grep - Unix, Linux Command

NAME

grep - print lines matching a pattern

SYNOPSIS

grep [OPTIONS] PATTERN [FILE...]
grep [OPTIONS] [-e PATTERN | -f FILE] [FILE...]

DESCRIPTION

grep searches the named input FILEs (or standard input if no files are named, or if a single hyphen-minus (-) is given as file name) for lines containing a match to the given PATTERN. By default, grep prints the matching lines.

OPTIONS

Tag	Description
-A NUM, --after-context=NUM	Print NUM lines of trailing context after matching lines. Places a line containing -- between contiguous groups of matches.
-a, --text	Process a binary file as if it were text; this is equivalent to the --binary-files=text option.
-B NUM, --before-context=NUM	Print NUM lines of leading context before matching lines. Places a line containing -- between contiguous groups of matches.
-C NUM, --context=NUM	Print NUM lines of output context. Places a line containing --between contiguous groups of matches.
-b, --byte-offset	Print the byte offset within the input file before each line of output.
--binary-files=TYPE	If the first few bytes of a file indicate that the file contains binary data, assume that the file is of type TYPE. By default, TYPE isbinary, and grep normally outputs either a one-line message saying that a binary file matches, or no message if there is no match. If TYPE is without-match, grep assumes that a binary file does not match; this is equivalent to the -I option. If TYPE is text, grep processes a binary file as if it were text; this is equivalent to the -a option. Warning: grep --binary-files=text might output binary garbage, which can have nasty side effects if the output is a terminal and if the terminal driver interprets some of it as commands.
--colour[=WHEN], --color[=WHEN]	Surround the matching string with the marker find inGREP_COLOR environment variable. WHEN may be ‘never’, ‘always’, or ‘auto’
-c, --count	Suppress normal output; instead print a count of matching lines for each input file. With the -v, --invert-match option (see below), count non-matching lines.
-D ACTION, --devices=ACTION	If an input file is a device, FIFO or socket, use ACTION to process it. By default, ACTION is read, which means that devices are read just as if they were ordinary files. If ACTION is skip, devices are silently skipped.
-d ACTION, --directories=ACTION	If an input file is a directory, use ACTION to process it. By default,ACTION is read, which means that directories are read just as if they were ordinary files. If ACTION is skip, directories are silently skipped. If ACTION is recurse, grep reads all files under each directory, recursively; this is equivalent to the -r option.
-E, --extended-regexp	Interpret PATTERN as an extended regular expression (see below).
-e PATTERN, --regexp=PATTERN	Use PATTERN as the pattern; useful to protect patterns beginning with -.
-F, --fixed-strings	Interpret PATTERN as a list of fixed strings, separated by newlines, any of which is to be matched.
-P, --perl-regexp	Interpret PATTERN as a Perl regular expression.
-f FILE, --file=FILE	Obtain patterns from FILE, one per line. The empty file contains zero patterns, and therefore matches nothing.
-G, --basic-regexp	Interpret PATTERN as a basic regular expression (see below). This is the default.
-H, --with-filename	Print the filename for each match.
-h, --no-filename	Suppress the prefixing of filenames on output when multiple files are searched.
--help	Output a brief help message.
-I	Process a binary file as if it did not contain matching data; this is equivalent to the --binary-files=without-match option.
-i, --ignore-case	Ignore case distinctions in both the PATTERN and the input files.
-L, --files-without-match	Suppress normal output; instead print the name of each input file from which no output would normally have been printed. The scanning will stop on the first match.
-l, --files-with-matches	Suppress normal output; instead print the name of each input file from which output would normally have been printed. The scanning will stop on the first match.
-m NUM, --max-count=NUM	Stop reading a file after NUM matching lines. If the input is standard input from a regular file, and NUM matching lines are output, grep ensures that the standard input is positioned to just after the last matching line before exiting, regardless of the presence of trailing context lines. This enables a calling process to resume a search. When grep stops after NUM matching lines, it outputs any trailing context lines. When the -c or --count option is also used, grep does not output a count greater than NUM. When the -v or --invert-match option is also used, grep stops after outputting NUM non-matching lines.
--mmap	If possible, use the mmap(2) system call to read input, instead of the default read(2) system call. In some situations, --mmap yields better performance. However, --mmap can cause undefined behavior (including core dumps) if an input file shrinks while grep is operating, or if an I/O error occurs.
-n, --line-number	Prefix each line of output with the line number within its input file.
-o, --only-matching	Show only the part of a matching line that matches PATTERN.
--label=LABEL	Displays input actually coming from standard input as input coming from file LABEL. This is especially useful for tools like zgrep, e.g. gzip -cd foo.gz \|grep -H --label=foo something
--line-buffered	Use line buffering, it can be a performance penality.
-q, --quiet, --silent	Quiet; do not write anything to standard output. Exit immediately with zero status if any match is found, even if an error was detected. Also see the -s or --no-messages option.
-R, -r, --recursive	Read all files under each directory, recursively; this is equivalent to the -d recurse option.
--include=PATTERN	Recurse in directories only searching file matching PATTERN.
--exclude=PATTERN	Recurse in directories skip file matching PATTERN.
-s, --no-messages	Suppress error messages about nonexistent or unreadable files. Portability note: unlike GNU grep, traditional grep did not conform to POSIX.2, because traditional grep lacked a -q option and its -s option behaved like GNU grep’s -q option. Shell scripts intended to be portable to traditional grep should avoid both -qand -s and should redirect output to /dev/null instead.
-U, --binary	Treat the file(s) as binary. By default, under MS-DOS and MS-Windows, grep guesses the file type by looking at the contents of the first 32KB read from the file. If grep decides the file is a text file, it strips the CR characters from the original file contents (to make regular expressions with ^ and $ work correctly). Specifying -U overrules this guesswork, causing all files to be read and passed to the matching mechanism verbatim; if the file is a text file with CR/LF pairs at the end of each line, this will cause some regular expressions to fail. This option has no effect on platforms other than MS-DOS and MS-Windows.
-u, --unix-byte-offsets	Report Unix-style byte offsets. This switch causes grep to report byte offsets as if the file were Unix-style text file, i.e. with CR characters stripped off. This will produce results identical to running grep on a Unix machine. This option has no effect unless-b option is also used; it has no effect on platforms other than MS-DOS and MS-Windows.
-V, --version	Print the version number of grep to standard error. This version number should be included in all bug reports (see below).
-v, --invert-match	Invert the sense of matching, to select non-matching lines.
-w, --word-regexp	Select only those lines containing matches that form whole words. The test is that the matching substring must either be at the beginning of the line, or preceded by a non-word constituent character. Similarly, it must be either at the end of the line or followed by a non-word constituent character. Word-constituent characters are letters, digits, and the underscore.
-x, --line-regexp	Select only those matches that exactly match the whole line.
-y	Obsolete synonym for -i.
-Z, --null	Output a zero byte (the ASCII NUL character) instead of the character that normally follows a file name. For example, grep -lZ outputs a zero byte after each file name instead of the usual newline. This option makes the output unambiguous, even in the presence of file names containing unusual characters like newlines. This option can be used with commands like find -print0, perl -0, sort -z, and xargs -0 to process arbitrary file names, even those that contain newline characters.

