Goals for This Hour
In this hour, you learn about: In this hour, you learn sophisticated ways to find specific information on the UNIX system. The powerful find command and its partner, xargs, are the contents of this hour.Finding What's Where
The more you use UNIX, the more likely you'll end up losing track of where some of your files are. In UNIX, however, there's a cool tool to help you find them again.Task 19.1: The find Command and Its Weird Options
The grep family can help you find files by their content. There are a
lot of other ways to look for things in UNIX, and that's where the find
command can help. This command has a notation that is completely
different from all other UNIX commands: it has full-word options rather
than single-letter options. Instead of -n pattern to match filenames,
for example, find uses -name pattern.
The general format for this command is to specify the starting point for a search through the file system, followed by any actions desired. The list of possible options, or flags, is shown in Table 19.1.
Option Meaning -atime n True if file was accessed n days ago. -ctime n True if the file was created n days ago. -exec command Execute command. -mtime n True if file was modified n days ago. -name pattern True if filename matches pattern. Print names of files found. -type c True if file is of type c (as shown in Table 19.2). -user name True if file is owned by user name.
Table 19.1. Useful options for the find command.
The find command checks the specified options, going from left to right, once for each file or directory encountered. Further, find with any of the time-oriented commands can search for files more recent than, older than, or exactly the same age as a specified date, with the specifications -n, +n, and n, respectively. Some examples will make this clear.
- At its simplest, find can be used to create a list of all files
and directories below the current directory:
% find . -print . ./OWL ./OWL/owl.h ./OWL/owl ./OWL/owl.c ./OWL/simple.editor.c ./OWL/ask.c ./OWL/simple.editor.o ./OWL/owl.o
lots and lots of output removed ./dead.letter ./who.is.who ./src.listing ./tmp.listing ./.wrongwords ./papert.article - To limit the output to just those files that are C source files
(those that have a .c suffix), I can use the -name option before
the -print option:
% find . -name "*.c" -print ./OWL/owl.c ./OWL/simple.editor.c ./OWL/ask.c ./OWL/handout.c ./OWL/WordMap/msw_to_txt.c ./OWL/WordMap/newtest.c ./OWL/feedback.c ./OWL/define.c ./OWL/spell.c ./OWL/submit.c ./OWL/utils.c ./OWL/parse.c ./OWL/sendmail.c ./owl.c ./src/calc.c ./src/info.c ./src/fixit.c ./src/massage.c
Using the -name option before the -print option can be very handy. - To find just those files that have been modified in the last
seven days, I can use -mtime with the argument -7 (include the
hyphen):
% find . -mtime -7 -name "*.c" -print ./OWL/owl.c ./OWL/simple.editor.c ./OWL/ask.c ./OWL/utils.c ./OWL/sendmail.c
If I just use the number 7 (without a hyphen), I will match only those files that were modified exactly seven days ago:% find . -mtime 7 -name "*.c" -print %
To find those C source files that I haven't touched for at least 30 days, I use +30:% find . -mtime +30 -name "*.c" -print ./OWL/WordMap/msw_to_txt.c ./OWL/WordMap/newtest.c ./src/calc.c ./src/info.c ./src/fixit.c ./src/massage.c
- With find, I now have a tool for looking across vast portions of
the file system for specific file types, filenames, and so on.
To look across the /bin and /usr directory trees for filenames that contain the pattern cp, I can use the following command:
% find /bin /usr -name "*cp*" -print /usr/diag/sysdcp /usr/spool/news/alt/bbs/pcbuucp /usr/spool/news/alt/sys/amiga/uucp /usr/spool/news/comp/mail/uucp /usr/spool/news/comp/os/cpm /usr/spool/news/comp/protocols/tcp-ip /usr/man/man4/tcp.4p /usr/man/man8/tcpd.8l /usr/man/cat3f/%unitcp.3f.Z /usr/man/cat3f/unitcp.3f.Z /usr/unsup/bin/cpio /usr/unsup/gnu/man/man1/cccp.1 /usr/news/cpulimits /usr/doc/local/form/cp /usr/doc/local/form/cpio /usr/doc/local/form/rcp /usr/doc/uucp
This type of search can take a long time on a busy system. When I ran this command on my system, it took almost an hour to complete!
