How do I find the current system timezone?

Question

On Linux, I need to find the currently configured timezone as an Olson location. I want my (C or C++) code to be portable to as many Linux systems as possible.

For e

Answer 1

I see two major linux cases:

1. Ubuntu. There should be a /etc/timezone file. This file should only contain the timezone and nothing else.
2. Red Hat. There should be a /etc/sysconfig/clock that contains something like: ZONE="America/Chicago"

In addition, Solaris should have an /etc/TIMEZONE file that contains a line like: TZ=US/Mountain

So based on the above, here is some straight C that I believe answers the OP's question. I have tested it on Ubuntu, CentOS (Red Hat), and Solaris (bonus).

#include <string.h>
#include <strings.h>
#include <stdio.h>

char *findDefaultTZ(char *tz, size_t tzSize);
char *getValue(char *filename, char *tag, char *value, size_t valueSize);

int main(int argc, char **argv)
{
  char tz[128];

  if (findDefaultTZ(tz, sizeof(tz)))
    printf("Default timezone is %s.\n", tz);
  else
    printf("Unable to determine default timezone.\n");
  return 0;
}


char *findDefaultTZ(char *tz, size_t tzSize)
{
  char *ret = NULL;
  /* If there is an /etc/timezone file, then we expect it to contain
   * nothing except the timezone. */
  FILE *fd = fopen("/etc/timezone", "r"); /* Ubuntu. */
  if (fd)
  {
    char buffer[128];
    /* There should only be one line, in this case. */
    while (fgets(buffer, sizeof(buffer), fd))
    {
      char *lasts = buffer;
      /* We don't want a line feed on the end. */
      char *tag = strtok_r(lasts, " \t\n", &lasts);
      /* Idiot check. */
      if (tag && strlen(tag) > 0 && tag[0] != '#')
      {
        strncpy(tz, tag, tzSize);
        ret = tz;
      }
    }
    fclose(fd);
  }
  else if (getValue("/etc/sysconfig/clock", "ZONE", tz, tzSize)) /* Redhat.    */
    ret = tz;
  else if (getValue("/etc/TIMEZONE", "TZ", tz, tzSize))     /* Solaris. */
    ret = tz;
  return ret;
}

/* Look for tag=someValue within filename.  When found, return someValue
 * in the provided value parameter up to valueSize in length.  If someValue
 * is enclosed in quotes, remove them. */
char *getValue(char *filename, char *tag, char *value, size_t valueSize)
{
  char buffer[128], *lasts;
  int foundTag = 0;

  FILE *fd = fopen(filename, "r");
  if (fd)
  {
    /* Process the file, line by line. */
    while (fgets(buffer, sizeof(buffer), fd))
    {
      lasts = buffer;
      /* Look for lines with tag=value. */
      char *token = strtok_r(lasts, "=", &lasts);
      /* Is this the tag we are looking for? */
      if (token && !strcmp(token, tag))
      {
        /* Parse out the value. */
        char *zone = strtok_r(lasts, " \t\n", &lasts);
        /* If everything looks good, copy it to our return var. */
        if (zone && strlen(zone) > 0)
        {
          int i = 0;
          int j = 0;
          char quote = 0x00;
          /* Rather than just simple copy, remove quotes while we copy. */
          for (i = 0; i < strlen(zone) && i < valueSize - 1; i++)
          {
            /* Start quote. */
            if (quote == 0x00 && zone[i] == '"')
              quote = zone[i];
            /* End quote. */
            else if (quote != 0x00 && quote == zone[i])
              quote = 0x00;
            /* Copy bytes. */
            else
            {
              value[j] = zone[i];
              j++;
            }
          }
          value[j] = 0x00;
          foundTag = 1;
        }
        break;
      }
    }
    fclose(fd);
  }
  if (foundTag)
    return value;
  return NULL;
}

Answer 2

I liked the post made by psmears and implemented this script to read the first output of the list. Of course there must have more elegant ways of doing this, but there you are...

    /**
     * Returns the (Linux) server default timezone abbreviation
     * To be used when no user is logged in (Ex.: batch job)
     * Tested on Fedora 12
     * 
     * @param void
     * @return String (Timezone abbreviation Ex.: 'America/Sao_Paulo')
     */
    public function getServerTimezone()
    {

        $shell = 'md5sum /etc/localtime';
        $q = shell_exec($shell);
        $shell = 'find /usr/share/zoneinfo -type f | xargs md5sum | grep ' . substr($q, 0, strpos($q, '/') - 2);
        $q = shell_exec($shell);
        $q = substr($q, strpos($q, 'info/') + 5, strpos($q, " "));
        return substr($q, 0, strpos($q, chr(10)));

    }

In my Brazilian Fedora 12, it returns:
Brazil/East

And does exactly what I need.

