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:. Another built-in variable and one that I cover extensively here is? At any point in a Bash session, this variable contains the return value of the last executed command.
The return value is always an integer. And specifically, this is the return value of the C program function main. Note: in any C program the function main must return an integer. By the UNIX convention, the return value of 0 denotes success, and any other value denotes failure. In the last example, note that the first? Because the second? Remember, the? You can use the commands true and false to set the value of? Now let's consider how Bash provides an impression of a seamlessly integrated command environment, even when the tasks it executes are inherently quite different.
First, note that running a Bash built-in command produces the same effect on the? This example shows that calling the built-in command echo changed? The point is that it behaves the same as calling the external program echo :. Yet, these two scenarios are quite different. The effect on the? From this, you can see that Bash treats a variable assignment as a command. If the variable assignment is not successful,? So this will fail:. Attempting to execute a non-existent command would also set?
In this case, Bash filled the special variable? This number is hard-wired in Bash, and it specifically means "command not found". To summarize, the above examples show three completely different scenarios: invoking an internal Bash command, running an external program and variable assignment.
Yet, Bash views all three as command execution and provides a common behavior with respect to the? Armed with these insights, now let's examine three basic programming constructs in Bash: the if statement, the while loop and the until loop. The fundamental element of almost every programming language is the conditional if statement. In the C language, it looks like this:.
This is sometimes called "truth value testing". Here's an example of this in Python:. This looks very much like the if conditional statement in any programming language.
However, it's not. In the above example, true is a command. In fact, true is a shell built-in:. Let that sink in: true is a command. In fact, that's the same true command that was run above from the command line to set the value of the? What then is the if statement evaluating? It's evaluating the return value of the true command.
Notice how the blank line was printed before the string Yay true! You actually can give an argument to the echo command:. Clearly, the second echo could be replaced with the external utility too. Moving on, I mentioned previously that Bash will treat the variable assignment as a command.
Thus, variable assignment can be used in the same place as the built-in command or external executable:. This has a good answer, I vote we leave it open. Add a comment. Active Oldest Votes. Just to add a bit more to this, there are many other shells to try, if you are interested, here's a few from this answer : Zsh has more advanced interactive facilities, but a few quirks when it comes to scripting less so now than back in the days. Improve this answer. Community Bot 1. Mark Kirby Mark Kirby When you copy-paste things from elsewhere, you're supposed to clearly indicate that this is the case.
What is the source of your comment that zsh has a few quirks with regard to scripting? Never mind, I see that the comment came from elsewhere. You also have fish which is incompatible with the sh syntax. Would you explain what was deficient about the Korn shell editing? It always seemed to work OK for me, even in the 90s. Show 3 more comments. Scooter Scooter 1 1 gold badge 8 8 silver badges 23 23 bronze badges. You do choose them when you want to quickly get simple stuff done.
For this, you need: Concise syntax, and enough consistency so you can actually memorise it it's hard to look up shorthand operators. It matters a lot if your shell is installed everywhere, because you really rather wouldn't memorise more than one of these kludge monsters.
Good interactive features, so you can hack right into the terminal and get stuff working without having to switch between multiple script files. The ability to grab any script snippet from anywhere and have it just work. Any language that cares about amounts of whitespace is not "well designed", but I agree that popularity is the key. Nagora: opinions vary. FWIW, you can always write Haskell with braces and semicolons instead of whitespace, and I suppose Python has similar fallbacks.
Only, nobody does this, because indentation-based grouping is just amazing for readability. It was there when it was needed It has enough eye candy that beginning users can spend a week customizing their prompt.
To know the bash version you can execute following commands in terminal: bash --version. Why is bash the default shell in Linux? Category: technology and computing operating systems. There are several reasons for changing a user's shell in Linux including the following: To block or disable normal user logins in Linux using a nologin shell. How do I check my default bash? The default shell runs when you open a terminal window.
How do I change shell in Linux? To change your shell with chsh:. Enter chsh for "change shell". Type in the path and name of your new shell. Type in su - and your userid to relog in to verify that everything works correctly. How do I know my shell? To find the current shell instance, look for the process shell having the PID of the current shell instance.
For example: I have bash as my default shell , which I use for my Terminal App. How do I find the bash shell in Linux? If you want to change the shell temporarily, then just type the name of the shell. Brace expansion was a feature that was missing from the Bourne shell that was implemented in Bash as a superset of the functionality found in the C shell. Arrays were improved by removing their size limits.
Parameter expansion in the command prompt allows users to customize their Bash prompt. Because of its long service life and massive user base, Bash is mature and very stable. There are many alternative shells available, from veterans like the C shell and the KornShell to newer shells like the Z shell zsh and the Friendly Interactive Shell fish. So why is Bash still the dominant shell? Unix machines, yes, but Linux boxes, no. That familiarity lets you get to work quickly and be effective straight away.
Using a shell that is—or is trying very hard to be—POSIX compliant matters to many Linux distributions, but what matters more is compatibility with previous releases. Making changes that could break existing scripts is obviously unattractive. Attractive or not, sometimes, you just have to bite the bullet. On Sept. At a. What might seem inconceivable now can actually happen later. Or maybe it will be Bash, but enhanced far beyond the shell that we use today.
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