Finding Things

Lesson

You can guess someone's age by how they talk about search: young people use "Google" as a verb, while crusty old Unix programmers use "grep". The word is a contraction of "global/regular expression/print", a common sequence of operations in early Unix text editors. It is also the name of a very useful command-line program.

grep finds and prints lines in files that match a pattern. For our examples, we will use a file that contains three haikus taken from a 1998 competition in Salon magazine:

The Tao that is seen
Is not the true Tao, until
You bring fresh toner.

With searching comes loss
and the presence of absence:
"My Thesis" not found.

Yesterday it worked
Today it is not working
Software is like that.

Let's find lines that contain the word "not":

$ grep not haiku.txt
Is not the true Tao, until
"My Thesis" not found
Today it is not working

Here, not is the pattern we're searching for. It's pretty simple: every alphanumeric character matches against itself. After the pattern comes the name or names of the files we're searching in. The output is the three lines in the file that contain the letters "not".

Let's try a different pattern: "day".

$ grep day haiku.txt
Yesterday it worked
Today it is not working

This time, the output is lines containing the words "Yesterday" and "Today", which both have the letters "day". If we give grep the -w flag, it restricts matches to word boundaries, so that only lines with the word "day" will be printed:

$ grep -w day haiku.txt

In this case, there aren't any, so grep's output is empty.

Another useful option is -n, which numbers the lines that match:

$ grep -n it haiku.txt
5:With searching comes loss
9:Yesterday it worked
10:Today it is not working

Here, we can see that lines 5, 9, and 10 contain the letters "it".

We can combine flags as we do with other Unix commands. For example, since -i makes matching case-insensitive and -v inverts the match, using them both only prints lines that don't match the pattern in any mix of upper and lower case:

$ grep -i -v the haiku.txt
You bring fresh toner.

With searching comes loss

Yesterday it worked
Today it is not working
Software is like that.

grep has lots of other options. To find out what they are, we can type man grep. man is the Unix "manual" command: it prints a description of a command and its options, and (if you're lucky) provides a few examples of how to use it:

$ man grep
GREP(1)                                                                                              GREP(1)

NAME
grep, egrep, fgrep - print lines matching a pattern

SYNOPSIS
grep [OPTIONS] PATTERN [FILE...]
grep [OPTIONS] [-e PATTERN | -f FILE] [FILE...]

DESCRIPTION
grep  searches the named input FILEs (or standard input if no files are named, or if a single hyphen-
minus (-) is given as file name) for lines containing a match to the given PATTERN.  By default, grep
prints the matching lines.
...        ...        ...

OPTIONS
Generic Program Information
--help Print  a  usage  message  briefly summarizing these command-line options and the bug-reporting
address, then exit.

-V, --version
Print the version number of grep to the standard output stream.  This version number should be
included in all bug reports (see below).

Matcher Selection
-E, --extended-regexp
Interpret  PATTERN  as  an  extended regular expression (ERE, see below).  (-E is specified by
POSIX.)

-F, --fixed-strings
Interpret PATTERN as a list of fixed strings, separated by newlines, any of  which  is  to  be
matched.  (-F is specified by POSIX.)
...        ...        ...

Wildcards

grep's real power doesn't come from its options, though; it comes from the fact that patterns can include wildcards. (The technical name for these is regular expressions, which is what the "re" in "grep" stands for.) Regular expressions are complex enough that we devoted an entire section of the website to them; if you want to do complex searches, please check it out. As a taster, we can find lines that have an 'o' in the second position like this:

$ grep -E '^.o' haiku.txt
You bring fresh toner.
Today it is not working
Software is like that.

We use the -E flag and put the pattern in quotes to prevent the shell from trying to interpret it. (If the pattern contained a '*', for example, the shell would try to expand it before running grep.) The '\^' in the pattern anchors the match to the start of the line. The '.' matches a single character (just like '?' in the shell), while the 'o' matches an actual 'o'.

While grep finds lines in files, the find command finds files themselves. Again, it has a lot of options; to show how the simplest ones work, we'll use the directory tree shown below.

Vlad's home directory contains one file called notes.txt and four subdirectories: thesis (which is sadly empty), data (which contains two files first.txt and second.txt), a tools directory that contains the programs format and stats, and an empty subdirectory called old.

