In this example, we shall learn to use AND operator to include multiple conditions in the expression. If count isn't 5 but 6, the system prints count is six. 1. if statement 2. if else statement 3. if elif statement 4. The aim is to improve readability and make it harder for us to make simple, silly mistakes. Sometimes we may have a series of conditions that may lead to different paths. Here's an example of the simplest form of an if statement: In this example, the variable count specifies a condition that is used as part of the if statement. FILE exists and it's size is greater than zero (ie. if statement when used with options , returns true if size of the file is greater than zero. it is not empty). It's a little hard to explain so here are some examples to illustrate: case
in If that is the case, the statement between the keywords then and fi are executed. www.tutorialkart.com - ©Copyright-TutorialKart 2018, # ^ ^ ^ ^ ^ ^ ^, # FALSE && TRUE || FALSE || TRUE evaluates to TRUE, # FALSE && TRUE || FALSE evaluates to FALSE, "Size of sample.txt is greater than zero", Options for IF statement in Bash Scripting, Example â Comparing strings using Bash If statement. It effectively gives the system the ability to make decisions. In this Bash Tutorial, we have learnt conditional branching in the sequential flow of execution of statements with bash if statement and example shell scripts. In this example, we shall learn to use OR operator to include multiple conditions in the expression. I would highly recommend you do indent your code however (especially as your scripts get larger) otherwise you will find it increasingly difficult to see the structure in your scripts. If elif if ladder appears like a conditional ladder. See also how to pass arguments to a bash script, which shows how to use conditionals to process parameters passed from the command line. You should print a certain message if true and another if false. You can nest as many if statements as you like but as a general rule of thumb if you need to nest more than 3 levels deep you should probably have a think about reorganising your logic. The keyword fi is if spelled backward. 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 [ ]thenelif [ ] thenelsefi. The simplest nested if statement is of the form: if...then...else...if...then...fi...fi. if statement when used with optionn , returns true if the length ofthe string is greater than zero. You can have any number of elif clauses. FILE exists and the read permission is granted. Indentation of the code between the keywords of the if statement improves readability but isn't necessary. In this example, we shall learn to include multiple conditions combined with AND and OR forming a single expression. Create a Bash script which will print a message based upon which day of the week it is (eg. The bash scripting language uses this convention to mark the end of a complex expression, such as an if statement or case statement. # Else, do nothing. If statements (and, closely related, case statements) allow us to make decisions in our Bash scripts. (Xzibit didn't actually say that but I'm sure he would have, had he hosted Pimp My Bash Script.). ) FILE exists and the execute permission is granted. If follows the format below: Anything between then and fi (if backwards) will be executed only if the test (between the square brackets) is true. Nested if statement 5. case statement Each type of statements is explained in this tutorial with an example. Note : In bash, respect each token/literal. if statement when used with optionz , returns true if the length ofthe string is zero. Bash Else If. You'll notice that in the if statement above we indented the commands that were run if the statement was true. We can accommodate these with boolean operators. In Bash else-if, there can be multiple elif blocks with a boolean expression for each of them. If marks are less than 80 and greater or equal to 50 then print 50 and so on. An example of multiple elif conditions is: A more compact way to write such statements with multiple conditions is the case method. The echo statement prints its argument, in this case, the value of the variable count, to the terminal window. Bash Else If is kind of an extension to Bash If Else statement. If it is neither 5 nor 6, the system prints none of the above. For example, the above piece of code can be re-written with the case statement as follows: if statements are often used inside for-loops or while-loops, as in this example: You can also have nested if statements. A basic if statement effectively says, if a particular test is true, then perform a given set of actions. Bash way of writing Single and Multiline Comments, Salesforce Visualforce Interview Questions. If you have a situation where a piece of code should only be executed if a condition is not true, use the keyword else in an if statement, as in this example: If the condition $count == 5 is true, the system prints the value of the variable count. if statement when used with optionf , returns true if the length ofthe string is zero. Bash If statement is used for conditional branching in the sequential flow of execution of statements. If it is not true then don't perform those actions. The lengh of STRING is zero (ie it is empty). elif (else if) is used for multiple if conditions. For example, input the marks of student and check if marks are greater or equal to 80 then print “Very Good”. Create a Bash script which will accept a file as a command line argument and analyse it in certain ways. If you want to differentiate between multiple conditions, use the keyword elif, which is derived from else if, as in this example: If count is 5, the system prints count is five. We shall learn about the syntax of if statement and get a thorough understanding of it with the help of examples. These options are used for file operations, string operations, etc. you could check if the file is executable or writable. ;; Other times we would like to perform the action if one of several condition is met. Sometimes we want to perform a certain set of actions if a statement is true, and another set of actions if it is false. Talking of indenting. For example it may be the case that if you are 18 or over you may go to the party. 