Added grep notes to the knowledge base

knowledge base/grep the standard error stream.md

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+# Grep the standard error stream
+
+If you're using `bash` or `zsh` you can employ anonymous pipes:
+
+```shell
+ffmpeg -i 01-Daemon.mp3 2> >(grep -i Duration)
+```
+
+If you want the filtered redirected output on `stderr` again, add the `>&2` redirection to grep:
+
+```shell
+command 2> >(grep something >&2)
+```
+
+`2>` redirects `stderr` to a pipe, while `>(command)` reads from it. This is _syntactic sugar_ to create a pipe (not a file) and remove it when the process completes. They are effectively anonymous, because they are not given a name in the filesystem.  
+Bash calls this _process substitution_:
+
+> Process substitution can also be used to capture output that would normally go to a file, and redirect it to the input of a process.
+
+You can exclude `stdout` and grep `stderr` redirecting it to `null`:
+
+```shell
+command 1>/dev/null 2> >(grep -oP "(.*)(?=pattern)")
+```
+
+> Do note that **the target command of process substitution runs asynchronously**.  
+> As a consequence, `stderr` lines that get through the grep filter may not appear at the place you would expect in the rest of the output, but even on your next command prompt.
+
+## See also
+
+- Knowledge base on [grep]
+
+## Further readings
+
+- Answer on [StackExchange] about [how to grep the standard error stream]
+
+[grep]: grep.md
+
+[stackexchange]: https://unix.stackexchange.com
+
+[how to grep the standard error stream]: https://unix.stackexchange.com/questions/3514/how-to-grep-standard-error-stream-stderr/#3657

knowledge base/grep.md

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+# Grep
+
+## TL;DR
+
+```shell
+# base search
+grep 'pattern' path/to/search
+
+# recursive search
+grep -R 'pattern' path/to/search/recursively
+grep -R --exclude-dir excluded/dir 'pattern' path/to/search/recursively  # gnu grep >= 2.5.2
+
+# show line numbers
+grep -n 'pattern' path/to/search
+```
+
+## Grep variants
+
+- [`egrep`](#egrep) to use regular expressions in search patterns, same as `grep -E`
+- [`fgrep`](#fgrep) to use patterns as fixed strings, same as `grep -F`
+- [archive-related variants](#archive-related-variants) for searching into compressed files
+- [`pdfgrep`](#pdfgrep) for searching into PDF files
+
+### Archive-related variants
+
+- [`xzgrep`](#xzgrep) (with `xzegrep` and `xzfgrep`)
+- [`zstdgrep`](#zstdgrep) for zstd archives
+- many many others
+
+### PDFgrep
+
+For simple searches, you might want to use [pdfgrep].
+
+Should you need more advanced grep capabilities not incorporated by pdfgrep, you might want to convert the file to text and search there.  
+You can to this using [`pdftotext`](pdfgrep.md) as shown in this example ([source][stackoverflow answer about how to search contents of multiple pdf files]):
+
+```sh
+find /path -name '*.pdf' -exec sh -c 'pdftotext "{}" - | grep --with-filename --label="{}" --color "your pattern"' ';'
+```
+
+## Further readings
+
+- Answer on [StackOverflow] about [how to search contents of multiple pdf files]
+- [Regular expressions in grep with examples]
+- [Grep the standard error stream]
+- Knowledge base on [pdfgrep]
+
+[grep the standard error stream]: grep\ the\ standard\ error\ stream.md
+[pdfgrep]: pdfgrep.md
+
+[stackoverflow]: https://stackoverflow.com
+
+[how to search contents of multiple pdf files]: https://stackoverflow.com/a/4643518
+[regular expressions in grep with examples]: https://www.cyberciti.biz/faq/grep-regular-expressions/