For our first command, let's run find . -type d. . (i.e., the current working directory) is where we want our search to start; -type d means "things that are directories". Sure enough, find's output is the names of the five directories in our little tree (including .):

$ find . -type d
./
./data
./thesis
./tools
./tools/old

If we change -type d to -type f, we get a listing of all the files instead:

$ find . -type f
./data/first.txt
./data/second.txt
./notes.txt
./tools/format
./tools/stats

find automatically goes into subdirectories, their subdirectories, and so on to find everything that matches the pattern we've given it. If we don't want it to, we can use -maxdepth to restrict the depth of search:

$ find . -maxdepth 1 -type f
./notes.txt

The opposite of -maxdepth is -mindepth, which tells find to only report things that are at or below a certain depth. -mindepth 2 therefore finds all the files that are two or more levels below us:

$ find . -mindepth 2 -type f
./data/first.txt
./data/second.txt
./tools/format
./tools/stats

Another option is -empty, which restricts matches to empty files and directories:

$ find . -empty
./thesis
./tools/old

Now let's try matching by name:

$ find . -name *.txt
./notes.txt

We expected it to find all the text files, but it only prints out ./notes.txt. The problem is that the shell expands wildcard characters like * before commands run. Since *.txt in the current directory expands to notes.txt, the command we actually ran was:

$ find . -name notes.txt

find did what we asked; we just asked for the wrong thing.

To get what we want, let's do what we did with grep: put *.txt in single quotes to prevent the shell from expanding the * wildcard. This way, find actually gets the pattern *.txt, not the expanded filename notes.txt:

$ find . -name '*.txt'
./data/first.txt
./data/second.txt
./notes.txt

As we said earlier, the command line's power lies in combining tools. We've seen how to do that with pipes; let's look at another technique. As we just saw, find . -name '*.txt' gives us a list of all text files in or below the current directory. How can we combine that with wc -l to count the lines in all those files?

The simplest way is to put the find command inside $():

$ wc -l $(find . -name '*.txt')
70  ./data/first.txt
420  ./data/second.txt
30  ./notes.txt
520  total
$

When the shell executes this command, the first thing it does is run whatever is inside the $(). It then replaces the $() expression with that command's output. Since the output of find is the three filenames ./data/first.txt, ./data/second.txt, and ./notes.txt, the shell constructs the command:

$ wc -l ./data/first.txt ./data/second.txt ./notes.txt

which is what we wanted. This expansion is exactly what the shell does when it expands wildcards like * and ?, but lets us use any command we want as our own "wildcard".

It's very common to use find and grep together. The first finds files that match a pattern; the second looks for lines inside those files that match another pattern. Here, for example, we can find PDB files that contain iron atoms by looking for the string "FE" in all the .pdb files below the current directory:

$ grep FE $(find . -name '*.pdb')
./human/heme.pdb:ATOM  25  FE  1  -0.924  0.535  -0.518

Binary Files

We have focused exclusively on finding things in text files. What if your data is stored as images, in databases, or in some other format? One option would be to extend tools like grep to handle those formats. This hasn't happened, and probably won't, because there are too many formats to support.

The second option is to convert the data to text, or extract the text-ish bits from the data. This is probably the most common approach, since it only requires people to build one tool per data format (to extract information). On the one hand, it makes simple things easy to do. On the negative side, complex things are usually impossible. For example, it's easy enough to write a program that will extract X and Y dimensions from image files for grep to play with, but how would you write something to find values in a spreadsheet whose cells contained formulas?

The third choice is to recognize that the shell and text processing have their limits, and to use a programming language such as Python instead. When the time comes to do this, don't be too hard on the shell: many modern programming languages, Python included, have borrowed a lot of ideas from it, and imitation is also the sincerest form of praise.

Conclusion

The Unix shell is older than most of the people who use it. It has survived so long because it is one of the most productive programming environments ever created—maybe even the most productive. Its syntax may be cryptic, but people who have mastered it can experiment with different commands interactively, then use what they have learned to automate their work. Graphical user interfaces may be better at the first, but the shell is still unbeaten at the second. And as Alfred North Whitehead wrote in 1911, "Civilization advances by extending the number of important operations which we can perform without thinking about them."