- To find a list of the directories I've created in my home
directory, I can use the -type specifier with one of the values
shown in Table 19.2. Here's one example:
% find . -type d -print . ./OWL ./OWL/Doc ./OWL/WordMap ./.elm ./Archives ./InfoWorld ./InfoWorld/PIMS ./Mail ./News ./bin ./src ./temp %
Letter Meaning d Directory f File l Link
Table 19.2. Helpful find -type file types. - To find more information about each of these directories, I can
use the -exec option to find. Unfortunately, I cannot simply
enter the command: the exec option must be used with {}, which
will be replaced by the matched filename, and \; at the end of
the command. (If the \ is left out, the C shell will interpret
the ; as the end of the find command.) You also must ensure that
there is a space between the {} and the \;.
% find . -type d -exec ls -ld {} \; drwx------ 11 taylor 1024 Dec 10 14:13 . drwx------ 4 taylor 532 Dec 6 18:31 ./OWL drwxrwx--- 2 taylor 512 Dec 2 21:18 ./OWL/Doc drwxrwx--- 2 taylor 512 Nov 7 11:52 ./OWL/WordMap drwx------ 2 taylor 512 Dec 10 13:30 ./.elm drwx------ 2 taylor 512 Nov 21 10:39 ./Archives drwx------ 3 taylor 512 Dec 3 02:03 ./InfoWorld drwx------ 2 taylor 512 Sep 30 10:38 ./InfoWorld/PIMS drwx------ 2 taylor 1024 Dec 9 11:42 ./Mail drwx------ 2 taylor 512 Oct 6 09:36 ./News drwx------ 2 taylor 512 Dec 10 13:58 ./bin drwx------ 2 taylor 512 Oct 13 10:45 ./src drwxrwx--- 2 taylor 512 Nov 8 22:20 ./temp - The find command is commonly used to remove core files that are
more than a few days old. These core files are copies of the
actual memory image of a running program when the program dies
unexpectedly. They can be huge, so occasionally trimming them is
wise:
% find . -name core -ctime +4 -exec /bin/rm -f {} \; %There's no output from this command because I didn't use the - print at the end of the command. What it does is find all files called "core" that have a creation time that's more than 4 days ago and remove them.
The find command is a powerful command in UNIX. It helps you find files by owner, type, filename, and other attributes. The most awkward part of the command is the required elements of the -exec option, and that's where the xargs command helps immensely.
Task 19.2: Using find with xargs
You can use find to search for files, and you can use grep to search
within files, but what if you want to search a combination? That's where
xargs is helpful.
- A few days ago, I was working on a file that was computing
character mappings of files. I'd like to find it again, but I
don't remember either the filename or where the file is located.
First off, what happens if I use find and have the -exec argument call grep to find files containing a specific pattern?
