What does the 'grep' command do?
I would like to learn how to use it so I can start to use the terminal more.
Solution 1:
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.
Solution 2:
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.
Solution 3:
You tagged your question with grep tag. But it was better if you read first the tag info referring to grep:
grepis 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
grepis 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.