In this tutorial, we will walk you through the basics of the Bash if statement and show you how to use it in your shell scripts. If the TEST-COMMAND evaluates to True, the STATEMENTS gets executed. If the decision to be made is a bit complex, multiple if statements will be needed. The elif bash (else if) statement allows you to check multiple conditions by consequence. In this section of our Bash Scripting Tutorial you will learn the ways you may use if statements in your Bash scripts to help automate tasks. IF statements are used in making decisions in bash scripting. The most fundamental construct in any decision-making structure is an if condition. The Bash if..elif..else statement takes the following form: if TEST-COMMAND1 then STATEMENTS1 elif TEST-COMMAND2 then STATEMENTS2 else STATEMENTS3 fi If the TEST-COMMAND1 evaluates to True , the STATEMENTS1 will be executed. Below is how the bash elif looks like: if [ [ test-command1 ] ] then echo elif [ [ test-command2 ] ] then echo else echo fi. Expressions may be combined using the following operators, listed in decreasing order of precedence: The [ (or test) built-in evaluates conditional expressions using a set of rules based on the number of arguments. For example, if a user enters the marks 90 or more, we want to display “Excellent”. Note that, more than two conditions can be specified, for which elif statement can be used. In this tutorial, we will be discussing some of the conditional statements (aka structured commands). Note: For explaining the usage of the Bash “if” statement, we have worked with Linux Mint 20. Doing so gives the user and developer much additional flexibility when it comes to writing Bash if statements. They allow us to decide whether or not to run a piece of code based upon conditions that we may set. The base for the 'if' constructions in bash is this: if [expression]; then echo 'even' –» it prints “even” to your screen else –» and this is the other “flag” that tells bash that if the statement above was not true (false), then execute the command here instead. Let’s see more about this in the syntax and examples below. then –» this is the “flag” that tells bash that if the statement above was true, then execute the commands from here. In this version, instead of the nested if statements, we’re using the logical AND (&&) operator. Using an else statement, we can write a two-part conditional. An if statement is always going to be true or false, either the operator is correct and it is true, or the operator is incorrect and it is false. [ -s FILE ] True if FILE exists and has a size greater than zero. The if else statement is the way to make decisions in Bash scripting. If, for example, you enter 15, the test command will evaluate to true because 15 is greater than 10, and the echo command inside the then clause will be executed. Tests commands in the bash if statement, and bash elif clauses, are executed in order until one test succeed. Boolean Operations with Bash Scripts Bash if statements are very useful. In the case statement, you may test several values for one variable. Method of Using the Bash “if” Statement. If the week number is even, it reminds you to put out the garbage cans: An example of comparing strings for testing the user ID: With Bash, you can shorten this type of construct. The “if then elif then else fi” example mentioned in above can be converted to the nested if as shown below. The condition in the if statement often involves a numerical or string test comparison, but it can also be any command that returns a status of 0 when it succeeds and some nonzero status when it fails. In a BASH scripts, commands run sequentially that means first line will be executed first, then next one & so on If the FILE argument to one of the primaries is of the form /dev/fd/N, then file descriptor "N" is checked. In if-else statements, the execution of a block of statement is decided based on the result of the if condition.. Bash If-Else Statement Syntax Syntax of if statement. Indentations and blank lines make your code more readable and organized. The new upgraded version of the test command [[ (double brackets) is supported on most modern systems using Bash, Zsh, and Ksh as a default shell. Unary expressions are often used to examine the status of a file. The following types of conditional statements can be used in bash. If you are not familiar yet (or would like to learn more about) Bash if statements, please see our Bash If Statements: If Elif Else Then Fi article. if elif and else statements are useful where we have more than two programs for execution, and need to execute only one based on results of if and elif condition. The Bash case statement can be taken as a nice and easily readable solution to the nested-if statements. 1. if statement 2. if else statement 3. if elif statement 4. Just like the if is closed with fi, the opening square bracket should be closed after the conditions have been listed. Once a condition returns True the remaining conditions are not performed and program control moves to the end of the if statements. (3 answers) Closed 5 years ago. variable holds the exit status of the previously executed command (the most recently completed foreground process). Here is how the output will look like:eval(ez_write_tag([[468,60],'linuxize_com-large-mobile-banner-1','ezslot_11',157,'0','0'])); The logical OR and AND operators allow you to use multiple conditions in the if statements. The table below contains an overview of the so-called "primaries" that make up the TEST-COMMAND command or list of commands. Bash if conditionals can have different forms. Otherwise, if TEST-COMMAND returns False, the STATEMENTS2 will be executed. You can have one or more elif clauses in the statement. Generally, it is more efficient to use the, How to Increment and Decrement Variable in Bash (Counter), How to Check if a String Contains a Substring in Bash. The following example shows a simple test: The following example demonstrates that TEST-COMMANDS might be any UNIX command that returns an exit status, and that if again returns an