Finding Things (aka Control F)

Overview

Teaching: 10 min
Exercises: 5 min
Questions
  • grep and find

Objectives
  • Use grep to select lines from text files that match simple patterns.

  • Use find to find files and directories whose names match simple patterns.

  • Use the output of one command as the command-line argument(s) to another command.

  • Explain what is meant by ‘text’ and ‘binary’ files, and why many common tools don’t handle the latter well.

In the same way that many of us now use ‘Google’ as a verb meaning ‘to find’, Unix programmers often use the word ‘grep’. ‘grep’ 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 within files that match a pattern. find finds files whose name matches a particular pattern.

For this example, let’s explore the scripts in Neele’s north pacific gyre directory

$ cd
$ cd ~/Desktop/shell-lesson-data/north-pacific-gyre
$ ls *.sh
goodiff.sh	goostats.sh

We can quickly view both script with cat.

cat *,sh
$ # check for the right number of input arguments
if [ $# -ne 2 ]
  then
    echo "goodiff file1 file2"
    echo "call goodiff with two arguments"
    exit 1
fi


# difference of two input files
# demo version, just return a random number or "files are identical"
if [ "$1" == "$2" ]
then
    echo "files are identical"
else
    echo 0.$RANDOM
fi
# check for the right number of input arguments
if [ $# -ne 2 ]
  then
    echo "goostats file1 file2"
    echo "call goostats with two arguments"
    exit 1
fi

sleep 2
head -n 3 $1 | cut -d , -f 1 | sort | uniq > $2

Cat is useful when looking at small files, but when working with large files, sometimes you want to focus on specific parts.

Many scripts have echo statements that print useful messages to the screen. Let’s use grep to print just the echo statements to the screen.

$ grep "echo" *sh
goodiff.sh:    echo "goodiff file1 file2"
goodiff.sh:    echo "call goodiff with two arguments"
goodiff.sh:    echo "files are identical"
goodiff.sh:    echo 0.$RANDOM
goostats.sh:    echo "goostats file1 file2"
goostats.sh:    echo "call goostats with two arguments"

We see four echo statements in goodiff.sh and two in goostats.sh. Notice that one echo statment has “random” and the other has “RANDOM”. By default grep match the case, but we can use the option -i to ignore case.

$ grep -i "random" *.sh
goodiff.sh:# demo version, just return a random number or "files are identical"
goodiff.sh:    echo 0.$RANDOM

When search for patterns, grep by default will match words that have more character that specified by the pattern. For instance, the pattern “file” is in “files” and “file1”.

$ grep "file" *sh
goodiff.sh:    echo "goodiff file1 file2"
goodiff.sh:# difference of two input files
goodiff.sh:# demo version, just return a random number or "files are identical"
goodiff.sh:    echo "files are identical"
goostats.sh:    echo "goostats file1 file2"

You can add the option -w to match the pattern as a word exactly. Because “file” doesn’t occur in either script, this will return nothing.

$ grep -w "file" *sh

We’ve now seen that you don’t have to have quotes around single words, but it is useful to use quotes when searching for multiple words. It also helps to make it easier to distinguish between the search term or phrase and the file being searched. We will use quotes in the remaining examples.

Another useful option is -n, which will print the line number were the pattern was found:

$ grep -n "file" *.sh
goodiff.sh:4:    echo "goodiff file1 file2"
goodiff.sh:10:# difference of two input files
goodiff.sh:11:# demo version, just return a random number or "files are identical"
goodiff.sh:14:    echo "files are identical"
goostats.sh:4:    echo "goostats file1 file2"

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

We can combine options (i.e. flags) as we do with other Unix commands. For example, let’s find the lines that contain the word ‘the’.

It is often useful to print the lines before or after the line with the matching pattern by adding the options -B 1 and -A 1.

$ grep -B 1 -A 1 "echo" *sh
goodiff.sh-3-  then
goodiff.sh:4:    echo "goodiff file1 file2"
goodiff.sh:5:    echo "call goodiff with two arguments"
goodiff.sh-6-    exit 1
--
--
goodiff.sh-13-then
goodiff.sh:14:    echo "files are identical"
goodiff.sh-15-else
--
--
goodiff.sh-15-else
goodiff.sh:16:    echo 0.$RANDOM
goodiff.sh-17-fi
--
--
goostats.sh-3-  then
goostats.sh:4:    echo "goostats file1 file2"
goostats.sh:5:    echo "call goostats with two arguments"
goostats.sh-6-    exit 1

grep has lots of other options. To find out what they are, we can type:

$ grep --help
Usage: grep [OPTION]... PATTERN [FILE]...
Search for PATTERN in each FILE or standard input.
PATTERN is, by default, a basic regular expression (BRE).
Example: grep -i 'hello world' menu.h main.c

