Performing recursive searches

BTW, these examples are all untested so far, but I do this stuff pretty often .

Why you might want to perform a recursive search:

The case study below talks about using recursive searches to locate someone's "lost" e-mail.
Another very important use is for finding unresolved externals in a large source tree, or under /usr/include or similar, but see also my find-sym program.
Another interesting possibility is to use a binary-capable grep like GNU grep to do a recursive search of programs under /bin, /usr/bin, etc. to locate the program that is outputting an error message, when it isn't clear where that error message is coming from. However, with *ix's increasing localization features, where a list of messages from a program are stored in separate files in a large number of languages, the usefulness of this method is decreasing.

A variety of examples of recursive searches

Using "locating lost e-mail" as a sort of case study:
1. Using GNU grep (disadvantage: May get stuck on named pipes. But the obvious advantage is convenience: there's simply less to type) :
  - cd ~username
  - grep -iRl '^Subject:.*unique subject'
2. Using a traditional grep with traditional xargs (GNU grep and GNU xargs should traditional usage as well):
  - cd ~username
  - find . -type f -print | xargs grep -il '^Subject:.*unique subject'
3. Using GNU find and xargs to contend with filenames containing whitespace:
  - cd ~username
  - find . -type f -print0 | xargs -0 grep -il '^Subject:.*unique subject'
4. Using agrep to conduct a fuzzy search with traditional find and xargs syntax:
  - cd ~username
  - find . -type f -print | xargs agrep -2 -il '^Subject:.*unique subject'
5. Using a traditional find and xargs to search a user's home directory, except for their ~/src and ~/bin directories:
  - find . -name bin -prune -o -name src -prune -o -type f -print | xargs egrep -il '^From:.*unique sender'
Using changing a hostname and/or IP address as an example. I'll be assuming you want to change hostname old1.old2.co.com and IP address 128.200.34.3 with a netmask of 255.255.255.0. If you follow what's happening, you'll most likely be able to do this for different kinds of changes too by changing what you search for slightly:
1. First, fix the hostname:
  - vi `find /etc -type f -print | xargs grep -il old1\.old2\.co\.com`
  - ...and go through and change the occurrences of old1.old2.co.com to new2.new2.co.com. You can use :n to go to the next file
2. Second, fix the ip address and/or routes:
  - vi `find /etc -type f -print | xargs grep -il '128\.200\.34\.'`
  - ...and go through and change occurrences of your old IP address to your new one Again, you can use :n to go to the next file
3. Note on *ix's this is known to work on, and known not to work on:
  - Won't work: AIX, because it stores this information in a database called the "ODM", rather than using the traditional flat files under /etc. The ODM is a bit like "the registry" on windows.
  - Will work: Just about any *ix that uses traditional flat files for these things under /etc, including Solaris, Linux, IRIX and Tru64, and probably just about all of the others
Getting feedback on how long your recursive grep is going to take.
- Traditional find+xargs when you know the number of files isn't huge:
  - find . -type f -print | count -b | xargs egrep -il pattern > /tmp/results
- Traditional find+xargs when you know the number of files is quite large:
  - find . -type f -print | count -e $(find . -type f -print | wc -l) | xargs egrep -il pattern > /tmp/results
- GNU find+xargs, when you know some of your filenames will have whitespace in them:
  - find . -type f -print0 | reblock -p -e $(expr $(find . -type f -print | wc -c) / 1024) $(expr 1024 \* 1024) | xargs -0 egrep -il pattern > /tmp/results
  - Actually, this method could be used with traditional find+xargs too, for a bit of a performance boost on huge numbers of files.