What is the difference between the stdout and the stderr channels

In this case, data flows from right to left: the reassigned descriptor is assumed to be 0 at the left of the less than symbol and the data source a path to a file must be at the right of the less than symbol. The command uniq , like most command line utilities for processing text, accepts data sent to stdin by default:. The -c option makes uniq display how many times a repeated line appears in the text. To use a file descriptor other than 0 in an input redirect only makes sense in specific contexts, because it is possible for a program to ask for data at file descriptors 3 , 4 , etc.

For example, the following C code reads data from file descriptor 3 and just replicates it to file descriptor 4 :. The program must handle such file descriptors correctly, otherwise it could attempt an invalid read or write operation and crash.

To test it, save the sample code as fd. This program needs file descriptors 3 and 4 to be available so it can read and write to them. The same file descriptor can even be used as input and output. Another way to redirect input involve the Here document and Here string methods. The Here document redirect allows to type multi-line text that will be used as the redirected content.

The insertion mode will finish as soon as a line containing only the ending term is entered. Any other term can be used as the ending term, but it is important to not put blank characters between the less than symbol and the ending term. In the example above, the two lines of text were sent to the stdin of wc -c command, which displays the characters count. As with input redirects for files, the stdin file descriptor 0 is assumed if the redirected file descriptor is suppressed.

In this example, the string at the right of the three less than signs is sent to the stdin of wc -c , which counts the number of characters. An easy way to access any file is by using the unique file descriptor number associated with it.

In the case of these streams, there are unique values assigned to each one of them. The command will require input from the keyboard. Here, the read tool is getting the input from stdin. Here, the ls command lists the file s in the current directory. The list is sent to stdout and the terminal prints it out. There are different ways an error may occur. For this example, sending ls an invalid argument will result in an error. This is a common technique that takes full advantage of the stdin and stdout streams.

Here, the sign is responsible for piping. In computer programming, standard streams are interconnected input and output communication channels between a computer program and its environment when it begins execution. In Linux, stdin is the standard input stream. This accepts text as its input. Text output from the command to the shell is delivered via the stdout standard out stream. Error messages from the command are sent through the stderr standard error stream. Standard output, sometimes abbreviated stdout, refers to the standardized streams of data that are produced by command line programs i.

The basic workflow of any Linux command is that it takes an input and give an output. The standard input stdin device is the keyboard. The standard output stdout device is the screen. Show 10 more comments. Jim Lewis Jim Lewis Thanks for your inputs. Would you happen to know how I could intercept the output data stream of a process and output it into a file of my own? As a complement of the answers above, here is a sum up about Redirections: EDIT: This graphic is not entirely correct.

Anthony Smith 5 3 3 bronze badges. Leopold Gault Leopold Gault 1, 13 13 silver badges 13 13 bronze badges. Your comment combining with the accepted answer makes perfect sense and clearly explains things! A picture is worth a thousand words! Actually, that first one is just plain wrong. In no way does "hello" interact with the standard input of echo. In fact, if you replaced echo with a program that read standard input, it would just sit there waiting for your terminal input.

They both do that but in a slightly different way. You would be better off rewriting the graphic as text so you could remove that first one easily :- — paxdiablo. I'm afraid your understanding is completely backwards. So is it when the process is being executed that it writes errors onto this stderr file or when the program is being compiled from its source. Also when we talk about these files from the perspective of compiler is it different than when it is compared with say a program?

Shouvik, the compiler is just another program, with its own stdin, stdout, and stderr. When the compiler needs to write warnings or errors, it will write them to stderr. When the compiler front-end outputs intermediate code for the assembler, it might write the intermediate code on stdout and the assembler might accept its input on stdin, but all that would be behind the scenes from your perspective as a user.

Once you have a compiled program, that program can write errors to its standard error as well, but it has nothing to do with being compiled. Thanks for that token of information. I guess it's pretty stupid of me not to see it in that perspective anyway So you are saying that standard helps us to print the program — babygame0ver.

Keyboard input. Leopold Gault 1, 13 13 silver badges 13 13 bronze badges. I think people saying stderr should be used only for error messages is misleading. See this for historical rationale: "All programs placed diagnostics on the standard output. The historical rationale link is broken - the domain is gone!

Replaced with archive. Sam Sam 91 1 1 silver badge 4 4 bronze badges. Each unix program has each one of those streams. Margach Chris Margach Chris 7 7 silver badges 15 15 bronze badges.