#!/bin/bash if [[ -f "/etc/passwd" ]] then echo "This file exists on your filesystem." The if...elsestatement in Bash lets the user manipulate the flow of code based on conditions. For example, input the marks of student and check if marks are greater or equal to 80 then print “Very Good”. And if the command returns a non-zero exit status code, then the commands written in the else section is executed. Like in any other programming language, if, if..else, if..elif..else and nested if statements in Bash can be used to execute code based on a certain condition. If the FILE argument to one of the primaries is of the form /dev/fd/N, then file descriptor "N" is checked. The if statement is closed with a fi (reverse of if). [ -p FILE ] True if FILE exists and is a named pipe (FIFO). Unary expressions are often used to examine the status of a file. [ -t FD ] True if file descriptor FD is open and refers to a terminal. Note that, more than two conditions can be specified, for which elif statement can be used. The table below contains an overview of the so-called "primaries" that make up the TEST-COMMAND command or list of commands. The base for the 'if' constructions in bash is this: if [expression]; then Firstly, what are Conditional statements ? IF statements are used in making decisions in bash scripting. Doing so gives the user and developer much additional flexibility when it comes to writing Bash if statements. 1. if statement 2. if else statement 3. if elif statement 4. To negate the test expression, use the logical NOT (!) linux bash if statement inside for loop [duplicate] Ask Question Asked 5 years ago. A simple If statement comparing strings. The general syntax of a basic if statement is as follows: if [ condition ]; then your code fi. fi. Therefore, when issued on the command line, they are separated by a semi-colon. For example, if a user enters the marks 90 or more, we want to display “Excellent”. [ -s FILE ] True if FILE exists and has a size greater than zero. 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. Welcome guys to our 5 th tutorials in BASH scripting series (other tutorials are here). In a script, you would write the following if statement. variable holds the exit status of the previously executed command (the most recently completed foreground process). If no test succeeds, and a bash else clause is provided, then the code portion of the final else clause will be executed.. What are the double Parentheses ((…)), single […], and double [[..]] Square Brackets? In case one if the condition goes false then check another if conditions. You can have one or more elif clauses in the statement. True if FILE exists and its sticky bit is set. If marks are less than 80 and greater or equal to 50 then print 50 and so on. Let’s add an elif clause to the previous script: Bash allows you to nest if statements within if statements. In this version, instead of the nested if statements, we’re using the logical AND (&&) operator. Bash is an interesting language. The ? Bash if elif Statement. For value more than or equal to 80 and less than 90, the displayed message is “Very Good”. AND operator returns true if both the operands are true, else it returns false. 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. User can specify separate code blocks to be executed, only one of which will finally get executed during runtime, based on the corresponding condition which is satisfied. 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. In the case statement, you may test several values for one variable. Bash if statements are very useful. 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. The Bash if..else statement takes the following form: If the TEST-COMMAND evaluates to True, the STATEMENTS1 will be executed. We shall learn about the syntax of if statement and get a thorough understanding of it with the help of examples. Bash AND logical operator can be used to form compound boolean expressions for conditional statements or looping statements. See the info pages for Bash for more information on pattern matching with the "(( EXPRESSION ))" and "[[ EXPRESSION ]]" constructs. We’ll never share your email address or spam you. 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. Bash If Statement with Bash, Bash Introduction, Bash Scripting, Bash Shell, History of Bash, Features of Bash, Filesystem and File Permissions, Relative vs Absolute Path, Hello World Bash Script, Bash Variables, Bash Functions, Bash Conditional Statements etc. The Bash elif statement enables us deciding from more choices than two; as we have seen in the above case. 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. They allow us to decide whether or not to run a piece of code based upon conditions that we may set. 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. 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. It is a conditional statement that allows a test before performing another statement. Indentations and blank lines make your code more readable and organized. [ -r FILE ] True if FILE exists and is readable. In Bash, the test command takes one of the following syntax forms: To make the script portable, prefer using the old test [ command which is available on all POSIX shells. If-then-else statement in bash. 2. The most basic if statement takes the following form: The if statement starts with the if keyword followed by the conditional expression and the then keyword. The echo statement prints its argument, in this case, the value of the variable count, to the terminal window. 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. If TEST-COMMAND returns False, nothing happens, the STATEMENTS gets ignored.eval(ez_write_tag([[728,90],'linuxize_com-box-3','ezslot_2',139,'0','0'])); In general, it is a good practice to always indent your code and separate code blocks with blank lines. You can place multiple if statement inside another if statement. Boolean Operations with Bash Scripts 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. Active 5 years ago. Using an else statement, we can write a two-part conditional. The following types of conditional statements can be used in bash. The TEST-COMMAND list is executed, and if its return status is zero, the CONSEQUENT-COMMANDS list is executed. elif (else if) is used for multiple if conditions. The -z and -n operators are used to verify whether the string is Null or not. then –» this is the “flag” that tells bash that if the statement above was true, then execute the commands from here. To achieve this, the elif statement is used. 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. For example, we ask the user to enter a … In this guide you will learn how to use if else in different ways as part of your scripts. Just like the if is closed with fi, the opening square bracket should be closed after the conditions have been listed. If statements (and, closely related, case statements) allow us to make decisions in our Bash scripts. In this guide you will learn about the following testing strings: Note: For explaining the usage of the Bash “if” statement, we have worked with Linux Mint 20. Viewed 31k times 5. 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 The most compact syntax of the if command is: if TEST-COMMANDS; then CONSEQUENT-COMMANDS; fi. The elif bash (else if) statement allows you to check multiple conditions by consequence. More information about this subject can be found in the Bash documentation. The bash scripting language uses this convention to mark the end of a complex expression, such as an if statement or case 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. What is a Bash if else statement? The most fundamental construct in any decision-making structure is an if condition. There are different string operators available in bash scripting language which can be used to test strings. Otherwise, if TEST-COMMAND returns False, the STATEMENTS2 will be executed. 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. Bash IF statement is used to execute a set of instructions or commands based on whether a certain condition is satisfied or not. If you have any questions or feedback, feel free to leave a comment. The return status is the exit status of the last command executed, or zero if no condition tested true. However, its syntax slightly differs. Pay attention to white space! In this guide, we will test these string operators using the if statement in Centos 8. When working with Bash and shell scripting, you might need to use conditions in your script.. Most people choose to use either 4-space or 2-space indentation. 1. The if construction allows you to specify such conditions. 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. If none of the test commands evaluate to True, the STATEMENTS2 is executed. Bash IF. 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. If-else statements in bash scripting is similar to any other programming languages; it is a method for a program to make decisions. 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. If you like our content, please consider buying us a coffee.Thank you for your support! The most basic form is: if expression then statement where 'statement' is only executed if 'expression' evaluates to true. It is important to remember that the then and fi are considered to be separated statements in the shell. We can also use Bash subshells inside if statements, inline with the statement. A piece of code based upon conditions bash if statement we may set a basic statement. 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