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What is stdout pipe?
Standard output and input. One of the most significant consequences of pipes in Unix is that Unix programs, whenever possible, are designed to read from standard input (stdin) and print to standard output (stdout). These jargony terms refer to streams of data, standardized as plain text.
What is pipe in subprocess?
A common use of pipes is to send data to or receive data from a program being run as a subprocess. It executes the shell command command as a subprocess. However, instead of waiting for the command to complete, it creates a pipe to the subprocess and returns a stream that corresponds to that pipe.
Why is it important to pipe stderr, and not stdout?
This is important because since the pipe was set up first, the FD1 (left side) and FD0 (right side) are already changed from what they might normally have been, and any duplication of these will reflect that fact. So, all output that “command” writes to its FD 2 (stderr) makes its way to the pipe and is read by “grep” on the other side.
What’s the difference between stdout and stderr in C?
One of the differences between stdout and stderr is the level of buffering. In §7.21.3 Files ¶7, the C11 standard says:
How to combine stderr and stdout in Bash?
To combine stdout and stderr you would redirect the latter to the former using 2>&1. This redirects stderr (file descriptor 2) to stdout (file descriptor 1), e.g.: $ { echo “stdout”; echo “stderr” 1>&2; } | grep -v std stderr $ stdout goes to stdout, stderr goes to stderr. grep only sees stdout, hence stderr prints to the terminal.
How does grep see both stdout and stderr?
stdout goes to stdout, stderr goes to stderr. grep only sees stdout, hence stderr prints to the terminal. On the other hand: After writing to both stdout and stderr, 2>&1 redirects stderr back to stdout and grep sees both strings on stdin, thus filters out both. You can read more about redirection here.