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First steps / Stage 11 of 15

Where a command's output goes

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Step 6 of 6 · summary · 2 min

Summary

  • A command prints its result to stream 1 and its error messages to stream 2; the terminal shows both interleaved.
  • > and >> redirect stream 1, 2> and 2>> stream 2. A single sign wipes the file before the command starts; a double one appends to the end.
  • Both streams in one file is > file 2>&1, and the order matters. An unwanted stream goes to /dev/null once you know what is in it.
  • At the start of the lesson the access refusals went to stream 2, and > took only stream 1 into the file — that is why the messages stayed on the screen.

Found an inaccuracy? Report an error in the lesson

Count the errors in the logs

Reference for this lesson

Articles

  • Output redirection and pipelines — A shell instruction that connects a command's stream to a file or another descriptor: > overwrites a file, >> appends, < reads from a file, and 2>&1 sends stderr wherever stdout goes.
  • find — searching for files — Finding files by name, size, time, permissions; acting on what's found.
  • Disk space: df, du, inode — Where the space and inodes went — df, du, finding large files, pitfalls.
  • head — the first lines of a file — The head command prints the beginning of a file or of command output — 10 lines by default, -n sets how many.
  • grep — searching text — The grep command finds lines matching a pattern in files and command output: regex, recursion, context.
Ubuntu 24.04
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