Grep Command in Linux/Unix With Examples | Code Factory


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grep is a command-line utility for searching plain-text data sets for lines that match a regular expression. Its name comes from the ed command g/re/p (globally search for a regular expression and print matching lines), which has the same effect.

grep was originally developed for the Unix operating system, but later available for all Unix-like systems and some others.

~/codeFactory$ grep --help
Usage: grep [OPTION]... PATTERNS [FILE]...
Search for PATTERNS in each FILE.
Example: grep -i 'hello world' menu.h main.c
PATTERNS can contain multiple patterns separated by newlines.

Pattern selection and interpretation:
  -E, --extended-regexp     PATTERNS are extended regular expressions
  -F, --fixed-strings       PATTERNS are strings
  -G, --basic-regexp        PATTERNS are basic regular expressions
  -P, --perl-regexp         PATTERNS are Perl regular expressions
  -e, --regexp=PATTERNS     use PATTERNS for matching
  -f, --file=FILE           take PATTERNS from FILE
  -i, --ignore-case         ignore case distinctions in patterns and data
      --no-ignore-case      do not ignore case distinctions (default)
  -w, --word-regexp         match only whole words
  -x, --line-regexp         match only whole lines
  -z, --null-data           a data line ends in 0 byte, not newline

Miscellaneous:
  -s, --no-messages         suppress error messages
  -v, --invert-match        select non-matching lines
  -V, --version             display version information and exit
      --help                display this help text and exit

Output control:
  -m, --max-count=NUM       stop after NUM selected lines
  -b, --byte-offset         print the byte offset with output lines
  -n, --line-number         print line number with output lines
      --line-buffered       flush output on every line
  -H, --with-filename       print file name with output lines
  -h, --no-filename         suppress the file name prefix on output
      --label=LABEL         use LABEL as the standard input file name prefix
  -o, --only-matching       show only nonempty parts of lines that match
  -q, --quiet, --silent     suppress all normal output
      --binary-files=TYPE   assume that binary files are TYPE;
                            TYPE is 'binary', 'text', or 'without-match'
  -a, --text                equivalent to --binary-files=text
  -I                        equivalent to --binary-files=without-match
  -d, --directories=ACTION  how to handle directories;
                            ACTION is 'read', 'recurse', or 'skip'
  -D, --devices=ACTION      how to handle devices, FIFOs and sockets;
                            ACTION is 'read' or 'skip'
  -r, --recursive           like --directories=recurse
  -R, --dereference-recursive  likewise, but follow all symlinks
      --include=GLOB        search only files that match GLOB (a file pattern)
      --exclude=GLOB        skip files that match GLOB
      --exclude-from=FILE   skip files that match any file pattern from FILE
      --exclude-dir=GLOB    skip directories that match GLOB
  -L, --files-without-match  print only names of FILEs with no selected lines
  -l, --files-with-matches  print only names of FILEs with selected lines
  -c, --count               print only a count of selected lines per FILE
  -T, --initial-tab         make tabs line up (if needed)
  -Z, --null                print 0 byte after FILE name

Context control:
  -B, --before-context=NUM  print NUM lines of leading context
  -A, --after-context=NUM   print NUM lines of trailing context
  -C, --context=NUM         print NUM lines of output context
  -NUM                      same as --context=NUM
      --color[=WHEN],
      --colour[=WHEN]       use markers to highlight the matching strings;
                            WHEN is 'always', 'never', or 'auto'
  -U, --binary              do not strip CR characters at EOL (MSDOS/Windows)

When FILE is '-', read standard input.  With no FILE, read '.' if
recursive, '-' otherwise.  With fewer than two FILEs, assume -h.
Exit status is 0 if any line (or file if -L) is selected, 1 otherwise;
if any error occurs and -q is not given, the exit status is 2.

Report bugs to: bug-grep@gnu.org
GNU grep home page: <http://www.gnu.org/software/grep/>
General help using GNU software: <https://www.gnu.org/gethelp/>

1. Case insensitive search: The -i option enables to search for a string case insensitively in the give file.

~/codeFactory$ cat test4.txt 
grep is a command-line utility for searching plain-text data sets for lines that match a regular expression.
Its name comes from the ed command g/re/p (globally search for a regular expression and print matching lines), which has the same effect.
grep was originally developed for the Unix operating system, but later available for all Unix-like systems and some others.