Regexp selection and interpretation:
  -E, --extended-regexp     PATTERN is an extended regular expression (ERE)
  -F, --fixed-strings       PATTERN is a set of newline-separated fixed strings
  -G, --basic-regexp        PATTERN is a basic regular expression (BRE)
  -P, --perl-regexp         PATTERN is a Perl regular expression
  -e, --regexp=PATTERN      use PATTERN for matching
  -f, --file=FILE           obtain PATTERN from FILE
  -i, --ignore-case         ignore case distinctions
  -w, --word-regexp         force PATTERN to match only whole words
  -x, --line-regexp         force PATTERN to match only whole lines
  -z, --null-data           a data line ends in 0 byte, not newline

Miscellaneous:
...        ...        ...

Using grep

Which command would result in the following output:

and the presence of absence:
  1. grep "of" haiku.txt
  2. grep -E "of" haiku.txt
  3. grep -w "of" haiku.txt
  4. grep -i "of" haiku.txt

Solution

The correct answer is 3, because the -w option looks only for whole-word matches. The other options will also match ‘of’ when part of another word.

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 both complex and powerful; if you want to do complex searches, please look at the lesson on our website. 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 option 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’.

Tracking a Species

Leah has several hundred data files saved in one directory, each of which is formatted like this:

2012-11-05,deer,5
2012-11-05,rabbit,22
2012-11-05,raccoon,7
2012-11-06,rabbit,19
2012-11-06,deer,2
2012-11-06,fox,4
2012-11-07,rabbit,16
2012-11-07,bear,1

She wants to write a shell script that takes a species as the first command-line argument and a directory as the second argument. The script should return one file called <species>.txt containing a list of dates and the number of that species seen on each date. For example using the data shown above, rabbit.txt would contain:

2012-11-05,22
2012-11-06,19
2012-11-07,16

Below, each line contains an individual command, or pipe. Arrange their sequence in one command in order to achieve Leah’s goal:

cut -d : -f 2
>
|
grep -w $1 -r $2
|
$1.txt
cut -d , -f 1,3

Hint: use man grep to look for how to grep text recursively in a directory and man cut to select more than one field in a line.

An example of such a file is provided in shell-lesson-data/exercise-data/animal-counts/animals.csv

Solution

grep -w $1 -r $2 | cut -d : -f 2 | cut -d , -f 1,3 > $1.txt

Actually, you can swap the order of the two cut commands and it still works. At the command line, try changing the order of the cut commands, and have a look at the output from each step to see why this is the case.

You would call the script above like this:

$ bash count-species.sh bear .

Little Women

You and your friend, having just finished reading Little Women by Louisa May Alcott, are in an argument. Of the four sisters in the book, Jo, Meg, Beth, and Amy, your friend thinks that Jo was the most mentioned. You, however, are certain it was Amy. Luckily, you have a file LittleWomen.txt containing the full text of the novel (shell-lesson-data/exercise-data/writing/LittleWomen.txt). Using a for loop, how would you tabulate the number of times each of the four sisters is mentioned?

Hint: one solution might employ the commands grep and wc and a |, while another might utilize grep options. There is often more than one way to solve a programming task, so a particular solution is usually chosen based on a combination of yielding the correct result, elegance, readability, and speed.

Solutions

for sis in Jo Meg Beth Amy
do
    echo $sis:
    grep -ow $sis LittleWomen.txt | wc -l
done

Alternative, slightly inferior solution:

for sis in Jo Meg Beth Amy
do
    echo $sis:
    grep -ocw $sis LittleWomen.txt
done

This solution is inferior because grep -c only reports the number of lines matched. The total number of matches reported by this method will be lower if there is more than one match per line.

Perceptive observers may have noticed that character names sometimes appear in all-uppercase in chapter titles (e.g. ‘MEG GOES TO VANITY FAIR’). If you wanted to count these as well, you could add the -i option for case-insensitivity (though in this case, it doesn’t affect the answer to which sister is mentioned most frequently).

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 shell-lesson-data/exercise-data directory tree shown below.

.
├── animal-counts/
│   └── animals.csv
├── creatures/
│   ├── basilisk.dat
│   ├── minotaur.dat
│   └── unicorn.dat
├── numbers.txt
├── proteins/
│   ├── cubane.pdb
│   ├── ethane.pdb
│   ├── methane.pdb
│   ├── octane.pdb
│   ├── pentane.pdb
│   └── propane.pdb
└── writing/
    ├── haiku.txt
    └── LittleWomen.txt

The exercise-data directory contains one file, numbers.txt and four directories: animal-counts, creatures, proteins and writing containing various files.

