GAWK: Effective AWK Programming

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146 GAWK: Effective AWK Programmingand not:first printsystem echosecond printsystem echofirst printsecond printIf awk did not flush its buffers before calling system, you would see the latter (undesirable)output.8.1.5 Using gawk’s Timestamp Functionsawk programs are commonly used to process log files containing timestamp information,indicating when a particular log record was written. Many programs log their timestamp inthe form returned by the time system call, which is the number of seconds since a particularepoch. On POSIX-compliant systems, it is the number of seconds since 1970-01-01 00:00:00UTC, not counting leap seconds. 8 All known POSIX-compliant systems support timestampsfrom 0 through 2 3 1 − 1, which is sufficient to represent times through 2038-01-19 03:14:07UTC. Many systems support a wider range of timestamps, including negative timestampsthat represent times before the epoch.In order to make it easier to process such log files and to produce useful reports, gawkprovides the following functions for working with timestamps. They are gawk extensions;they are not specified in the POSIX standard, nor are they in any other known version ofawk. 9 Optional parameters are enclosed in square brackets ([ ]):systime()This function returns the current time as the number of seconds since thesystem 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 numberon other systems.mktime(datespec)This function turns datespec into a timestamp in the same form as is returnedby systime. It is similar to the function of the same name in ISO C. Theargument, datespec, is a string of the form "YYYY MM DD HH MM SS [DST]". Thestring consists of six or seven numbers representing, respectively, the full yearincluding 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 0to 60, 10 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 Gregoriancalendar is assumed, with year 0 preceding year 1 and year −1 preceding year0. The time is assumed to be in the local timezone. If the daylight-savings flag8 See [Glossary], page 306, especially the entries “Epoch” and “UTC.”9 The GNU date utility can also do many of the things described here. Its use may be preferable forsimple time-related operations in shell scripts.10 Occasionally there are minutes in a year with a leap second, which is why the seconds can go up to 60.
Chapter 8: Functions 147is positive, the time is assumed to be daylight savings time; if zero, the time isassumed to be standard time; and if negative (the default), mktime attemptsto 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 ofrange, mktime returns −1.strftime([format [, timestamp [, utc-flag]]])This function returns a string. It is similar to the function of the same name inISO C. The time specified by timestamp is used to produce a string, based onthe contents of the format string. If utc-flag is present and is either non-zero ornon-null, the value is formatted as UTC (Coordinated Universal Time, formerlyGMT or Greenwich Mean Time). Otherwise, the value is formatted for the localtime zone. The timestamp is in the same format as the value returned by thesystime function. If no timestamp argument is supplied, gawk uses the currenttime of day as the timestamp. If no format argument is supplied, strftimeuses "%a %b %d %H:%M:%S %Z %Y". This format string produces output that is(almost) equivalent to that of the date utility. (Versions of gawk prior to 3.0require the format argument.)The systime function allows you to compare a timestamp from a log file with thecurrent time of day. In particular, it is easy to determine how long ago a particular recordwas 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 timeinto 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 asa log file.The strftime function allows you to easily turn a timestamp into human-readableinformation. It is similar in nature to the sprintf function (see Section 8.1.3 [String-Manipulation Functions], page 132), in that it copies nonformat specification charactersverbatim to the returned string, while substituting date and time values for format specificationsin the format string.strftime is guaranteed by the 1999 ISO C standard 11 to support the following dateformat 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. This is the year divided by 100 and truncated to the next lowerinteger.11 As this is a recent standard, not every system’s strftime necessarily supports all of the conversionslisted here.
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