[grep] grep stream 2, the error message

 

In Unix-like systems, stdout (standard output) is stream 1, and stderr (standard error) is stream 2.
By default, grep reads from stdin, which typically receives stdout — not stderr.

To have grep search the error stream (stderr) instead of stdout, you must redirect stderr (2) into stdin (0), like this:

command 2>&1 >/dev/null | grep "pattern"

Let’s break that down precisely:

• command — the command whose stderr you want to inspect.

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

• >/dev/null — discards the original stdout so that only stderr ends up going into the pipe.

• | grep "pattern" — filters the combined (now pure stderr) stream for the given text pattern.

Example

gcc bad_code.c 2>&1 >/dev/null | grep "error"

This will show only error messages (from stderr) that contain the word “error”.

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If you want to only redirect stderr to grep (without destroying stdout), the canonical, cleaner form is:

command 2> >(grep "pattern")

This uses process substitution (a Bash feature) to send just the stderr stream to grep, leaving stdout untouched.

Example

gcc bad_code.c 2> >(grep "warning")

→ grep filters warnings from stderr, while stdout (if any) still prints normally.

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Summary of key variants

Purpose	Command
Search stderr only, discard stdout	`command 2>&1 >/dev/null
Search stderr only, keep stdout intact	command 2> >(grep "pattern")
Search both stdout and stderr together	`command 2>&1

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Would you like me to show how to capture the filtered stderr lines in a file as well (for logging or diagnostics)?