Finding Things

Overview

Teaching: 15 min
Exercises: 0 min

Questions

• How can I find files?

• How can I find things in files?

Objectives

Using grep

Referring to haiku.txt presented at the begin of this topic, 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 flag looks only for whole-word matches. The other options will all match “of” when part of another word.

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 in the current directory
2. Count the number of lines each of these files contains
3. Sort the output from step 2. numerically

Matching and Subtracting

The -v flag 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 files in /data whose names end in s.txt (e.g., animals.txt or planets.txt), but do not contain the word net? Once you have thought about your answer, you can test the commands in the data-shell directory.

1. find data -name '*s.txt' | grep -v net
2. find data -name *s.txt | grep -v net
3. grep -v "temp" $(find data -name '*s.txt')
4. None of the above.

Solution

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

Option 2 is incorrect because the shell expands *s.txt instead of passing the wildcard expression to find.

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

Tracking a Species

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

2013-11-05,deer,5
2013-11-05,rabbit,22
2013-11-05,raccoon,7
2013-11-06,rabbit,19
2013-11-06,deer,2

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, rabbits.txt would contain:

2013-11-05,22
2013-11-06,19

Put these commands and pipes in the right order to achieve this:

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 data-shell/data/animal-counts/animals.txt

Solution

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

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 (data-shell/writing/data/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.

Finding Files With Different Properties

The find command can be given several other criteria known as “tests” to locate files with specific attributes, such as creation time, size, permissions, or ownership. Use man find to explore these, and then write a single command to find all files in or below the current directory that were modified by the user ahmed in the last 24 hours.

Hint 1: you will need to use three tests: -type, -mtime, and -user.

Hint 2: The value for -mtime will need to be negative—why?

Solution

Assuming that Nelle’s home is our working directory we type:

$ find ./ -type f -mtime -1 -user ahmed

Key Points