The if condition is the most commonly used conditional statement in bash and other general-purpose programming languages. The if condition evaluates the given condition and makes the decision. The given condition is also called a test expression. There are numerous ways to use the if condition in bash.
The if condition is used with else block. In case, if the given condition is true, then the statements inside the if block is executed, otherwise the else block is executed. There are multiple ways of using the if condition statement in Bash which are the following:.
The if statement The if statement only evaluates the given condition, if the given condition is true, then the statements or commands inside the if block is executed, otherwise the program is terminated. In bash, if the condition starts with the if keyword and ends with the fi keyword. The then keyword is used to define the block of statements or commands which execute when a certain condition is true.
The -gt is used to evaluate the greater than condition whereas, the -lt is used to evaluate the less than condition. The if else statement The if else statement is also used as a conditional statement. The statements or commands after the if condition is executed if the given condition is true. Otherwise, the else block is executed if the given condition is not true.
The else block is followed by the if block and starts with the else keyword. Multiple conditions can be evaluated by using the if condition.
The nested if statement In nested if statement, we have an if statement inside an if statement. The first if statement is evaluated, if it is true then the other if statement is evaluated. The if elif statement The if elif statement is used to evaluate multiple conditions. The first condition starts with the if block and the other conditions are followed by the elif keyword.
The eq is used as an equal operator. Looping Loops are the essential and fundamental part of any programming language. Unlike, the other programming languages, the loops are also used in Bash to perform a task repeatedly until the given condition is true.
The loops are iterative, they are a great tool for automation of similar types of tasks. The for loop, while loop, and until loop is used in Bash. The while loop The while loop executes the same statements or commands repeatedly. It evaluates the condition, and run the statements or commands until the condition is true. This is the basic syntax of using a while loop in Bash. We have a variable VAR whose value is equal to zero. In the while loop, we have put a condition, that the loop should run until the value of the VAR is less than The variable value is incremented by 1 after each iteration.
So, in this case, the loop will start executing until the variable value is less than The for loop The for loop is the most commonly used loop in every programming language. It is used to execute the iterative task. It is the best way to perform repetitive tasks. The until loop The other type of loop that is used in Bash is until loop. It also performs or executes the same set of repeatedly. The until loop evaluates the condition and start executing until the given condition is false. The until loop terminates when the given condition is true.
The syntax of the until loop is as follows:. The until loop will run unless the condition is false The value of the variable is less than Reading from the user and writing it on screen The Bash gives the liberty to the user to enter some string value or data on the terminal. The user entered string or data can be read from the terminal, it can be stored in the file, and can be printed on the terminal. In the Bash file, the input from the user can be read using the read keyword and we store it in a variable.
The variable content can be displayed on the terminal by using the echo command. Multiple options can be used with the read command. The most commonly used options are -p and -s. The -p displays the prompt and the input can be taken in the same line.
The —s takes the input in the silent mode. The characters of the input are displayed on the terminal. It is useful to enter some sensitive information i. Reading and writing text files Text files are the essential components to read and write the data.
The data is stored in the text files temporarily and it can be read from the text file easily. Although Bash is primarily a command interpreter, it's also a programming language. Bash supports variables, functions and has control flow constructs, such as conditional statements and loops.
However, all of this comes with some unusual quirks. This is because Bash attempts to fulfill two roles at the same time: to be a command interpreter and a programming language—and there is tension between the two.
This trait has a deep history, stretching all the way to the very first shell and the first UNIX system. Over time, UNIX shells acquired the programming capabilities by evolution, and this has led to some unusual solutions for the programming environment. As many people come to Bash already having some background in traditional programming languages, the unusual perspective that Bash takes with programming constructs is a source of much confusion, as evidenced by many questions posted on Bash forums.
In this article, I discuss how programming constructs in Bash differ from traditional programming languages. For a true understanding of Bash, it's useful to understand how UNIX shells evolved, so I first review the relevant history, and then introduce several Bash features.
The majority of this article shows how the unusual aspects of Bash programming originate from the need to blend the command interpreter function seamlessly with the capabilities of a programming language. Figure 1. Between —, John R. Mashey extended the original Thompson shell and added several programming capabilities, making it a high-level programming language.
In Mashey's own words:. Modifications have been aimed at improving the use of the shell In line with the philosophy of much existing UNIX software, an attempt has been made to add new features only when they are shown necessary by actual user experience in order to avoid contaminating a compact, elegant system through "creeping featurism". From J. Stephen Bourne started working on a new shell early in The Bourne shell benefited from the concepts introduced by the Mashey shell, and it brought some new ideas of its own.
The original Thompson shell, the Mashey shell and the Bourne shell were all called sh, and they overlapped or replaced one another in the years — as they were refined and gained additional capabilities. With the development of UNIX, shells were constantly developed and refined.
At the time when the Bourne shell already was in use, Bill Joy at Berkeley developed the C shell csh. In the early s, David Korn developed the Korn shell ksh.
Compared to the Bourne shell, the C shell emphasized the command interpreter mode, and the Korn shell came with more extensive programming capabilities. In fact, the original letter from Stallman, sent on the net. Brian Fox, the Free Software Foundation's first paid programmer, started working on a shell This became Bash, first released as beta in Bash is mostly a clone of the Bourne shell hence "Bourne-Again" , but it also includes additional features inspired by the C shell and Korn shell.
