[Unix] pipe error to less, grep, tail

 

默认是bash

 

To pipe only the error output (stderr) of a command to tail on a Unix system, use this syntax:

command 2>&1 >/dev/null | tail

Explanation:

• command is any Unix command.

• 2>&1 redirects stderr (file descriptor 2) to stdout (file descriptor 1).

• >/dev/null discards the original stdout, so only stderr is left.

• The result is that only stderr is passed to tail.

Example:

ls /nonexistent 2>&1 >/dev/null | tail

This will show only the error message from ls (since /nonexistent does not exist) and pipe it through tail.

If you want to see both stdout and stderr, just do:

command 2>&1 | tail

Want to pipe stderr and stdout separately or to different places?

 

If you want to pipe stdout and stderr separately (e.g., to different commands or files), here are several canonical idioms in Unix shell (assuming bash or similar Bourne-style shell):

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1. Send stdout to one command, stderr to another

You can use process substitution:

command > >(stdout_handler) 2> >(stderr_handler)

Example:

make > >(tee stdout.log) 2> >(tee stderr.log >&2)

• make runs

• stdout goes to tee, which saves to stdout.log

• stderr goes to another tee, which saves to stderr.log

• >&2 sends tee's output back to stderr so it behaves correctly

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2. Pipe only stderr to tail and discard stdout

command 2>&1 1>/dev/null | tail

As discussed previously.

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3. Pipe stdout to tail, discard stderr

command 2>/dev/null | tail

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4. Send stderr to a file, stdout to another

command > out.log 2> err.log

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5. Interleave both stdout and stderr, and pipe them together

command 2>&1 | tee combined.log | tail

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Unix shell redirection is exacting and low-level; once you learn the logic of file descriptors (0: stdin, 1: stdout, 2: stderr), you can route anything anywhere. Need an example with grep, awk, or curl?

 

例如：

ffprobe abc.mp3 2>&1 1>/dev/null | tail -4