Create a bash file with the following script to see the use of reading command line argument value. The script is executed for four times for four types of arithmetic operators. Argument values can be passed with name-value pair in bash. Create a bash file with the following script to show how to read argument values with the name. The script will read two argument variables. The argument values with name are printed in the first statement of the script. Next, a for loop is used to iterate the array that contains command line argument values.
Next, case statement is used to print a particular message based on the key value. Two command line arguments are provided in the above commands.
Processing command line arguments by using getopts function is not discussed in this tutorial. If you want to know about command line processing by using getopts function then you can visit this tutorial. Doing arithmetic operation is a common requirement of any programming language. We assure that you will not find any problem in this Shell Scripting tutorial.
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Executing Shell Script. Shell Sourcing Config File. Shell if then else Shell if then elif Shell for loop Shell while loop Shell until loop.