To demystify the difference between command line arguments and stdin. I am going to divide my answer into multiple parts explaining the underling concepts.
Then answer your question. Standard Input is a file Linux which is accessed by programs to take input from the user. Any program cannot directly interact with Keyboard instead they interact with stdin. So if a program needs to take an input from user it will access stdin. When you specify these special characters they instruct stdin and stdout how to function.
Command line arguments are basically a method to take input from used along with the invocation of command to make the command do some specific task.
It is not necessary for each and every program to have command line arguments , as it is up to the developer of the following program to use them. Command line arguments are passed to a command as an array. Array is a data structure consisting of a collection of elements, each identified by at least one array index or key.
Hence the values in that array can be used by the programmer to add more functionality to their program. In the above program, the program does not take any standard input from user however it does take command line arguments and works differently depending upon the argument is provided or not. After that array will contain the arguments passed. Now this does not mean this program cannot take input. Below is modified program which takes both command line argument and standard input.
The above takes a command line argument then wait for user to enter password through standard input. If we don't specify any argument it will throw an error just like in previous C code. Below is the output when program is executed :.
After passing the command line argument this program waited for password which was supposed to be entered by the user, hence through stdin. If you didn't understand the C program it's alright. This function gets the name of the file to process from the first element returned by commandArgs. Hint: Everything argument read from the command-line is interpreted as a character string.
You can convert from a string to a number using the function as. The next step is to teach our program how to handle multiple files. Using small data files as input also allows us to check our results more easily: here, for example, we can see that our program is calculating the mean correctly for each line, whereas we were really taking it on faith before. This is yet another rule of programming: test the simple things first.
We want our program to process each file separately, so we need a loop that executes once for each filename. Note : at this point, we have created three versions of our script called readings R , readings R , and readings R that we committed to version control every time we got an enhancement working.
For teaching, though, we need all the successive versions side by side. Write a program called check. R that takes the names of one or more inflammation data files as arguments and checks that all the files have the same number of rows and columns.
What is the best way to test your program? The next step is to teach our program to pay attention to the --min , --mean , and --max flags. Silent failures like this are always hard to debug. This version pulls the processing of each file out of the loop into a function of its own. It also uses stopifnot to check that action is one of the allowed flags before doing any processing, so that the program fails fast.
This is four lines longer than its predecessor, but broken into more digestible chunks of 8 and 12 lines. R has a package named argparse that helps handle complex command-line flags it utilizes a Python module of the same name.
With each word as one element in the array. How the program, you are going to invoke interprets the arguments located in the array depends on how it is programmed. There are some quasi standards of how the syntax of the arguments should look like but in general the programmer is entire free.
So the first argument can be interpreted as a name of a file or whatever the programmer thoughts of at the time he wrote the program.
A shell is a program that reads commands that are typed on a keyboard and then executes i. A process is an instance of a program in execution. This terminology can be confusing to new users. However, it is useful to become familiar with it because it is commonly employed in the documentation for Unix-like operating systems, including the online man i.
Users familiar with the C programming language , in which the Linux kernel i. Examples include printf , which allows text to be sent to standard output, and scanf , which allows a program to read from standard input. Standard input is by default any text entered from the keyboard. It is the counterpart of standard output and standard error, which are by default the display monitor. Standard input can be redirected through the use of arguments and redirection operators so that it becomes a source other than the keyboard just as standard output and standard error can be redirected so that they go to a destination other than the display monitor.
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