Thank you psmears

Answer 3

It's hard to get a reliable answer. Relying on things like /etc/timezone may be the best bet.

(The variable tzname and the tm_zone member of struct tm, as suggested in other answers, typically contains an abbreviation such as GMT/BST etc, rather than the Olson time string as requested in the question).

• On Debian-based systems (including Ubuntu), /etc/timezone is a file containing the right answer.
• On some Redhat-based systems (including at least some versions of CentOS, RHEL, Fedora), you can get the required information using readlink() on /etc/localtime, which is a symlink to (for example) /usr/share/zoneinfo/Europe/London.
• OpenBSD seems to use the same scheme as RedHat.

However, there are some issues with the above approaches. The /usr/share/zoneinfo directory also contains files such as GMT and GB, so it's possible the user may configure the symlink to point there.

Also there's nothing to stop the user copying the right timezone file there instead of creating a symlink.

One possibility to get round this (which seems to work on Debian, RedHat and OpenBSD) is to compare the contents of the /etc/localtime file to the files under /usr/share/zoneinfo, and see which ones match:

eta:~% md5sum /etc/localtime
410c65079e6d14f4eedf50c19bd073f8  /etc/localtime
eta:~% find /usr/share/zoneinfo -type f | xargs md5sum | grep 410c65079e6d14f4eedf50c19bd073f8
410c65079e6d14f4eedf50c19bd073f8  /usr/share/zoneinfo/Europe/London
410c65079e6d14f4eedf50c19bd073f8  /usr/share/zoneinfo/Europe/Belfast
410c65079e6d14f4eedf50c19bd073f8  /usr/share/zoneinfo/Europe/Guernsey
410c65079e6d14f4eedf50c19bd073f8  /usr/share/zoneinfo/Europe/Jersey
410c65079e6d14f4eedf50c19bd073f8  /usr/share/zoneinfo/Europe/Isle_of_Man
...
...

Of course the disadvantage is that this will tell you all timezones that are identical to the current one. (That means identical in the full sense - not just "currently at the same time", but also "always change their clocks on the same day as far as the system knows".)

Your best bet may be to combine the above methods: use /etc/timezone if it exists; otherwise try parsing /etc/localtime as a symlink; if that fails, search for matching timezone definition files; if that fails - give up and go home ;-)

(And I have no idea whether any of the above applies on AIX...)

Answer 4

Since tzselect was not mentioned by anyone and you do need a nearly goof-proof solution, work with what Olson did. Get the tzcode and tzdata files from elsie, plus tab files.

ftp://elsie.nci.nih.gov

In March 2017, the correct location to download from would be ftp://ftp.iana.org/tz/releases (and download tzcode2017a.tar.gz and tzdata2017a.tar.gz).

Then get tzselect.ksh from the glibc download. Then you can see how to reverse engineer timezones. One sticking point: you WILL sometimes have to ask what country and city the linux box is in. You can serialize that data if you want, and then verify it against the timezone data you can find.

There is no way to do this reliably all the time without the possibility of user intervention, for example, as part of program installation.

Good luck on Arizona in general, and in Western Indiana.... hopefully your code is going to run elsewhere.

Answer 5

According to this page, it looks like if you #include <time.h> it will declare the following.

void tzset (void);
extern char *tzname[2];
extern long timezone;
extern int daylight;

Does that give you the information that you need?

Answer 6

The libc accesses the Olson database when tzset is called, and uses simplified time zones afterwards. tzset looks at the TZ environment variable first, and falls back to parsing the binary data in /etc/localtime.

At first systemd standardised on having the Olson time zone name in /etc/timezone, Debian-style. After systemd 190 and the /usr merge, systemd only reads and updates /etc/localtime, with the extra requirement that the file be a symlink to /usr/share/zoneinfo/${OLSON_NAME}.

Looking at TZ, then readlink("/etc/localtime"), is the most reliable way to match the libc's tzset logic and still keep symbolic Olson names. For systems that don't follow the systemd symlink convention, reading /etc/timezone (and possibly checking that /usr/share/zoneinfo/$(</etc/timezone) is the same as /etc/localtime) is a good fallback.

If you can live without symbolic names, parsing the /etc/localtime tzfile is as portable as it gets, though a lot more complex. Reading just the last field gets you a Posix time zone (for example: CST5CDT,M3.2.0/0,M11.1.0/1), which can interoperate with a few time-handling libraries, but drops some of the metadata (no historical transition info).