EXAMPLES

Example-1:

To Search for the given string in a single file test.sh

$ cat test.sh
#!/bin/bash
fun()
        echo "This is a test."
        # Terminate our shell script with success message
        exit 1

fun()

from above file grep exit:

$ grep "exit" demo_file

output:

exit 1

Example-2:

To Checking for the given string in multiple files: in this case test.sh and test1.sh

$ cat test.sh
#!/bin/bash
fun()
        echo "This is a test."
        # Terminate our shell script with success message
        exit 1

fun()

$ cat test1.sh
#!/bin/bash
fun()
        echo "This is a test1."
        # Terminate our shell script with success message
        exit 0

fun()

grep exit in both files test.sh and test1.sh:

$ grep exit test*

output:

test1.sh: exit 0
test.sh: exit 1

Example-3:

To Case insensitive search using grep -i, added EXIT in test1.sh

$ cat test1.sh
#!/bin/bash
fun()
        echo "This is a test1."
        # Terminate our shell script with success message, EXIT with 0
        exit 0

fun()

$ grep exit test1.sh
test1.sh: exit 0

$ grep -i exit test*

output:

test1.sh: # Terminate our shell script with success message, EXIT with 0
test1.sh: exit 0

two lines with -i option, as its case insensitive.

Example-4:

To Match regular expression in files

$ grep "This.*test" test1.sh

output:

echo "This is a test1."

Example-5:

* To Checking for full words, not for sub-strings using grep -w

$ grep -w "exit" test.sh

output:

exit 1

Note:

$ grep -w ex test.sh -> no output, as full word with ex is not in test.sh

for substring search we should not use -w option.

$ grep ex test.sh

exit 1

Example-6:

To Invert match using grep -v

$ cat test.sh
#!/bin/bash
fun()
        echo "This is a test."
        # Terminate our shell script with success message
        exit 1

fun()

$ grep -v exit test.sh

output:

#!/bin/bash
fun()
echo "This is a test."
# Terminate our shell script with success message

Note: exit statement is not seen in output.

Example-7:

To print line number of searched string

$ grep -n exit test.sh

output:

5: exit 1

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