exit status of zero: The same result can be obtained as follows: The examples below use numerical comparisons: This script is executed by cron every Sunday. We’ll never share your email address or spam you. The CONSEQUENT-COMMANDS list that follows the then statement can be any valid UNIX command, any executable program, any executable shell script or any shell statement, with the exception of the closing fi. In a script, the different parts of the if statement are usually well-separated. The if construction allows you to specify such conditions. The compact equivalent of the above test is as follows: Similar to the "&&" expression which indicates what to do if the test proves true, "||" specifies what to do if the test is false. You can place multiple if statement inside another if statement. Let’s add an else clause to the previous example script: If you run the code and enter a number, the script will print a different message based on whether the number is greater or less/equal to 10.eval(ez_write_tag([[728,90],'linuxize_com-medrectangle-4','ezslot_0',160,'0','0'])); The Bash if..elif..else statement takes the following form: If the TEST-COMMAND1 evaluates to True, the STATEMENTS1 will be executed. There are a ton of operators in bash that will take more than 8 examples to get you through but for now, the examples in today's article should get you started. elif (else if) is used for multiple if conditions. Bash if elif Statement. In this guide, we will test these string operators using the if statement in Centos 8. For value more than or equal to 80 and less than 90, the displayed message is “Very Good”. You can have only one else clause in the statement. The bash scripting language uses this convention to mark the end of a complex expression, such as an if statement or case statement. User can give any number of conditions using elif, and finally a default condition using elseblock. To negate the test expression, use the logical NOT (!) In Bash, the Case statement is one of the decision making statements that let us execute a block of code. When an IF statement is introduced in a bash script, it is intended to check for certain conditions before running different commands in the script. Check the below script and execute it on the shell with different-2 inputs. The -z and -n operators are used to verify whether the string is Null or not. If-else statements in bash scripting is similar to any other programming languages; it is a method for a program to make decisions. The else clause is optional.eval(ez_write_tag([[728,90],'linuxize_com-box-4','ezslot_3',143,'0','0'])); eval(ez_write_tag([[336,280],'linuxize_com-banner-1','ezslot_4',161,'0','0']));The conditions are evaluated sequentially. If marks are less than 80 and greater or equal to 50 then print 50 and so on. The first example checks for the existence of a file: The above example will work when entered on the command line: However, if you use testing of conditions that depend on the environment, you might get different results when you enter the same command in a script, because the script will open a new shell, in which expected variables and options might not be set automatically. 1. Therefore, when issued on the command line, they are separated by a semi-colon. Regular expressions may also be used in comparisons: Most programmers will prefer to use the test built-in command, which is equivalent to using square brackets for comparison, like this: If you invoke the exit in a subshell, it will not pass variables to the parent. When comparing strings , always use single or double quotes to avoid word splitting and globbing issues.eval(ez_write_tag([[300,250],'linuxize_com-large-leaderboard-2','ezslot_5',146,'0','0'])); Below are some of the most commonly used operators: The if, if..else and if..elif..else statements allow you to control the flow of the Bash script’s execution by evaluating given conditions. Below, a couple of simple examples. The keyword “fi” indicates the end of the inner if statement and all if statement should end with the keyword “fi”. The most basic form is: if expression then statement where 'statement' is only executed if 'expression' evaluates to true. The TEST-COMMAND list is executed, and if its return status is zero, the CONSEQUENT-COMMANDS list is executed. If the command runs successfully and returns an exit status code of zero, then the commands listed between then and else will be executed. If you have any questions or feedback, feel free to leave a comment. stdin, stdout and stderr and their respective file descriptors may also be used for tests. We can also use Bash subshells inside if statements, inline with the statement. Here is another version of the script to print the largest number among the three numbers. If none of the test commands evaluate to True, the STATEMENTS2 is executed. Pay attention to white space! The ? Bash AND logical operator can be used to form compound boolean expressions for conditional statements or looping statements. #!/bin/bash if [[ -f "/etc/passwd" ]] then echo "This file exists on your filesystem." There are different string operators available in bash scripting language which can be used to test strings. In a script, you would write the following if statement. To achieve this, the elif statement is used. In programming, conditions are crucial : they are used to assert whether some conditions are true or not.. The most compact syntax of the if command is: if TEST-COMMANDS; then CONSEQUENT-COMMANDS; fi. The if statement is closed with a fi (reverse of if). The TEST-COMMAND often involves numerical or string comparison tests, but it can also be any command that returns a status of zero when it succeeds and some other status when it fails. Most people choose to use either 4-space or 2-space indentation. This post should serve you as a cheat sheet to most of the scenarios that you might encounter in your everyday scripting tasks that uses conditional execution. fi. [ -t FD ] True if file descriptor FD is open and refers to a terminal. 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