For our first command, let’s run find . (remember to run this command from the shell-lesson-data/exercise-data folder).

$ find .
.
./writing
./writing/LittleWomen.txt
./writing/haiku.txt
./creatures
./creatures/basilisk.dat
./creatures/unicorn.dat
./creatures/minotaur.dat
./animal-counts
./animal-counts/animals.csv
./numbers.txt
./proteins
./proteins/ethane.pdb
./proteins/propane.pdb
./proteins/octane.pdb
./proteins/pentane.pdb
./proteins/methane.pdb
./proteins/cubane.pdb

As always, the . on its own means the current working directory, which is where we want our search to start. find’s output is the names of every file and directory under the current working directory. This can seem useless at first but find has many options to filter the output and in this lesson we will discover some of them.

The first option in our list is -type d that means ‘things that are directories’. Sure enough, find’s output is the names of the five directories (including .):

$ find . -type d
.
./writing
./creatures
./animal-counts
./proteins

Notice that the objects find finds are not listed in any particular order. If we change -type d to -type f, we get a listing of all the files instead:

$ find . -type f
./writing/LittleWomen.txt
./writing/haiku.txt
./creatures/basilisk.dat
./creatures/unicorn.dat
./creatures/minotaur.dat
./animal-counts/animals.csv
./numbers.txt
./proteins/ethane.pdb
./proteins/propane.pdb
./proteins/octane.pdb
./proteins/pentane.pdb
./proteins/methane.pdb
./proteins/cubane.pdb

Now let’s try matching by name:

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

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

$ find . -name numbers.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 quotes to prevent the shell from expanding the * wildcard. This way, find actually gets the pattern *.txt, not the expanded filename numbers.txt:

$ find . -name "*.txt"
./writing/LittleWomen.txt
./writing/haiku.txt
./numbers.txt

Listing vs. Finding

ls and find can be made to do similar things given the right options, but under normal circumstances, ls lists everything it can, while find searches for things with certain properties and shows them.

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")
  21022 ./writing/LittleWomen.txt
     11 ./writing/haiku.txt
      5 ./numbers.txt
  21038 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 ./writing/LittleWomen.txt, ./writing/haiku.txt, and ./numbers.txt, the shell constructs the command:

$ wc -l ./writing/LittleWomen.txt ./writing/haiku.txt ./numbers.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 txt files that contain the word “searching” by looking for the string ‘searching’ in all the .txt files in the current directory:

$ grep "searching" $(find . -name "*.txt")
./writing/LittleWomen.txt:sitting on the top step, affected to be searching for her book, but was
./writing/haiku.txt:With searching comes loss

Matching and Subtracting

The -v option to grep inverts pattern matching, so that only lines which do not match the pattern are printed. Given that, which of the following commands will find all .dat files in creatures except unicorn.dat? Once you have thought about your answer, you can test the commands in the shell-lesson-data/exercise-data directory.

  1. find creatures -name "*.dat" | grep -v unicorn
  2. find creatures -name *.dat | grep -v unicorn
  3. grep -v "unicorn" $(find creatures -name "*.dat")
  4. None of the above.

Solution

Option 1. is correct. Putting the match expression in quotes prevents the shell expanding it, so it gets passed to the find command.

Option 2 is also works in this instance because the shell tries to expand *.dat but there are no *.dat files in the current directory, so the wildcard expression gets passed to find. We encounter this in episode 6.

Option 3 is incorrect because it searches the contents of the files for lines which do not match ‘unicorn’, rather than searching the file names.

Binary Files

We have focused exclusively on finding patterns in text files. What if your data is stored as images, in databases, or in some other format?

A handful of tools extend grep to handle a few non text formats. But a more generalizable approach is to convert the data to text, or extract the text-like elements from the data. On the one hand, it makes simple things easy to do. On the other hand, 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?

A last option is to recognize that the shell and text processing have their limits, and to use another programming language. When the time comes to do this, don’t be too hard on the shell: many modern programming languages have borrowed a lot of ideas from it, and imitation is also the sincerest form of praise.

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 easier to use at first, but once learned, the productivity in the shell is unbeatable. 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.’

find Pipeline Reading Comprehension

Write a short explanatory comment for the following shell script:

wc -l $(find . -name "*.dat") | sort -n

Solution

  1. Find all files with a .dat extension recursively from the current directory
  2. Count the number of lines each of these files contains
  3. Sort the output from step 2. numerically

Key Points

  • find finds files with specific properties that match patterns.

  • grep selects lines in files that match patterns.

  • --help is an option supported by many bash commands, and programs that can be run from within Bash, to display more information on how to use these commands or programs.

  • man [command] displays the manual page for a given command.

  • $([command]) inserts a command’s output in place.