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Going with a typical Apache access log, you can run:
tail -f access_log | grep "127.0.0.1" Which will only show you the logs (as they are created) for the specified IP address.
But why does this fail when you pipe it though grep a second time, to further limit the results?
For example, a simple exclude for ".css":
tail -f access_log | grep "127.0.0.1" | grep -v ".css" won't show any output.
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The tail command is being executed on the output of the grep command and so will print the last 4 lines of the output of grep. If for example there are 50 occurrences of "&" in input. txt, only the last 4 will print.
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 " | ".
grep -v “pattern” file. The -v flags invert the search results, as it displays those strings in file. txt which do not contain pattern .
The grep command searches a text file based on a series of options and search string and returns the lines of the text file which contain the matching search string.
I believe the problem here is that the first grep is buffering the output which means the second grep won't see it until the buffer is flushed.
Try adding the --line-buffered option on your first grep:
tail -f access_log | grep --line-buffered "127.0.0.1" | grep -v ".css" For more info, see "BashFAQ/009 -- What is buffering? Or, why does my command line produce no output: tail -f logfile | grep 'foo bar' | awk ..."
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