linux의 grep은 파일 또는 표준 입력(Standard input)에 대해서 특정 패턴을 통해 검색 해주는 명령어 입니다.

패턴에 대해서는 기본값으로 기본 정규 표현식(basic regular expression - BRE)을 지원 합니다.

기본적인 사용 법은 다음과 같습니다.

$> grep
Usage: grep [OPTION]... PATTERN [FILE]...
Try 'grep --help' for more information.

$> grep Error /var/log/syslog
Aug  3 10:20:25 geeksaga org.a11y.Bus[4449]: ** (process:4467): WARNING **: Failed to register client: GDBus.Error:org.freedesktop.DBus.Error.ServiceUnknown: The name org.gnome.SessionManager was not provided by any .service files
Aug  3 10:20:25 geeksaga org.freedesktop.systemd1[4449]: ** (process:4476): WARNING **: cgmanager method call org.linuxcontainers.cgmanager0_0.MovePidAbs failed: GDBus.Error:org.freedesktop.DBus.Error.InvalidArgs: invalid request.  Use G_DBUS_DEBUG=message for more info.

표준 입력(Standard input)을 이용한 방법

$> dmesg | grep random
[    0.000000] random: get_random_bytes called from start_kernel+0x99/0x51e with crng_init=0
[    1.035100] random: systemd-udevd: uninitialized urandom read (16 bytes read)
[    1.035156] random: systemd-udevd: uninitialized urandom read (16 bytes read)
[    1.035164] random: systemd-udevd: uninitialized urandom read (16 bytes read)
[    1.991103] random: fast init done
[   45.515726] random: crng init done
[   45.515727] random: 7 urandom warning(s) missed due to ratelimiting

help grep

$> grep --help
Usage: grep [OPTION]... PATTERN [FILE]...
Search for PATTERN in each FILE or standard input.
PATTERN is, by default, a basic regular expression (BRE).
Example: grep -i 'hello world' menu.h main.c

Regexp selection and interpretation:
  -E, --extended-regexp     PATTERN is an extended regular expression (ERE)
  -F, --fixed-strings       PATTERN is a set of newline-separated strings
  -G, --basic-regexp        PATTERN is a basic regular expression (BRE)
  -P, --perl-regexp         PATTERN is a Perl regular expression
  -e, --regexp=PATTERN      use PATTERN for matching
  -f, --file=FILE           obtain PATTERN from FILE
  -i, --ignore-case         ignore case distinctions
  -w, --word-regexp         force PATTERN to match only whole words
  -x, --line-regexp         force PATTERN to match only whole lines
  -z, --null-data           a data line ends in 0 byte, not newline

  -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 matches
  -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 the file name for each match
  -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 the part of a line matching PATTERN
  -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=FILE_PATTERN  search only files that match FILE_PATTERN
      --exclude=FILE_PATTERN  skip files and directories matching FILE_PATTERN
      --exclude-from=FILE   skip files matching any file pattern from FILE
      --exclude-dir=PATTERN  directories that match PATTERN will be skipped.
  -L, --files-without-match  print only names of FILEs containing no match
  -l, --files-with-matches  print only names of FILEs containing matches
  -c, --count               print only a count of matching 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
      --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)
  -u, --unix-byte-offsets   report offsets as if CRs were not there

'egrep' means 'grep -E'.  'fgrep' means 'grep -F'.
Direct invocation as either 'egrep' or 'fgrep' is deprecated.
When FILE is -, read standard input.  With no FILE, read . if a command-line
-r is given, - otherwise.  If fewer than two FILEs are given, assume -h.
Exit status is 0 if any line is selected, 1 otherwise;
if any error occurs and -q is not given, the exit status is 2.

Report bugs to:
GNU grep home page: <>
General help using GNU software: <>

tldr grep

$> tldr grep


  Matches patterns in input text.
  Supports simple patterns and regular expressions.

  - Search for an exact string:
    grep search_string path/to/file

  - Search in case-insensitive mode:
    grep -i search_string path/to/file

  - Search recursively (ignoring non-text files) in current directory for an exact string:
    grep -RI search_string .

  - Use extended regular expressions (supporting ?, +, {}, () and |):
    grep -E ^regex$ path/to/file

  - Print 3 lines of [C]ontext around, [B]efore, or [A]fter each match:
    grep -C|B|A 3 search_string path/to/file

  - Print file name with the corresponding line number for each match:
    grep -Hn search_string path/to/file

  - Use the standard input instead of a file:
    cat path/to/file | grep search_string

  - Invert match for excluding specific strings:
    grep -v search_string

참고 링크