Last updated on 2023-08-05 | Edit this page
Estimated time 45 minutes
- How can I find files?
- How can I find things in files?
grepto select lines from text files that match simple patterns.
findto 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 in files that match a
pattern. For our examples, we will use a file that contains three haiku
taken from a 1998
competition in Salon magazine (Credit to authors Bill
Torcaso, Howard Korder, and Margaret Segall, respectively. See Haiku
Error Messsages archived Page
1 and Page
2 .). For this set of examples, we’re going to be working in the
$ cd $ cd Desktop/shell-lesson-data/exercise-data/writing $ cat haiku.txt
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
not is the pattern we’re searching for. The grep
command searches through the file, looking for matches to the pattern
specified. To use it type
grep, then the pattern we’re
searching for and finally the name of the file (or files) we’re
The output is the three lines in the file that contain the letters ‘not’.
By default, grep searches for a pattern in a case-sensitive way. In addition, the search pattern we have selected does not have to form a complete word, as we will see in the next example.
Let’s search for the pattern: ‘The’.
$ grep The haiku.txt
The Tao that is seen "My Thesis" not found.
This time, two lines that include the letters ‘The’ are outputted, one of which contained our search pattern within a larger word, ‘Thesis’.
To restrict matches to lines containing the word ‘The’ on its own, we
-w option. This will limit
matches to word boundaries.
Later in this lesson, we will also see how we can change the search behavior of grep with respect to its case sensitivity.
$ grep -w The haiku.txt
The Tao that is seen
Note that a ‘word boundary’ includes the start and end of a line, so
not just letters surrounded by spaces. Sometimes we don’t want to search
for a single word, but a phrase. We can also do this with
grep by putting the phrase in quotes.
$ grep -w "is not" haiku.txt
Today it is not working
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 numbers the lines
$ 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 options (i.e. flags) as we do with other Unix
commands. For example, let’s find the lines that contain the word ‘the’.
We can combine the option
-w to find the lines that contain
the word ‘the’ and
-n to number the lines that match:
$ grep -n -w "the" haiku.txt
2:Is not the true Tao, until 6:and the presence of absence:
Now we want to use the option
-i to make our search
$ grep -n -w -i "the" haiku.txt
1:The Tao that is seen 2:Is not the true Tao, until 6:and the presence of absence:
Now, we want to use the option
-v to invert our search,
i.e., we want to output the lines that do not contain the word
$ grep -n -w -v "the" haiku.txt
1:The Tao that is seen 3:You bring fresh toner. 4: 5:With searching comes loss 7:"My Thesis" not found. 8: 9:Yesterday it worked 10:Today it is not working 11:Software is like that.
If we use the
-r (recursive) option,
can search for a pattern recursively through a set of files in
Let’s search recursively for
Yesterday in the
$ grep -r Yesterday .
./LittleWomen.txt:"Yesterday, when Aunt was asleep and I was trying to be as still as a ./LittleWomen.txt:Yesterday at dinner, when an Austrian officer stared at us and then ./LittleWomen.txt:Yesterday was a quiet day spent in teaching, sewing, and writing in my ./haiku.txt:Yesterday it worked
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: ... ... ...
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.
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
*, for example, the shell would try to expand it before
^ 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’.
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
containing a list of dates and the number of that species seen on each
date. For example using the data shown above,
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
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
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 .
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
LittleWomen.txt containing the full text of the
for loop, how would you tabulate the number of
times each of the four sisters is mentioned?
Hint: one solution might employ the commands
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
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
grep finds lines in files, the
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
. ├── animal-counts/ │ └── animals.csv ├── creatures/ │ ├── basilisk.dat │ ├── minotaur.dat │ └── unicorn.dat ├── numbers.txt ├── alkanes/ │ ├── cubane.pdb │ ├── ethane.pdb │ ├── methane.pdb │ ├── octane.pdb │ ├── pentane.pdb │ └── propane.pdb └── writing/ ├── haiku.txt └── LittleWomen.txt
exercise-data directory contains one file,
numbers.txt and four directories:
writing containing various files.
For our first command, let’s run
find . (remember to run
this command from the
$ 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 ./alkanes ./alkanes/ethane.pdb ./alkanes/propane.pdb ./alkanes/octane.pdb ./alkanes/pentane.pdb ./alkanes/methane.pdb ./alkanes/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,
is the names of the five directories (including
$ find . -type d
. ./writing ./creatures ./animal-counts ./alkanes
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 ./alkanes/ethane.pdb ./alkanes/propane.pdb ./alkanes/octane.pdb ./alkanes/pentane.pdb ./alkanes/methane.pdb ./alkanes/cubane.pdb
Now let’s try matching by name:
$ find . -name *.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.
*.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
To get what we want, let’s do what we did with
*.txt in quotes to prevent the shell from expanding the
* wildcard. This way,
find actually gets the
*.txt, not the expanded filename
$ find . -name "*.txt"
./writing/LittleWomen.txt ./writing/haiku.txt ./numbers.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
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
find is the three filenames
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
but lets us use any command we want as our own ‘wildcard’.
It’s very common to use
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
-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
you have thought about your answer, you can test the commands in the
find creatures -name "*.dat" | grep -v unicorn
find creatures -name *.dat | grep -v unicorn
grep -v "unicorn" $(find creatures -name "*.dat")
- None of the above.
Option 1. is correct. Putting the match expression in quotes prevents
the shell expanding it, so it gets passed to the
Option 2 is also works in this instance because the shell tries to
*.dat but there are no
*.dat files in
the current directory, so the wildcard expression gets passed to
find. We first encountered this in episode 3.
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.
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 all files with a
.datextension recursively from the current directory
- Count the number of lines each of these files contains
- Sort the output from step 2. numerically
findfinds files with specific properties that match patterns.
grepselects lines in files that match patterns.
--helpis 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.