% find . -type f -exec grep -i mapping {} \; typedef struct mappings { map_entry character_mapping[] = { int long_mappings = FALSE; case 'l': long_mappings = TRUE; if (long_mappings) /** do a short mapping **/ /** do a long mapping **/ /** Look up the specified character in the mapping database **/ while ((character_mapping[pointer].key < ch) && (character_mapping[pointer].key > 0)) if (character_mapping[pointer].key == ch) return ( (map_entry *) &character_mapping[pointer]); # map,uucp-map = The UUCP Mapping Project = nca-maps@apple.com grep -i "character*mapping" * */* */*/* to print PostScript files produced by a mapping application that [ccc]runs on the bionet.genome.chromosomes Mapping and sequencing of [ccc]eucaryote chromosomes. ./bin/my.new.cmd: Permission denied typedef struct mappings { map_entry character_mapping[] = { int long_mappings = FALSE; case 'l': long_mappings = TRUE; if (long_mappings) /** do a short mapping **/ /** do a long mapping **/ /** Look up the specified character in the mapping database **/ while ((character_mapping[pointer].key < ch) && (character_mapping[pointer].key > 0)) if (character_mapping[pointer].key == ch) return ( (map_entry *) &character_mapping[pointer]); or lower case values. The table mapping upper toThe output is interesting, but it doesn't contain any filenames! - A second, smarter strategy would be to use the -l flag to grep
so that grep specifies only the matched filename:
% find . -type f -exec grep -l -i mapping {} \; ./OWL/WordMap/msw_to_txt.c ./.elm/aliases.text ./Mail/mark ./News/usenet.alt ./bin/my.new.cmd: Permission denied ./src/fixit.c ./temp/attach.msg - That's a step in the right direction, but the problem with this
approach is that each time find matches a file, it invokes grep,
which is a very resource-intensive strategy. Instead, you use
the xargs to read the output of find and build calls to grep
(remember that each time a file is seen, the grep program will
check through it) that specify a lot of files at once. This way,
grep is called only four or five times even though it might
check through 200 or 300 files. By default, xargs always tacks
the list of filenames to the end of the specified command, so
using it is as easy as can be:
% find . -type f -print | xargs grep -l -i mapping ./OWL/WordMap/msw_to_txt.c ./.elm/aliases.text ./Mail/mark ./News/usenet.alt ./bin/my.new.cmd: Permission denied ./src/fixit.c ./temp/attach.msg
This gave the same output, but it was a lot faster. - What's nice about this approach to working with find is that
because grep is getting multiple filenames, it will
automatically include the filename of any file that contains a
match when grep shows the matching line. Removing the -l flag
results in exactly what I want:
% ^-l^ find . -type f -print | xargs grep -i mapping ./OWL/WordMap/msw_to_txt.c:typedef struct mappings { ./OWL/WordMap/msw_to_txt.c:map_entry character_mapping[] = { ./OWL/WordMap/msw_to_txt.c:int long_mappings = FALSE; ./OWL/WordMap/msw_to_txt.c: case 'l': long_mappings = TRUE; ./OWL/WordMap/msw_to_txt.c: if (long_mappings) ./OWL/WordMap/msw_to_txt.c: /** do a short mapping **/ ./OWL/WordMap/msw_to_txt.c: /** do a long mapping **/ ./OWL/WordMap/msw_to_txt.c: /** Look up the specified character in [ccc]the mapping database **/ ./OWL/WordMap/msw_to_txt.c: while ((character_mapping[pointer].key ./src/fixit.c: /** do a long mapping **/ ./src/fixit.c: /** Look up the specified character in the [ccc]mapping database **/ ./src/fixit.c: while ((character_mapping[pointer].key < ch) && ./src/fixit.c: (character_mapping[pointer].key > 0)) ./src/fixit.c: if (character_mapping[pointer].key == ch) ./src/fixit.c: return ( (map_entry *) &character_mapping[pointer]); ./temp/attach.msg:or lower case values. The table mapping upper to
When used in combination, find, grep, and xargs are a potent team to help find files lost or misplaced anywhere in the UNIX file system. I encourage you to experiment further with these important commands to find ways they can help you work with UNIX.
Summary
The find command is one of the more potent commands in UNIX. It has a lot of esoteric options, and to get the full power out of find, xargs, and grep, you need to experiment.Workshop
This Workshop poses some questions about the topics presented in this chapter. It also provides you with a preview of what you will learn in the next hour.Questions
- Use find and wc -l to count how many files you have. Be sure to
include the -type f option so that you don't include directories
in the count.
- Use the necessary commands to list the following:
- ALL filenames that contain abc
- ALL files that contain abc
