Detailed explanation of the usage of grep command in Linux

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 fixed 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

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 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 bytes 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
  --group-separator=SEP use SEP as a group separator
  --no-group-separator use empty string as a group separator
  --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)
 -u, --unix-byte-offsets report offsets as if CRs were not there
       (MSDOS/Windows)

'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: [email protected]
GNU Grep home page: <http://www.gnu.org/software/grep/>
General help using GNU software: http://www.gnu.org/gethelp/

grep 'wordA\|wordB' *.py
grep -E 'wordA|wordB' *.doc
grep -e wordA -e wordB *.py
egrep "wordA|wordB" *.c

grep -e pattern1 *.py |grep -e pattern2

grep -w 'warning\|error\|critical' /home/logs

egrep -wi --color 'warning|error|critical' /home/logs

egrep -Rwi --color 'warning|error' /home/logs/