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A Very Valuable Find

Unless you're superbly organized or have a great memory, chances are you "misplace" files from time to time. Find answers the question, "Now, where did I put that?"

Unless you’re superbly organized or have a great memory, chances are you “misplace” files from time to time. Find answers the question, “Now, where did I put that?”

Of the many file management utilities in Linux, find is worth understanding in depth because it does so much more that just locate files. Find lets you write decision-making logic, specify filters, and run commands on the contents of entire directory trees. Unfortunately, find is not documented well, and its syntax can appear complex. But, as we’ll see, find is actually simple to use, elegant, and a great Linux power tool.

In Search Of…

More on find features soon — first, let’s look at a simple example and some find fundamentals. (If you’ve never used find or want to see more examples, see “Finding Stuff” in the April 2001 issue, or online at http://www.linux-mag.com/2001-04/newbies_01.html.)

The following command looks for all files named foo in the current directory (through the shortcut .) and all of its subdirectories:

$ find . -name foo -print
./junk/foo
./proj/2002/06/foo

The first argument to find is the pathname of the directory where find starts its search. Here we’re using . (dot), the relative path for the current directory. find searches . and all subdirectories within it. For each directory entry (including files, symbolic links, sockets, and “hidden” entries whose names start with a dot, like .mozilla) find performs a test, which is expressed with operators.

Some operators require arguments; others stand alone. find tests each directory entry against the operators, from first to last (in command line order). If any of the operators don’t match (if the result isn’t “true”), find skips the directory entry and goes on to the next.

In this case, there are two operators. -name foo is true if the file’s name is exactly foo. If true, find will evaluate the next operator, -print, to see if it’s “true.” Actually, -print is always “true,” and always prints the pathname of the current directory entry. (There’s more about -print in the sidebar “To -print or not to -print.”) As you can see, the example find command discovered two entries named foo in subdirectories of . and printed their names.




To -print or not to -print


Your Linux version of find probably doesn’t require -print. If you don’t use any “action” operator like -ok, -exec, or -print, the GNU find will add -print automatically. This might irritate people who love the beauty of find expressions, but it solves an old problem: if you forgot -print, find would seem not to have “found” anything.

Logic and Order

If you’re familiar with any programming or scripting language, you’re probably familiar with short-circuit evaluation of logical expressions. In logic, an expression “A and B” is true only if both A and B are true. If expression A isn’t true, there’s no way the whole expression can be true, so there’s no need to test B. Hence, you can “short-circuit” the process.

You can think of find‘s operators as logical expressions that are joined by invisible “and”s (and, as we’ll see later, by visible “or”s) into logical expressions. And, like a smart programming language, find short-circuits as soon as it’s obvious that an expression isn’t true.

In the previous example, you can think of -name foo as the “A” and -print as the “B”, with an implicit “and” between them. If -name foo isn’t true, then find will short-circuit the evaluation and not bother to test -print. In other words, if the file isn’t named foo, then find won’t print its name.

Combining operators to express what you want to find is the key to unlocking find‘s power. And, because find short-circuits, the order of operators is very important. For instance, what would you expect to happen if you put -print first (before -name foo)? Every pathname would be printed. Remember, -print always evaluates to true. If -print is first in a logical expression, it always prints a pathname as a side effect of being evaluated. The logical expression may short-circuit later, but that doesn’t change the output.

You can put any number of operators on the command line. For instance, to find only files named foo — not directories, and so on — add -type f (before -print, of course). Then, the -print operator will be evaluated only if both the -name and -type operators are true.

Efficient Ordering

Choosing the right order can also make find more efficient. That’s important if find is searching a lot of files. For example, testing a file’s name is easy: find already knows all the filenames in a directory. Using -exec, which executes an entirely new Linux process, is slower. So, as you order the operators for a deep search, try to think about how find might implement it.

