How does touch -t work internally

touch creates a new, empty file if the file doesn't exist because that's what it was designed to do. The utility has to contain code to handle that case specifically. The utility appeared in Unix V7; its manual described it thus:

touch — update date last modified of a file

touch attempts to set the modified date of each file. This is done by reading a character from the file and writing it back. If a **file* does not exist, an attempt will be made to create it unless the -c option is specified.

(I don't know what touch did if the file was empty. The underlying system call came later.)

I don't know for sure why touch was designed to make the file exist, but I suspect it's because of make. Why would you want to set a file's modification time to the current time? There are cases where it could be useful to set the modification time to a particular time, but that ability came later, the original touch could only set the modification time to the current time. A reason to do that is to re-run a make rule that depends on the file.

That is, suppose you have a file foo, and a makefile that declares a command to generate bar from foo. When you type make bar, the command is executed and bar is created. If bar exists and is newer than foo, make bar does nothing, because make assumes that bar has already been generated. However, if bar is older than foo, make thinks that bar is not up-to-date and needs to be regenerated.

But what if the rules to generate bar have changed? Then you have two options:

• rm bar; make bar
• touch foo; make bar

You would need foo to exist in order to generate bar, otherwise the command would typically not work.

The “touch” terminology was also present in the make utility: make -t bar would only pretend to run the commands, that is, it would set the modification time of bar to the current time without actually running the command to generate bar (you would do this if you thought that the changes to foo shouldn't affect bar). The touch utility was therefore a standalone version of the make -t feature.

-t doesn't accept epoch time, -d does

   -d, --date=STRING
          parse STRING and use it instead of current time

   -t STAMP
          use [[CC]YY]MMDDhhmm[.ss] instead of current time

You need to use -d or --date instead of -t and you need to put @ before epochtime format is used, as described in date manpages:

   EXAMPLES
       Convert seconds since the epoch (1970-01-01 UTC) to a date

              $ date --date='@2147483647'

Example:

touch --date=@1442968132 test.txt

If you want to change modify time only, use -m or --time modify or --time mtime, without it both modify and access times are changed.

   -m     change only the modification time

   --time=WORD
          change the specified time: WORD is access, atime, or use: equivalent to -a WORD is modify or mtime: equivalent to -m

Examples:

$ touch --date=@1442968132 test
$ stat test
  File: test
  Size: 0           Blocks: 0          IO Block: 4096   regular empty file
Device: fd03h/64771d    Inode: 43266017    Links: 1
Access: (0664/-rw-rw-r--)  Uid: ( 1000/    user1)   Gid: ( 1000/    user1)
Context: unconfined_u:object_r:user_home_t:s0
Access: 2015-09-23 02:28:52.000000000 +0200
Modify: 2015-09-23 02:28:52.000000000 +0200
Change: 2018-11-23 11:34:59.893888360 +0100
 Birth: -

$ touch --date=@1542968132 test
$ stat test
  File: test
  Size: 0           Blocks: 0          IO Block: 4096   regular empty file
Device: fd03h/64771d    Inode: 43266017    Links: 1
Access: (0664/-rw-rw-r--)  Uid: ( 1000/    user1)   Gid: ( 1000/    user1)
Context: unconfined_u:object_r:user_home_t:s0
Access: 2018-11-23 11:15:32.000000000 +0100
Modify: 2018-11-23 11:15:32.000000000 +0100
Change: 2018-11-23 11:35:06.893888073 +0100
Birth: -

$ touch -m --date=@1342968132 test
$ stat test
  File: test
  Size: 0           Blocks: 0          IO Block: 4096   regular empty file
Device: fd03h/64771d    Inode: 43266017    Links: 1
Access: (0664/-rw-rw-r--)  Uid: ( 1000/    user1)   Gid: ( 1000/    user1)
Context: unconfined_u:object_r:user_home_t:s0
Access: 2018-11-23 11:15:32.000000000 +0100
Modify: 2012-07-22 16:42:12.000000000 +0200
Change: 2018-11-23 11:35:22.300887441 +0100