Linux/Unix “find” by example

This article is a reproduction of Wyane Pollock’s post from: http://content.hccfl.edu/pollock/Unix/FindCmd.htm
I posted this here for my own referrence, because i always “find” myself repeatedly “finding” for this document while doing some sysadmin tasks.
All rights reserved for Wayne Poolock.

 A Unix/Linux “find” Command Tutorial
 
©2002–2008 by Wayne Pollock, Tampa Florida USA.  All rights reserved.
Locating Files:

The find command is used to locate files on a Unix or Linux system.  find will search any set of directories you specify for files that match the supplied search criteria.  You can search for files by name, owner, group, type, permissions, date, and other criteria.  The search is recursive in that it will search all subdirectories too.  The syntax looks like this:

find where-to-look criteria what-to-do

All arguments to find are optional, and there are defaults for all parts.  (This may depend on which version of find is used.  Here we discuss the freely available GNU version of find, which is the version available on YborStudent.)  For example where-to-look defaults to . (that is, the current working directory), criteria defaults to none (that is, show all files), and what-to-do (known as the find action) defaults to -print (that is, display found files to standard output).

For example:

find

will display all files in the current directory and all subdirectories.  The commands

find . -print
find -print
find .

do the exact same thing.  Here’s an example find command using a search criteria and the default action:

find / -name foo

will search the whole system for any files named foo and display them.  Here we are using the criteria -name with the argument foo to tell find to perform a name search for the filename foo.  The output might look like this:

/home/wpollock/foo
/home/ua02/foo
/tmp/foo


If find doesn’t locate any matching files, it produces no output.

The above example said to search the whole system, by specifying the root directory (“/”) to search.  If you don’t run this command as root, find will display a error message for each directory on which you don’t have read permission.  This can be a lot of messages, and the matching files that are found may scroll right off your screen.  A good way to deal with this problem is to redirect the error messages so you don’t have to see them at all:

find / -name foo 2>/dev/null

You can specify as many places to search as you wish:

find /tmp /var/tmp . $HOME -name foo

Advanced Features And Applications:

The “-print” action lists the files separated by a space when the output is piped to another command.  This can lead to a problem if any found files contain spaces in their names, as the output doesn’t use any quoting.  In such cases, when the output of find contains a file name such as “foo bar” and is piped into another command, that command “sees” two file names, not one file name containing a space.

In such cases you can specify the action “-print0” instead, which lists the found files separated not with a space, but with a null character (which is not a legal character in Unix or Linux file names).  Of course the command that reads the output of find must be able to handle such a list of file names.  Many commands commonly used with find (such as tar or cpio) have special options to read in file names separated with NULLs instead of spaces.

You can use shell-style wildcards in the -name search argument:

find . -name foo\*bar

This will search from the current directory down for foo*bar (that is, any filename that begins with foo and ends with bar).  Note that wildcards in the name argument must be quoted so the shell doesn’t expand them before passing them to find.  Also, unlike regular shell wildcards, these will match leading periods in filenames.  (For example “find -name \*.txt”.)

You can search for other criteria beside the name.  Also you can list multiple search criteria.  When you have multiple criteria any found files must match all listed criteria.  That is, there is an implied Boolean AND operator between the listed search criteria.  find also allows OR and NOT Boolean operators, as well as grouping, to combine search criteria in powerful ways (not shown here.)

Here’s an example using two search criteria:

find / -type f -mtime -7 | xargs tar -rf weekly_incremental.tar
gzip weekly_incremental.tar


will find any regular files (i.e., not directories or other special files) with the criteria “-type f”, and only those modified seven or fewer days ago (“-mtime -7”).  Note the use of xargs, a handy utility that coverts a stream of input (in this case the output of find) into command line arguments for the supplied command (in this case tar, used to create a backup archive).

Using the tar option “-c” is dangerous here;  xargs may invoke tar several times if there are many files found and each “-c” will cause tar to over-write the previous invocation.  The “-r” option appends files to an archive.  Other options such as those that would permit filenames containing spaces would be useful in a “production quality” backup script.

Another use of xargs is illustrated below.  This command will efficiently remove all files named core from your system (provided you run the command as root of course):

find / -name core | xargs /bin/rm -f
find / -name core -exec /bin/rm -f ‘{}’ \; # same thing
find / -name core -delete                  # same if using Gnu find


(The last two forms run the rm command once per file, and are not as efficient as the first form.)

One of my favorite find criteria is to locate files modified less than 10 minutes ago.  I use this right after using some system administration tool, to learn which files got changed by that tool:

find / -mmin -10

(This search is also useful when I’ve downloaded some file but can’t locate it.)

Another common use is to locate all files owned by a given user (“-user username”).  This is useful when deleting user accounts.

You can also find files with various permissions set.  “-perm /permissions” means to find files with any of the specified permissions on, “-perm -permissions” means to find files with all of the specified permissions on, and “-perm permissions” means to find files with exactly permissions.  Permisisons can be specified either symbolically (preferred) or with an octal number.  The following will locate files that are writeable by “others” (including symlinks, which should be writeable by all):

find . -perm -o=w

(Using -perm is more complex than this example shows.  You should check both the POSIX documentation for find (which explains how the symbolic modes work) and the Gnu find man page (which describes the Gnu extensions).

