grep prints lines that contain a match for a pattern. The general synopsis of the grep command line is grep options pattern input_file_names
There can be zero or more options. pattern will only be seen as such (and not as an input_file_name ) if it wasn’t already specified within options (by using the ‘ -e pattern ’ or ‘ -f file’ options). There can be zero or more input_file_names.
Matching Control
-e pattern
--regexp= pattern
Use pattern as the pattern. This can be used to specify multiple search patterns, or to protect a pattern beginning with a ‘ - ’. (-e is specified by POSIX.)
-f file
--file= file
Obtain patterns from file, one per line. The empty file contains zero patterns, and therefore matches nothing. (-f is specified by POSIX.)
-i
-y
--ignore-case
Ignore case distinctions, so that characters that differ only in case match each other. Although this is straightforward when letters differ in case only via lowercase-uppercase pairs, the behavior is unspecified in other situations. For example, uppercase “S” has an unusual lowercase counterpart “ſ” (Unicode character U+017F, LATIN SMALL LETTER LONG S) in many locales, and it is unspecified whether this unusual character matches “S” or “s” even though uppercasing it yields “S”. Another example: the lowercase German letter “ß” (U+00DF, LATIN SMALL LETTER SHARP S) is normally capitalized as the two-character string “SS” but it does not match “SS”, and it might not match the uppercase letter “ẞ” (U+1E9E, LATIN CAPITAL LETTER SHARP S) even though lowercasing the latter yields the former.
-y is an obsolete synonym that is provided for compatibility. (-i is specified by POSIX.)
-v
--invert-match
Invert the sense of matching, to select non-matching lines. (-v is specified by POSIX.)
-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. This option has no effect if -x is also specified.
-x
--line-regexp
Select only those matches that exactly match the whole line. For a regular expression pattern, this is like parenthesizing the pattern and then surrounding it with ‘ ^ ’ and ‘ $’. (-x is specified by POSIX.)
Examples:
to find authentication logs for “root” on an debian system:
# grep "root" /var/log/auth.log
For example, we can see that when someone failed to login as an admin, they also failed the reverse mapping which means they might not have a valid domain name.
# grep -B 3 -A 2 'Invalid user' /var/log/auth.log
To find authentication logs at current system date
# grep "$(date +%b) $(date +%e)" /var/log/auth.log | grep 'fail\|preauth'
To find authentication logs at current system hour
# grep "$(date +%b) $(date +%e) $(date +%H:)" /var/log/auth.log
To find mail logs at current system date
# grep "$(date +%b) $(date +%e)" /var/log/mail.info
To find mail logs at one hour before current system date
# grep "$(date --date="1 hours ago" +%b) $(date --date="1 hours ago" +%e)" /var/log/mail.info
%% a literal %
%a locale's abbreviated weekday name (e.g., Sun)
%A locale's full weekday name (e.g., Sunday)
%b locale's abbreviated month name (e.g., Jan)
%B locale's full month name (e.g., January)
%c locale's date and time (e.g., Thu Mar 3 23:05:25 2005)
%C century; like %Y, except omit last two digits (e.g., 20)
%d day of month (e.g., 01)
%D date; same as %m/%d/%y
%e day of month, space padded; same as %_d
%F full date; same as %Y-%m-%d
%g last two digits of year of ISO week number (see %G)
%G year of ISO week number (see %V); normally useful only with %V
%h same as %b
%H hour (00..23)
%I hour (01..12)
%j day of year (001..366)
%k hour, space padded ( 0..23); same as %_H
%l hour, space padded ( 1..12); same as %_I
%m month (01..12)
%M minute (00..59)
%n a newline
%N nanoseconds (000000000..999999999)
%p locale's equivalent of either AM or PM; blank if not known
%P like %p, but lower case
%r locale's 12-hour clock time (e.g., 11:11:04 PM)
%R 24-hour hour and minute; same as %H:%M
%s seconds since 1970-01-01 00:00:00 UTC
%S second (00..60)
%t a tab
%T time; same as %H:%M:%S
%u day of week (1..7); 1 is Monday
%U week number of year, with Sunday as first day of week (00..53)
%V ISO week number, with Monday as first day of week (01..53)
%w day of week (0..6); 0 is Sunday
%W week number of year, with Monday as first day of week (00..53)
%x locale's date representation (e.g., 12/31/99)
%X locale's time representation (e.g., 23:13:48)
%y last two digits of year (00..99)
%Y year
%z +hhmm numeric time zone (e.g., -0400)
%:z +hh:mm numeric time zone (e.g., -04:00)
%::z +hh:mm:ss numeric time zone (e.g., -04:00:00)
%:::z numeric time zone with : to necessary precision (e.g., -04,
+05:30)
%Z alphabetic time zone abbreviation (e.g., EDT)
By default, date pads numeric fields with zeroes. The following
optional flags may follow '%':
- (hyphen) do not pad the field
_ (underscore) pad with spaces
0 (zero) pad with zeros
^ use upper case if possible
# use opposite case if possible
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