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. It is always good practice to test your scripts with input that covers the different scenarios that are possible. It will print to the screen the larger of the two numbers. However, an if statement can be nested with arbitrary complexity. The if statement then checks whether the value of count is 5. esac. The bash scripting language uses this convention to mark the end of a complex expression, such as an if statement or case statement. It is also possible to have an if statement inside of another if statement. If statement can accept options to perform a specific task. When we refer to FILE above we are actually meaning a, Because [ ] is just a reference to the command, if [ $USER == 'bob' ] || [ $USER == 'andy' ], space_free=$( df -h | awk '{ print $5 }' | sort -n | tail -n 1 | sed 's/%//' ). Following example proves the same. Here's a perfect example of when it makes life easier for you. Also the semicolon at the end of first line. The square brackets ( [ ] ) in the if statement above are actually a reference to the command test. Otherwise, any statements following the if statement are executed. - Socrates. #!/bin/bash if [ $(whoami) = 'root' ]; then echo "You are root" fi The whoami command outputs the username. This means that all of the operators that test allows may be used here as well. For example, we may want to analyse a number given on the command line like so: Yo dawg, I herd you like if statements so I put an if statement inside your if statement. Like what we have looked at in previous sections, their syntax is very specific so stay on top of all the little details. If not, this is an even week so send a message. For compound expressions, following if syntax is allowed. Note : Observe the mandatory spaces required, in the first line, marked using arrows. if [ ]thenelsefi. The bash shell provides other programming constructs, such as for-loops, while-loops, and arithmetic expressions. Following is the syntax of Else If statement in Bash Shell Scripting. From the bash variables tutorial , you know that $(command) syntax is used for command substitution and it gives you the output of the command. Now let's look at a slightly more complex example where patterns are used a bit more. Otherwise you cannot go. Check the below script and execute it on the shell with different-2 inputs. For instance maybe we only want to perform an operation if the file is readable and has a size greater than zero. Bash IF statement is used for conditional branching in the sequential flow of execution of statements. Observe the spaces provided after if [ string literal "hello" and ==, When you run the above bash if example script file in a shell program like Terminal, the result would be, This example demonstrates on how to compare numbers, When you run the above bash script file in shell program like Terminal, the result would be. bash的if语句用法之：基本用法 从一个问题引发. Create a Bash script which will take 2 numbers as command line arguments. Before the if statement is executed, the variable count is assigned the value 5. In the following example, we demonstrate the usage of if statement with a simple scenario of comparing two strings. This is referred to as indenting and is an important part of writing good, clean code (in any language, not just Bash scripts). There aren't any rules regarding indenting in Bash so you may indent or not indent however you like and your scripts will still run exactly the same. Sometimes we may wish to take different paths based upon a variable matching a series of patterns. We can accommodate this with the else mechanism. The final else is also optional. INTEGER1 is numerically equal to INTEGER2, INTEGER1 is numerically greater than INTEGER2, INTEGER1 is numerically less than INTEGER2. eg. Former Lifewire writer Juergen Haas is a software developer, data scientist, and a fan of the Linux operating system. In case one if the condition goes false then check another if conditions. Bash if statements are very useful. What extension is given to a Bash Script File? The length of STRING is greater than zero. The following types of conditional statements can be used in bash. FILE exists and the write permission is granted. 最近我们项目碰到一个regression bug究其原因是因为对bash语言语句的理解不深刻。我们有一个脚本接收一个数字参数，指定操作的类型。 #!/bin/bash if [ $1 -eq 1 ]; then echo "do_something_1" else echo "do_other" fi ;; In this topic, we shall provide examples for some mostly used options. If you aren't but you have a letter from your parents you may go but must be back before midnight. Bash if statements are very useful. The echo statement prints its argument, in this case, the value of the variable count , to the terminal window. We could use a series of if and elif statements but that would soon grow to be unweildly. If statements, combined with loops (which we'll look at in the next section) allow us to make much more complex scripts which may solve larger tasks. ) Fortunately there is a case statement which can make things cleaner. You can have as many elif branches as you like. Otherwise, it prints the string count is not 5. #!/bin/bash # Calculate the week number using the date command: WEEKOFFSET=$[ $(date +"%V") % 2 ] # Test if we have a remainder. Look up the man page for test to see all of the possible operators (there are quite a few) but some of the more common ones are listed below. They allow us to decide whether or not to run a piece of code based upon conditions that we may set. They allow us to decide whether or not to run a piece of code based upon conditions that we may set. 'Happy hump day' for Wedensday, 'TGIF' for Friday etc). By Ryan Chadwick © 2020 Follow @funcreativity, Education is the kindling of a flame, not the filling of a vessel. It functions similarly to the if statement with multiple elif clauses but is more concise. With an if statement, which is a type of conditional statement, you can perform different actions depending on specified conditions. And if conditional statement ends with fi, For including multiple conditions with AND or OR operators. You may have as many if statements as necessary inside your script. Following example proves the same. If statements (and, closely related, case statements) allow us to make decisions in our Bash scripts. Maybe we would like to perform something slightly different if the user is either bob or andy. Get the Latest Tech News Delivered Every Day, Learn How to Properly Run Subshells Using Bash Scripts, Beginners Guide to BASH—Conditions and Variables, Hello World: Your First Raspberry Pi Project, How to Use the Linux Sleep Command to Pause a BASH Script, Beginners Guide To BASH - Part 1 - Hello World, How to Count Database Table Values With SQL COUNT, How to Use the Google Sheets If( ) Function, How to Use Test Conditions Within a Bash Script, How to Run the Bash Command Line in Windows 10. Now we could easily read from a file if it is supplied as a command line argument, else read from STDIN. Learn bash if syntax with example shell scripts. Syntax of Bash Else IF – elif. Sometimes we only want to do something if multiple conditions are met.