(gawk.info) Time Functions
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(gawk.info) I/O Functions
(gawk.info) Built-in
Functions for Dealing with Time Stamps
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A common use for `awk' programs is the processing of log files
containing time stamp information, indicating when a particular log
record was written. Many programs log their time stamp in the form
returned by the `time' system call, which is the number of seconds
since a particular epoch. On POSIX systems, it is the number of
seconds since Midnight, January 1, 1970, UTC.
In order to make it easier to process such log files, and to produce
useful reports, `gawk' provides two functions for working with time
stamps. Both of these are `gawk' extensions; they are not specified in
the POSIX standard, nor are they in any other known version of `awk'.
Optional parameters are enclosed in square brackets ("[" and "]").
`systime()'
This function returns the current time as the number of seconds
since the system epoch. On POSIX systems, this is the number of
seconds since Midnight, January 1, 1970, UTC. It may be a
different number on other systems.
`strftime([FORMAT [, TIMESTAMP]])'
This function returns a string. It is similar to the function of
the same name in ANSI C. The time specified by TIMESTAMP is used
to produce a string, based on the contents of the FORMAT string.
The TIMESTAMP is in the same format as the value returned by the
`systime' function. If no TIMESTAMP argument is supplied, `gawk'
will use the current time of day as the time stamp. If no FORMAT
argument is supplied, `strftime' uses `"%a %b %d %H:%M:%S %Z %Y"'.
This format string produces output (almost) equivalent to that of
the `date' utility. (Versions of `gawk' prior to 3.0 require the
FORMAT argument.)
The `systime' function allows you to compare a time stamp 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 `strftime' function allows you to easily turn a time stamp into
human-readable information. It is similar in nature to the `sprintf'
function ( Built-in Functions for String Manipulation String
Functions.), in that it copies non-format 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 ANSI C standard 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.
`%d'
The day of the month as a decimal number (01-31).
`%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).
`%p'
The locale's equivalent of the AM/PM designations associated with
a 12-hour clock.
`%S'
The second as a decimal number (00-60).(1)
`%U'
The week number of the year (the first Sunday as the first day of
week one) as a decimal number (00-53).
`%w'
The weekday as a decimal number (0-6). Sunday is day zero.
`%W'
The week number of the year (the first Monday as the first day of
week one) as a decimal number (00-53).
`%x'
The locale's "appropriate" date representation.
`%X'
The locale's "appropriate" time representation.
`%y'
The year without century as a decimal number (00-99).
`%Y'
The year with century as a decimal number (e.g., 1995).
`%Z'
The time zone name or abbreviation, or no characters if no time
zone is determinable.
`%%'
A literal `%'.
If a conversion specifier is not one of the above, the behavior is
undefined.(2)
Informally, a "locale" is the geographic place in which a program is
meant to run. For example, a common way to abbreviate the date
September 4, 1991 in the United States would be "9/4/91". In many
countries in Europe, however, it would be abbreviated "4.9.91". Thus,
the `%x' specification in a `"US"' locale might produce `9/4/91', while
in a `"EUROPE"' locale, it might produce `4.9.91'. The ANSI C standard
defines a default `"C"' locale, which is an environment that is typical
of what most C programmers are used to.
A public-domain C version of `strftime' is supplied with `gawk' for
systems that are not yet fully ANSI-compliant. If that version is used
to compile `gawk' ( Installing `gawk' Installation.), then the
following additional format specifications are available:
`%D'
Equivalent to specifying `%m/%d/%y'.
`%e'
The day of the month, padded with a space if it is only one digit.
`%h'
Equivalent to `%b', above.
`%n'
A newline character (ASCII LF).
`%r'
Equivalent to specifying `%I:%M:%S %p'.
`%R'
Equivalent to specifying `%H:%M'.
`%T'
Equivalent to specifying `%H:%M:%S'.
`%t'
A tab character.
`%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.
`%C'
The century, as a number between 00 and 99.
`%u'
The weekday as a decimal number [1 (Monday)-7].
`%V'
The week number of the year (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 week 53 of the previous year and
the next week is week one).
`%G'
The year with century of the ISO week number, as a decimal number.
For example, January 1, 1993, is in week 53 of 1992. Thus, the year
of its ISO week number is 1992, even though its year is 1993.
Similarly, December 31, 1973, is in week 1 of 1974. Thus, the year
of its ISO week number is 1974, even though its year is 1973.
`%g'
The year without century of the ISO week number, as a decimal
number (00-99).
`%Ec %EC %Ex %Ey %EY %Od %Oe %OH %OI'
`%Om %OM %OS %Ou %OU %OV %Ow %OW %Oy'
These are "alternate representations" for the specifications that
use only the second letter (`%c', `%C', and so on). They are
recognized, but their normal representations are used.(3) (These
facilitate compliance with the POSIX `date' utility.)
`%v'
The date in VMS format (e.g., 20-JUN-1991).
`%z'
The timezone offset in a +HHMM format (e.g., the format necessary
to produce RFC-822/RFC-1036 date headers).
This 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' will copy non-format specifier characters to
the standard output, and will interpret the current time according to
the format specifiers in the string. For example:
$ date '+Today is %A, %B %d, %Y.'
-| Today is Thursday, July 11, 1991.
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 was set to UTC.
#! /bin/sh
#
# date --- approximate the P1003.2 'date' command
case $1 in
-u) TZ=GMT0 # use UTC
export TZ
shift ;;
esac
gawk 'BEGIN {
format = "%a %b %d %H:%M:%S %Z %Y"
exitval = 0
if (ARGC > 2)
exitval = 1
else if (ARGC == 2) {
format = ARGV[1]
if (format ~ /^\+/)
format = substr(format, 2) # remove leading +
}
print strftime(format)
exit exitval
}' "$@"
---------- Footnotes ----------
(1) Occasionally there are minutes in a year with a leap second,
which is why the seconds can go up to 60.
(2) This is because ANSI C leaves the behavior of the C version of
`strftime' undefined, and `gawk' will use the system's version of
`strftime' if it's there. Typically, the conversion specifier will
either not appear in the returned string, or it will appear literally.
(3) If you don't understand any of this, don't worry about it; these
facilities are meant to make it easier to "internationalize" programs.
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