For instance, the next example looks for every file in the /prj/reports directory whose name ends with .bak that also contains the word DRAFT:


$ find /prj/reports -type f -name ‘*.bak’ \
-exec grep -s DRAFT {} \; -print

(Backslashes and quotes are often needed when using find. For more information, see the sidebar “Special Characters: Shells vs. find.”) Let’s focus on the arguments that find gets from the shell. That rather long operator, -exec grep -s DRAFT {} ;, runs the Linux utility grep -s to search the current file, which find abbreviates as {}. The grep option -s (“silent”) tells grep not to output matching lines, but simply return an exit status of 0 if it finds the word DRAFT. An -exec operator is “true” if the program it runs returns a 0 exit status. So, if grep finds a match, it returns 0, the -exec succeeds, and then find evaluates -print, which prints the filename.




Special Characters: Shells vs. Find

The find command


$ find archive -type f \
! -name ‘*.gz’ ! -name ‘*.bz2′ \
\( -exec gzip {} \; -o -exec bas

shows an important thing about find: the backslash (\) characters. Actually, find never sees those backslashes. They’re there for the shell, which reads the command line and passes (most of) its arguments on to find. You’ll use lots of backslashes with find because find command lines tend to be long and because a lot of find‘s special characters (like * and ;) are also special to the shell.

The first backslash, at the end of the first line, tells the shell to keep reading arguments on the following line; it’s not passed on to find. On the third line, backslashes before the parentheses tell the shell not to treat them as subshell operators; the parentheses are passed on to find as grouping operators. In the same way, backslashed semicolons (;) aren’t interpreted as shell command separators; the backslashes are removed and the semicolons are passed to find.

(We also could have used backslashes before the stars (*) in the filename patterns and written \*.gz instead of ‘*.gz’. Both have the same effect: telling the shell not to interpret the star (not to expand *.gz at the time that the command line is interpreted) but to pass it on to find (so find will interpret the *.gz as it checks the name of each directory entry.)

If you’re new to the shell and the command line, all the backslashes may seem intimidating. But they’re actually just another way to extend the syntax and give the Linux command line even more power. Learn how Unix shells work — they’re one of the most important power tools.

If we had put the -type and -name operators after -exec, then find would have run grep on every entry — even on subdirectories and on files with the wrong names — which would have wasted a lot of time!

Grouping and Multiple Expressions

As mentioned above, find puts an implicit “and” operator between all operators. (You can write find . -name -foo -a -type f -a -print, but that’s not required.) Find also has “or”: it’s the -o operator. “Or” — which you always have to write explicitly — lets a test succeed if any of a chain of or-joined operators is true.

Here’s an example. If your username is ed and your group is staff, you could find all directories where you have write permission by user and group with the command shown in Figure One.




Figure One: Using an “or” operator


$ find / -type d \( -user ed -o -group staff \) -perm -220 -ls
927441 4 rwxrwxrwx 39 ed staff 4096 Jun 25 10:51 /home/ed/tmp
134001 4 rwxrwx— 2 jan staff 4096 Jun 23 18:23 /prj/adir

The parentheses are find grouping operators. (The backslashes are required to keep the shell from interpreting the parentheses.) Why do we need them? You have to tell find which comparisons to make. “And” has higher precedence than “or” — so, without the parentheses, the expression would be true either if both the -type d and -user ed tests are true, or if both the -group staff and -perm -220 tests are true. We don’t want that!

Grouping with parentheses makes the test true if -type d is true and if either -user ed or -group staff is true. Of course, the -perm -220 test (which checks for the “write” permission, bit 2, for user and group) also has to succeed before -ls can happen.

You can use this “either-or” behavior in another, less obvious way: to make a long find command line with multiple expressions, only one of which will be used for a particular directory entry. Figure Two shows an example that an administrator might run nightly, from a shell script, to clean up a directory tree.




