What does the 'grep' command do?

I would like to learn how to use it so I can start to use the terminal more.

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5 Answers

grep is used to search for text from a file or another command's output. It can return the lines where it finds a match or the lines where it doesn't.

NAME

 grep, egrep, fgrep - 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 the file name - is given) for lines containing a match to the given PATTERN. By default, grep prints the matching lines. In addition, two variant programs egrep and fgrep are available. Egrep is the same as grep -E. Fgrep is the same as grep -F.

OPTIONS

 -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 is binary, and grep normally outputs either a one-line mes- sage 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 termi- nal driver interprets some of it as commands. --colour[=WHEN], --color[=WHEN] Surround the matching string with the marker find in GREP_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 pro- cess 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, direc- tories 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 new- lines, 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 con- tains 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 com- ing 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 immedi- ately 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 equiv- alent 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 con- form 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 -q and -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-Win- dows. -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 ver- sion 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 con- stituent 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.

SOURCE:MANPAGE FOR GREP
You can learn about commands by typing

info <command>
man <command>
whatis <command>
apropos <command> 

in the terminal.

You tagged your question with grep tag. But it was better if you read first the tag info referring to grep:

grep is a command-line utility that is used for searching text from standard input or a file for specific expressions, returning the lines where matches occur.

A common use for grep is to locate and print out certain lines from log files or program output.

See this Ubuntu Documentation page for more information on how to use grep.

Also, almost any command has a manual page that you can read it. In your case, use this command:

man grep

Even more, you can get information about grep using one of these commands: info grep, grep -h, grep --help, whatis grep or apropos grep.

grep searches the content of files based on a regular expression. The easiest way to use it is something like grep "word" file.txt, in which case it will just return every line where "word" appears in file.txt (or nothing if it's not there). To make it do more advanced things, learn about regular expressions. You can find plenty of tutorials online by searching.

Personally, I don't recommend using the terminal just for the sake of using the terminal. You might go through a lot of trouble to learn complicated tools like grep only to discover that you use graphical programs or scripts you wrote in your favorite interpreted language. Most people who recommend learning old-fashioned terminal commands are themselves old-fashioned hackers.

2

The grep command searches one or more input files for lines containing a match to a specified pattern. By default, grep prints the matching lines.

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

In addition, two variant programs egrep and fgrep are available. Egrep is the same as grep -E. Fgrep is the same as grep -F.^{Man Page}

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