3. Grep/Awk/Sed

Materials to download

1. Linux word dictionary
2. Mary had a little lamb
3. BRITE students

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Grep

Grep (an acronym for “Global Regular Expression Print”), finds a string in a given file or input.

Grep format:

grep [options] [regexp] [filename]

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Grep usecases:

1. Case-insensitive search (grep -i):

grep -i 'mary' mary-lamb.txt

2. Whole-word search (grep -w):

grep -w 'as' mary-lamb.txt

3. recursively search through sub-folders (grep -r <pattern> <path>):

grep -r '456' /<your_working_directory>/

4. Inverted search (grep -v):

grep -v ‘the’ mary-lamb.txt

5. Print additional (trailing) context lines after match (grep -A <NUM>):

grep -A1 'School'  mary-lamb.txt

6. Print additional (leading) context lines before match (grep -B <NUM>):

grep -B2 'School'  mary-lamb.txt

7. Print additional (leading and trailing) context lines before and after the match (grep -C <NUM>):

grep -C3 'School' mary-lamb.txt

8. Print the filename for each match (grep -H <pattern> filename):

grep -H 'School' mary-lamb.txt

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Regexp or regular expression:

Regexp is how we specify that we find to see a particular pattern (it could be words or characters).

• The period . matches any single character.
• * when the previous pattern could be matched zero or more times.

grep 'M.a' mary-lamb.txt
grep 'M*y' Mary_Lamb_lyrics.txt

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AWK:

awk [options] [filename]

Named after the authors: Aho, Weinberger, Kernighan

• Print everything in the text file:

awk '{print}' BRITE_students.txt

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• Now, let’s get the more specific. Let’s ask for names only:

awk '{print $1}' BRITE_students.txt

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• What if we want to see two columns at the same time, let’s say name and GPA?

awk '{print $1" "$3}' BRITE_students.txt

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• Now what let’s see what your info is (exact match):

awk '$1=="Ali"' BRITE_students.txt

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• How can we see a particular pattern in our cohort (not an exact match):

awk '/Kat/ {print $0}' BRITE_students.txt

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• Question for you: How do you print the name and favorite sport of students whose names contain the letter “u”?

<insert code here>

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• How many students are there whose name begins with “Kat”?

awk '/Kat/{++cnt} END {print "Count = ", cnt}' BRITE_students.txt

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• You could also run loops in awk, print all :

awk 'BEGIN {
   sum = 0; for (i = 0; i < 20; ++i) {
       sum += i; if (sum > 50) exit(10); else print "Sum =", sum
   }
}'

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SED:

sed [options] [filename]

SED stands for “Stream EDitor”. It is a widely used text processing Linux tool.

• I want to read the first three lines of a text file:

cat BRITE_students.txt | sed -n 3p

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• What if we want to replace one word with another:

cat mary-lamb.txt | sed 's/Mary/Maria/g'

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• Let’s remove the 1st, 2nd and 5th lines from a text file:

sed -e '1d' -e '2d' -e '5d' BRITE_students.txt

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• But what if we had a much longer list and wanted to remove more lines?

echo -e "1d\n2d\n5d" > my_lines.txt
cat my_lines.txt
sed -f my_lines.txt BRITE_students.txt

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• Now let’s print the 2nd line to last:

cat BRITE_students.txt | sed -n 2,'$p'

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