gawk: Time Functions
9.1.5 Time Functions
--------------------
'awk' programs are commonly used to process log files containing
timestamp information, indicating when a particular log record was
written. Many programs log their timestamps in the form returned by the
'time()' system call, which is the number of seconds since a particular
epoch. On POSIX-compliant systems, it is the number of seconds since
1970-01-01 00:00:00 UTC, not counting leap seconds.(1) All known
POSIX-compliant systems support timestamps from 0 through 2^31 - 1,
which is sufficient to represent times through 2038-01-19 03:14:07 UTC.
Many systems support a wider range of timestamps, including negative
timestamps that represent times before the epoch.
In order to make it easier to process such log files and to produce
useful reports, 'gawk' provides the following functions for working with
timestamps. They are 'gawk' extensions; they are not specified in the
POSIX standard.(2) However, recent versions of 'mawk' (
Other
Versions) also support these functions. Optional parameters are
enclosed in square brackets ([ ]):
'mktime(DATESPEC' [', UTC-FLAG' ]')'
Turn DATESPEC into a timestamp in the same form as is returned by
'systime()'. It is similar to the function of the same name in ISO
C. The argument, DATESPEC, is a string of the form
'"YYYY MM DD HH MM SS [DST]"'. The string consists of six or seven
numbers representing, respectively, the full year including
century, the month from 1 to 12, the day of the month from 1 to 31,
the hour of the day from 0 to 23, the minute from 0 to 59, the
second from 0 to 60,(3) and an optional daylight-savings flag.
The values of these numbers need not be within the ranges
specified; for example, an hour of -1 means 1 hour before midnight.
The origin-zero Gregorian calendar is assumed, with year 0
preceding year 1 and year -1 preceding year 0. If UTC-FLAG is
present and is either nonzero or non-null, the time is assumed to
be in the UTC time zone; otherwise, the time is assumed to be in
the local time zone. If the DST daylight-savings flag is positive,
the time is assumed to be daylight savings time; if zero, the time
is assumed to be standard time; and if negative (the default),
'mktime()' attempts to determine whether daylight savings time is
in effect for the specified time.
If DATESPEC does not contain enough elements or if the resulting
time is out of range, 'mktime()' returns -1.
'strftime('[FORMAT [',' TIMESTAMP [',' UTC-FLAG] ] ]')'
Format the time specified by TIMESTAMP based on the contents of the
FORMAT string and return the result. It is similar to the function
of the same name in ISO C. If UTC-FLAG is present and is either
nonzero or non-null, the value is formatted as UTC (Coordinated
Universal Time, formerly GMT or Greenwich Mean Time). Otherwise,
the value is formatted for the local time zone. The TIMESTAMP is
in the same format as the value returned by the 'systime()'
function. If no TIMESTAMP argument is supplied, 'gawk' uses the
current time of day as the timestamp. Without a FORMAT argument,
'strftime()' uses the value of 'PROCINFO["strftime"]' as the format
string (Built-in Variables). The default string value is
'"%a %b %e %H:%M:%S %Z %Y"'. This format string produces output
that is equivalent to that of the 'date' utility. You can assign a
new value to 'PROCINFO["strftime"]' to change the default format;
see the following list for the various format directives.
'systime()'
Return the current time as the number of seconds since the system
epoch. On POSIX systems, this is the number of seconds since
1970-01-01 00:00:00 UTC, not counting leap seconds. It may be a
different number on other systems.
The 'systime()' function allows you to compare a timestamp from a log
file with the current time of day. In particular, it is easy to
determine how long ago a particular record was logged. It also allows
you to produce log records using the "seconds since the epoch" format.
The 'mktime()' function allows you to convert a textual
representation of a date and time into a timestamp. This makes it easy
to do before/after comparisons of dates and times, particularly when
dealing with date and time data coming from an external source, such as
a log file.
The 'strftime()' function allows you to easily turn a timestamp into
human-readable information. It is similar in nature to the 'sprintf()'
function (String Functions), in that it copies nonformat
specification characters verbatim to the returned string, while
substituting date and time values for format specifications in the
FORMAT string.
'strftime()' is guaranteed by the 1999 ISO C standard(4) to support
the following date format specifications:
'%a'
The locale's abbreviated weekday name.
'%A'
The locale's full weekday name.
'%b'
The locale's abbreviated month name.
'%B'
The locale's full month name.
'%c'
The locale's "appropriate" date and time representation. (This is
'%A %B %d %T %Y' in the '"C"' locale.)
'%C'
The century part of the current year. This is the year divided by
100 and truncated to the next lower integer.
'%d'
The day of the month as a decimal number (01-31).
'%D'
Equivalent to specifying '%m/%d/%y'.
'%e'
The day of the month, padded with a space if it is only one digit.
'%F'
Equivalent to specifying '%Y-%m-%d'. This is the ISO 8601 date
format.
'%g'
The year modulo 100 of the ISO 8601 week number, as a decimal
number (00-99). For example, January 1, 2012, is in week 53 of
2011. Thus, the year of its ISO 8601 week number is 2011, even
though its year is 2012. Similarly, December 31, 2012, is in week
1 of 2013. Thus, the year of its ISO week number is 2013, even
though its year is 2012.
