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Is that possible to use grep on a continuous stream?
What I mean is sort of a tail -f <file> command, but with grep on the output in order to keep only the lines that interest me.
I've tried tail -f <file> | grep pattern but it seems that grep can only be executed once tail finishes, that is to say never.
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1 Answer. When using non-interactively, most standard commands, include grep , buffer the output, meaning it does not write data immediately to stdout . It collects large amount of data (depend on OS, in Linux, often 4096 bytes) before writing.
grep is very often used as a "filter" with other commands. It allows you to filter out useless information from the output of commands. To use grep as a filter, you must pipe the output of the command through grep . The symbol for pipe is " | ".
I use the tail -f <file> | grep <pattern> all the time.
It will wait till grep flushes, not till it finishes (I'm using Ubuntu).
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Turn on grep's line buffering mode when using BSD grep (FreeBSD, Mac OS X etc.)
tail -f file | grep --line-buffered my_pattern It looks like a while ago --line-buffered didn't matter for GNU grep (used on pretty much any Linux) as it flushed by default (YMMV for other Unix-likes such as SmartOS, AIX or QNX). However, as of November 2020, --line-buffered is needed (at least with GNU grep 3.5 in openSUSE, but it seems generally needed based on comments below).
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