Brian Fox was the official maintainer of Bash until At the time, Chet Ramey already was involved with the work on Bash, and he became the official maintainer in Chet Ramey continued to maintain and develop Bash for the next 25 years, and he's still Bash's current maintainer.
The original Thompson shell was a simple command interpreter whose mode of operation was as follows:. The Thompson shell had no programming capabilities. This changed with the development of the Mashey shell and later the Bourne shell.
The UNIX shell is both a programming language and a command language. As a programming language, it contains control-flow primitives and string-valued variables. As a command language, it provides a user interface to the process-related facilities of the UNIX operating system.
Note the emphasis on the different functionality: a programming language and a command language. In fact, it was the Mashey and Bourne shells that extended the capabilities of the Thompson shell beyond the command interpreter. The shell's original role was a command interpreter, and the programming capabilities of shells were added later.
UNIX shells evolved some ingenious ways of consolidating the programming capabilities with the original command interpreter role. Today's Bash is more powerful compared to the original Mashey shell and the Bourne shell. However, the purpose of the shell remains exactly the same. Arguably, the most important function of the shell is running commands that is, submitting an executable file to the kernel for execution. This has several profound ramifications. For a start, Bash treats almost anything that is given to it as a command.
Consider the following Bash session:. This shows that Bash splits the input into words, then attempts to execute the first word as a command the "words" VAR and 9. It's important to note that Bash keeps all words as strings and has no concept of numbers until forced to do an arithmetic evaluation. As a rather simplified summary, Bash operates as follows:. This view ignores several intermediate steps. For example, Bash scans the input line and performs all sorts of expansions and replacements.
It also checks for a built-in command with the name given, and executes that, if it exists. Not to lose sight of the big picture, I often ignore these details. So, Bash's most essential purpose is to execute commands, and this has some profound implications. Notably, the programming constructs in Bash, which at first sight may look like a programming language, are derived from this mode of operation.
And, that is the central theme of this article. The point that's often confusing to Bash newcomers is the difference between Bash built-in commands and external commands. However, there are also subtle differences try echo --version. Why this duplication of commands? There are several reasons. The built-in version typically exists for performance reasons: Bash built-ins execute within the shell process that's already running.
In contrast, executing an external utility involves loading and executing the external binary by the kernel, which is a much slower process. At this point, it's useful to note that some shell commands, by their nature, cannot be external utilities in other words, they must be shell built-ins. Consider the cd command that changes the current working directory. An external utility wouldn't be able to change the shell's current working directory, so cd must be a Bash built-in.
Because invoking a command as an external utility would make the shell its parent process, and a child process cannot change the current working directory of the parent process.
A practical problem users often face is this: how do you know whether the command you just called is the shell built-in or an external utility with the same name? The Bash command type which is itself a shell built-in indicates what command would be used if executed. For example:. The basic rule is as follows: if the built-in command with a given name exists, it will be executed. If the built-in command doesn't exist, Bash will search for an external program, and if found, will execute it.
If you want to be sure to use the executable, which happens to have the same name as a shell built-in, calling the executable with the full path will do. When a command is entered in Bash, Bash expects that the first word it encounters is a command.
This has assigned the value 7 to the variable named VAR. To retrieve the value of a variable, you need to prefix the variable name with the dollar sign. Thus, to view the value of a variable, you can combine the dollar-sign prefix with echo :. The following will fail:. This clearly isn't what was intended here. Although Bash allows you to create arbitrary variables on the fly simply by assigning the values to them, it also has a number of built-in variables.
This contains the process ID of the Bash shell itself:. Be sure that you have completed the instructions on Setting up Git, Bash, and Conda on your computer to install the tools for your operating system Windows, Mac, Linux. In the early days of computing, the computer that processed data or performed operations was separate from the tool that gave it the instructions to do the processing.
There was:. Today, there are computers that can both provide commands AND perform the computation, and these computers have graphical user interfaces known as GUIs that make it easy to perform tasks. However, accessing the command line or terminal can often be more efficient than using GUIs for certain tasks, and you can send commands via the terminal to programmatically accomplish these tasks.
For example, working with files in the terminal is faster and more efficient than working with files in a graphic environment like Windows Explorer or the Finder on a MAC. You can also use the terminal to launch and execute open reproducible science tools such as Jupyter Notebook , Python , and git , which you will use throughout this textbook. In the chapter on Open Reproducible Science, you learned that Shell is the primary program that computers use to receive code i. These commands can be entered and executed via the terminal.
For example, you can use Bash to perform operations on multiple files quickly via the command line. You can also write and execute scripts in Bash , just like you can in R or Python , that can be executed across different operating systems. Sometimes you will hear Bash and Shell used interchangeably; this textbook uses the term Bash. Using Bash in the Terminal is a powerful way of interacting with your computer.
GUIs and command line Bash are complementary—by knowing both, you will greatly expand the range of tasks you can accomplish with your computer. With Bash commands, you will be able to perform many tasks more efficiently and automate and replicate workflows across different operating systems. Common tasks that you can run at the command line include checking your current working directory, changing directories, making a new directory, extracting files, and finding files on your computer.
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