'%G'
The full year of the ISO week number, as a decimal number.
'%h'
Equivalent to '%b'.
'%H'
The hour (24-hour clock) as a decimal number (00-23).
'%I'
The hour (12-hour clock) as a decimal number (01-12).
'%j'
The day of the year as a decimal number (001-366).
'%m'
The month as a decimal number (01-12).
'%M'
The minute as a decimal number (00-59).
'%n'
A newline character (ASCII LF).
'%p'
The locale's equivalent of the AM/PM designations associated with a
12-hour clock.
'%r'
The locale's 12-hour clock time. (This is '%I:%M:%S %p' in the
'"C"' locale.)
'%R'
Equivalent to specifying '%H:%M'.
'%S'
The second as a decimal number (00-60).
'%t'
A TAB character.
'%T'
Equivalent to specifying '%H:%M:%S'.
'%u'
The weekday as a decimal number (1-7). Monday is day one.
'%U'
The week number of the year (with the first Sunday as the first day
of week one) as a decimal number (00-53).
'%V'
The week number of the year (with the first Monday as the first day
of week one) as a decimal number (01-53). The method for
determining the week number is as specified by ISO 8601. (To wit:
if the week containing January 1 has four or more days in the new
year, then it is week one; otherwise it is the last week [52 or 53]
of the previous year and the next week is week one.)
'%w'
The weekday as a decimal number (0-6). Sunday is day zero.
'%W'
The week number of the year (with the first Monday as the first day
of week one) as a decimal number (00-53).
'%x'
The locale's "appropriate" date representation. (This is '%A %B %d
%Y' in the '"C"' locale.)
'%X'
The locale's "appropriate" time representation. (This is '%T' in
the '"C"' locale.)
'%y'
The year modulo 100 as a decimal number (00-99).
'%Y'
The full year as a decimal number (e.g., 2015).
'%z'
The time zone offset in a '+HHMM' format (e.g., the format
necessary to produce RFC 822/RFC 1036 date headers).
'%Z'
The time zone name or abbreviation; no characters if no time zone
is determinable.
'%Ec %EC %Ex %EX %Ey %EY %Od %Oe %OH'
'%OI %Om %OM %OS %Ou %OU %OV %Ow %OW %Oy'
"Alternative representations" for the specifications that use only
the second letter ('%c', '%C', and so on).(5) (These facilitate
compliance with the POSIX 'date' utility.)
'%%'
A literal '%'.
If a conversion specifier is not one of those just listed, the
behavior is undefined.(6)
For systems that are not yet fully standards-compliant, 'gawk'
supplies a copy of 'strftime()' from the GNU C Library. It supports all
of the just-listed format specifications. If that version is used to
compile 'gawk' (Installation), then the following additional
format specifications are available:
'%k'
The hour (24-hour clock) as a decimal number (0-23). Single-digit
numbers are padded with a space.
'%l'
The hour (12-hour clock) as a decimal number (1-12). Single-digit
numbers are padded with a space.
'%s'
The time as a decimal timestamp in seconds since the epoch.
Additionally, the alternative representations are recognized but
their normal representations are used.
The following example is an 'awk' implementation of the POSIX 'date'
utility. Normally, the 'date' utility prints the current date and time
of day in a well-known format. However, if you provide an argument to
it that begins with a '+', 'date' copies nonformat specifier characters
to the standard output and interprets the current time according to the
format specifiers in the string. For example:
$ date '+Today is %A, %B %d, %Y.'
-| Today is Monday, September 22, 2014.
Here is the 'gawk' version of the 'date' utility. It has a shell
"wrapper" to handle the '-u' option, which requires that 'date' run as
if the time zone is set to UTC:
#! /bin/sh
#
# date --- approximate the POSIX 'date' command
case $1 in
-u) TZ=UTC0 # use UTC
export TZ
shift ;;
esac
gawk 'BEGIN {
format = PROCINFO["strftime"]
exitval = 0
if (ARGC > 2)
exitval = 1
else if (ARGC == 2) {
format = ARGV[1]
if (format ~ /^\+/)
format = substr(format, 2) # remove leading +
}
print strftime(format)
exit exitval
}' "$@"
---------- Footnotes ----------
(1) Glossary, especially the entries "Epoch" and "UTC."
(2) The GNU 'date' utility can also do many of the things described
here. Its use may be preferable for simple time-related operations in
shell scripts.
(3) Occasionally there are minutes in a year with a leap second,
which is why the seconds can go up to 60.
(4) Unfortunately, not every system's 'strftime()' necessarily
supports all of the conversions listed here.
(5) If you don't understand any of this, don't worry about it; these
facilities are meant to make it easier to "internationalize" programs.
Other internationalization features are described in
Internationalization.
(6) This is because ISO C leaves the behavior of the C version of
'strftime()' undefined and 'gawk' uses the system's version of
'strftime()' if it's there. Typically, the conversion specifier either
does not appear in the returned string or appears literally.
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