grep.md

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grep

grep is a linux command that processes text and prints any lines that match a specified pattern. grep stands for Global Regular Expression Print.

$grep [OPTIONS] PATTERN [FILE...]

Examples

Imagine a file called test.txt:

Linux is fun
C4CS is cool
CS is awesome

Running grep with the pattern "C4CS" will return the line all lines containg that pattern.

$grep C4CS test.txt
C4CS is cool

Regular Expressions

The real power of grep comes from its ability to use regular expressions as a pattern. For the folowing examples consider a dictionary called dict.txt.

Bracket Expressions

A bracket expression is a list of characters enclosed by [ and ]. A bracket expression will match any single character in the list.

$grep c[aou]t dict.txt
cat
cot
cut

bracket expressions can also be used with a hypen to match a range. [a-e] is equivlent to [abcde]. And [0-9] is equivlent to [0123456789].

Negating a Bracket Expression

A carat (^) at the begining of a bracketed expression will match characters not in the list.

$grep c[^u]t dict.txt
cat
cot

Anchoring

The ^ symbol is used to match at the begining of the string. The $ symbol is used to match at the end of the string.

$grep t dict.txt
# returns all words containing an 't'
$grep ^t dict.txt
# returns all words begining with 't'
$grep t$ dict.txt
# returns all words ending in 't'
### Wildcard
The . symbol is used as a wildcard. It can match any single character.
```bash
$grep .oat dict.txt
boat
coat
goat
moat

Repetition

The ? symbol means the preceding item may be matched zero or one times. ab?c matches both "ac" and "abc". The * symbol means the preceding item may be matched zero or more times. ab*c mathces "ac", "abc", "abbc", and so on. The + symbol means the preceding item may be matched one or more times. ab+c matches "abc", "abbc", "abbbc", and so on. {n} means the preceding item is matched exactly n times. a{5} matches "aaaaa". {n,} means the preceding item is matched n or more times. a{3,} matches "aaa", "aaaa", and so on. {,n} means the preceding item is matched at most n times. a{,3} matches "","a","aa","aaa". {n,m} means the preceding item is matched at least n times and at most m times. a{1,3} matches "a","aa","aaa".

grep Options

-i, --ignore-case

Uses case insesitive matching for the PATTERN.

-v, --invert-match

Prints lines that do not match the PATTERN.

-c, --count

Does not print matching lines, instead prints the number of matching lines. Can be used in combination with -v as -cv to print the number of non-matching lines.

-q, --quiet, --silent

Do not print anything. Exit 0 if any match is found, even if there is an error.

-H, --with-filename

Print the filename for eaach match. This option is default when provided with more than one file.

-h, --no-filename

Suppress output of file names. This is default when provided with only one file to search

-n, --line-number

Prefix each line of output with the 1-based line number within its file.

-R, -r, --recursive

Read all files under a directory