Figure Two: Testing three things with one find command


$ find /prj -type f \
\( \
\( -name ‘*~’ -mtime +7 -exec rm -f {} \; \) -o \
\( -name core -atime +30 -exec rm -f {} \; \) -o \
\( -name ‘*.bak’ ! -perm 444 -exec chmod 444 {} \; \)
\)

What’s happening there? Let’s take it line by line:


  • The first line tells find to search the /prj tree. The -type f test is applied to all entries. If the entry isn’t a file, then short-circuit evaluation skips the rest of the test. (Remember: because there’s no operator after -type f, that’s an implicit “and”.) The second half of the “and” is the entire expression after the first grouping parenthesis.

  • The next three lines are three subexpressions with “or” (-o) operators between them. An “aerial” view looks like this: ((first-expr) -o (second-expr) -o (third-expr)). The outer parentheses surround all expressions because the expressions are also joined with “or”s, any one can be true.

  • find evaluates each subexpression, starting with first-expr. If it’s true — if the filename ends with a tilde (~) and it was modified more than 7 days ago — then the rm -f command is executed to remove the file. Assuming that rm succeeds (returns a status of 0, which the -f option makes very likely), then -exec is true, the whole subexpression is true, and find stops evaluation for this file.

  • If “first-expr” wasn’t true, find tries second-expr. Is the file named core and was it last accessed more than 30 days ago? If so, remove it and “second-expr” is true.

  • If neither of the first two subexpressions succeeds, the third can. This one looks for filenames ending in .bak (backup files, perhaps) that are not read-only for all users. (The ! operator reverses the value of the expression after it: if -perm 444 isn’t true, ! -perm 444 is true.) If the permissions aren’t 444, find executes chmod to make it so.

The advantage of these long expressions is that find traverses the directory tree only once — as opposed to your other choice, running a series of find commands that have to separately traverse the whole directory tree. The disadvantage comes if you want more than one subexpression to be executed for a particular file: joining them with “or” means that only one succeeds.

Phenomenal finds

Now that you’ve seen the most important parts of how find works, here are some examples.

1. Find uncompressed files (whose name doesn’t end with .gz or .bz2) with more than 10,000 characters, show a long listing of the file information, then ask if you want to compress the file with gzip:

$ find archive -type f \
! -name ‘*.gz’ ! -name ‘*.bz2′ \
-size +10000c -ls -ok gzip {} \;

In this example, we’re using two “action” operators. The first (-ls) always succeeds, so the second (-ok) always happens after -ls shows the file listing.

2. Same as above, but always try to compress files. If gzip fails (doesn’t return 0 status), start a shell to (optionally) fix the problem. When you type exit to end the shell, find will go on to the next entry:


$ find archive -type f \
! -name ‘*.gz’ ! -name ‘*.bz2′ \
-size +10000c \
\( -exec gzip {} \; -o -exec bash \; \)

This depends on -o: if the first -exec fails (because gzip failed), find will try to make the parenthesized expression “true” by evaluating the second -exec to run the bash shell.

3. Duplicate the subdirectory tree under the current directory (don’t copy files, just make empty subdirectories):

$ cd /prj/daily/20020626
$ find . -type d ! -name . \
-exec mkdir /prj/daily/20020627/{} \;

This example depends on find visiting a directory before its subdirectories, which it always does (unless you use the -depth operator). This actually executes commands like mkdir /prj/daily/20020627/./adir, but you can ignore the ./ (which stands for the current directory along the pathname); the result is mkdir /prj/daily/20020627/ adir). The “! -name .” prevents trying to re-create the destination directory with mkdir /prj/daily/20020627/.. Note that this last example doesn’t work on non-GNU versions of find that expect {} to stand alone.

If you’ll be using find on more than one system, be sure they all support the operators you want to use. But all versions work the way you’ve seen here: treating their operators logically, with short-circuit evaluation. If you keep that in mind, you’ll know how to use the real power of find.

Next month we’ll see what’s really behind pathnames and the current directory, and also see lots of tips for getting what you want from the filesystem.



Jerry Peek is a freelance writer and instructor who has used Unix and Linux for over 20 years. He’s happy to hear from readers at jpeek@jpeek.com.

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