~/codeFactory$ cat test4.txt | grep -i GREP
grep is a command-line utility for searching plain-text data sets for lines that match a regular expression.
grep was originally developed for the Unix operating system, but later available for all Unix-like systems and some others.

2. Displaying the count of number of matches: We can find the number of lines that matches the given string/pattern.

~/codeFactory$ grep -c "grep" test4.txt 
2

3. Display the file names that matches the pattern: We can just display the files that contains the given string/pattern.

~/codeFactory$ grep -l "grep" test*
test4.txt

4. Checking for the whole words in a file: By default, grep matches the given string/pattern even if it found as a substring in a file. The -w option to grep makes it match only the whole words.

~/codeFactory$ grep -w "grep" test4.txt 
grep is a command-line utility for searching plain-text data sets for lines that match a regular expression.
grep was originally developed for the Unix operating system, but later available for all Unix-like systems and some others.

5. Displaying only the matched pattern: By default, grep displays the entire line which has the matched string. We can make the grep to display only the matched string by using the -o option.

~/codeFactory$ grep -o "grep" test4.txt 
grep
grep

6. Show line number while displaying the output using grep -n: To show the line number of file with the line matched.

~/codeFactory$ grep -n "grep" test4.txt 
1:grep is a command-line utility for searching plain-text data sets for lines that match a regular expression.
3:grep was originally developed for the Unix operating system, but later available for all Unix-like systems and some others.

7. Inverting the pattern match: You can display the lines that are not matched with the specified search sting pattern using the -v option.

~/codeFactory$ grep -v "grep" test4.txt 
Its name comes from the ed command g/re/p (globally search for a regular expression and print matching lines), which has the same effect.

8. Matching the lines that start with a string: The ^ regular expression pattern specifies the start of a line. This can be used in grep to match the lines which start with the given string or pattern.

~/codeFactory$ grep "^grep" test4.txt 
grep is a command-line utility for searching plain-text data sets for lines that match a regular expression.
grep was originally developed for the Unix operating system, but later available for all Unix-like systems and some others.

9. Matching the lines that end with a string: The $ regular expression pattern specifies the end of a line. This can be used in grep to match the lines which end with the given string or pattern.

~/codeFactory$ grep "expression.$" test4.txt 
grep is a command-line utility for searching plain-text data sets for lines that match a regular expression.

10. Specifies expression with -e option. Can use multiple times:

~/codeFactory$ grep -e "grep" -e "command" test4.txt 
grep is a command-line utility for searching plain-text data sets for lines that match a regular expression.
Its name comes from the ed command g/re/p (globally search for a regular expression and print matching lines), which has the same effect.
grep was originally developed for the Unix operating system, but later available for all Unix-like systems and some others.

11. -f file option Takes patterns from file, one per line:

~/codeFactory$ cat grep.txt 
grep
command

~/codeFactory$ grep -f grep.txt test4.txt 
grep is a command-line utility for searching plain-text data sets for lines that match a regular expression.
Its name comes from the ed command g/re/p (globally search for a regular expression and print matching lines), which has the same effect.
grep was originally developed for the Unix operating system, but later available for all Unix-like systems and some others.

12. Print n specific lines from a file: -A prints the searched line and n lines after the result, -B prints the searched line and n lines before the result, and -C prints the searched line and n lines after and before the result.

Syntax:

$grep -A[NumberOfLines(n)] [search] [file]  

$grep -B[NumberOfLines(n)] [search] [file]  

$grep -C[NumberOfLines(n)] [search] [file]

Example:

~/codeFactory$ grep -A1 grep test4.txt 
grep is a command-line utility for searching plain-text data sets for lines that match a regular expression.
Its name comes from the ed command g/re/p (globally search for a regular expression and print matching lines), which has the same effect.
grep was originally developed for the Unix operating system, but later available for all Unix-like systems and some others.

~/codeFactory$ grep -A0 grep test4.txt 
grep is a command-line utility for searching plain-text data sets for lines that match a regular expression.
--
grep was originally developed for the Unix operating system, but later available for all Unix-like systems and some others.

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