When using find to locate files for backups, it often pays to use the “-depth” option (really a criteria that is always true), which forces the output to be depth-first—that is, files first and then the directories containing them.  This helps when the directories have restrictive permissions, and restoring the directory first could prevent the files from restoring at all (and would change the time stamp on the directory in any case).  Normally, find returns the directory first, before any of the files in that directory.  This is useful when using the “-prune” action to prevent find from examining any files you want to ignore:

find / -name /dev -prune | xargs tar …

When specifying time with find options such as -mmin (minutes) or -mtime (24 hour periods, starting from now), you can specify a number “n” to mean exactly n, “-n” to mean less than n, and “+n” to mean more than n.

Fractional 24-hour periods are truncated!  That means that “find -mtime +1” says to match files modified two or more days ago.

For example:

find . -mtime 0   # find files modified between now and 1 day ago
                  # (i.e., within the past 24 hours)
find . -mtime -1  # find files modified less than 1 day ago
                  # (i.e., within the past 24 hours, as before)
find . -mtime 1   # find files modified between 24 and 48 hours ago
find . -mtime +1  # find files modified more than 48 hours ago

find . -mmin +5 -mmin -10 # find files modifed between
                          # 6 and 9 minutes ago

Using the “-printf” action instead of the default “-print” is useful to control the output format better than you can with ls or dir.  You can use find with -printf to produce output that can easily be parsed by other utilities, or imported into spreadsheets or databases.  (See the man page for the dozens of possibilities with the -printf action.)  The following displays non-hidden (no leading dot) files in the current directory only (no subdirectories), with an custom output format:

find . -maxdepth 1 -name ‘[!.]*’ -printf ‘Name: %16f Size: %6s\n’

“-maxdepth” is a Gnu extension.  On a modern, POSIX version of find you could use this:

find . -path ‘./*’ -prune …

On any version of find you can use this more complex (but portable) code:

find . ! -name . -prune …

which says to “prune” (don’t decend into) any directories except “.”.

Note that “-maxdepth 1” will include “.” unless you also specify “-mindepth 1”.  A portable way to include “.” is:

 find . \( -name . -o -prune \) …

[This information posted by Stephane Chazelas, on 3/10/09 in newsgroup comp.unix.shell.]

As a system administrator you can use find to locate suspicious files (e.g., world writable files, files with no valid owner and/or group, SetUID files, files with unusual permissions, sizes, names, or dates).  Here’s a final more complex example (which I save as a shell script):

find / -noleaf -wholename ‘/proc’ -prune \
     -o -wholename ‘/sys’ -prune \
     -o -wholename ‘/dev’ -prune \
     -o -wholename ‘/windows-C-Drive’ -prune \
     -o -perm -2 ! -type l  ! -type s \
     ! \( -type d -perm -1000 \) -print

This says to seach the whole system, skipping the directories /proc, /sys, /dev, and /windows-C-Drive (presumably a Windows partition on a dual-booted computer).  The Gnu -noleaf option tells find not to assume all remaining mounted filesystems are Unix file systems (you might have a mounted CD for instance).  The “-o” is the Boolean OR operator, and “!” is the Boolean NOT operator (applies to the following criteria).

So this criteria says to locate files that are world writable (“-perm -2”, same as “-o=w”) and NOT symlinks (“! -type l”) and NOT sockets (“! -type s”) and NOT directories with the sticky (or text) bit set (“! \( -type d -perm -1000 \)”).  (Symlinks, sockets and directories with the sticky bit set are often world-writable and generally not suspicious.)

A common request is a way to find all the hard links to some file.  Using “ls -li file” will tell you how many hard links the file has, and the inode number.  You can locate all pathnames to this file with:

  find mount-point -xdev -inum inode-number

Since hard links are restricted to a single filesystem, you need to search that whole filesystem so you start the search at the filesystem’s mount point.  (This is likely to be either “/home” or “/” for files in your home directory.)  The “-xdev” options tells find to not search any other filesystems.

(While most Unix and all Linux systems have a find command that supports the “-inum” criteria, this isn’t POSIX standard.  Older Unix systems provided the “ncheck” utility instead that could be used for this.)
Using -exec Efficiently:

The -exec option to find is great, but since it runs the command listed for every found file, it isn’t very efficient.  On a large system this makes a difference!  One solution is to combine find with xargs as discussed above:

  find whatever… | xargs command

However this approach has two limitations.  Firstly not all commands accept the list of files at the end of the command.  A good example is cp:

find . -name \*.txt | xargs cp /tmp  # This won’t work!

(Note the Gnu version of cp has a non-POSIX option “-t” for this, and xargs has options to handle this too.)

Secondly filenames may contain spaces or newlines, which would confuse the command used with xargs.  (Again Gnu tools have options for that, “find … -print0 |xargs -0 …”.)

There are POSIX (but non-obvious) solutions to both problems.  An alternate form of -exec ends with a plus-sign, not a semi-colon.  This form collects the filenames into groups or sets, and runs the command once per set.  (This is exactly what xargs does, to prevent argument lists from becoming too long for the system to handle.)  In this form the {} argument expands to the set of filenames.  For example:

find / -name core -exec /bin/rm -f ‘{}’ +

This form of -exec can be combined with a shell feature to solve the other problem (names with spaces).  The POSIX shell allows us to use:

sh -c ‘command-line’ [ command-name [ args… ] ]

(We don’t usually care about the command-name, so “X”, “dummy”, or “inline cmd” is often used.)  Here’s an example of efficiently copying found files to /tmp, in a POSIX-compliant way (Posted on comp.unix.shell netnews newsgroup on Oct. 28 2007 by Stephane CHAZELAS):

find . -name ‘*.txt’ -type f \
  -exec sh -c ‘exec cp -f “$@” /tmp’ find-copy {} +

The find command can be amazingly useful.  See the man page to learn all the criteria and options you can use.

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