The use of the -f option for `touch`

coreutilstouch

From man touch:

-f     (ignored)

But I don't get what is meant by ignored.

I've tried following:

$ ls -l file
-rw-rw-r-- 1 pandya pandya 0 Mar 20 16:17 file

$ touch -f file
$ ls -l file
-rw-rw-r-- 1 pandya pandya 0 Mar 20 16:18 file

And noticed that it changes timestamps in spite of -f.

So, I want to know what -f stands for, or what it does.

Best Answer

For GNU utilities, the full documentation is in the info page, where you can read:

-f
Ignored; for compatibility with BSD versions of `touch'.

See historic BSD man pages for touch, where -f was to force the touch.

If you look at the source of those old BSDs, there was no utimes() system call, so touch would open the file in read+write mode, read one byte, seek back and write it again so as to update the last access and last modification time.

Obviously, you needed both read and write permissions (touch would avoid trying to do that if access(W_OK|R_OK) returned false). -f tried to work around that by temporarily changing the permissions temporarily to 0666!

0666 means read and write permission to everybody. It had to be that as otherwise (like with a more restrictive permission such as 0600 that still would have permitted the touch) that could mean during that short window, processes that would otherwise have read or write permission to the file couldn't any more, breaking functionality.

That means however that processes that would not otherwise have access to the file now have a short opportunity to open it, breaking security.

That's not a very sensible thing to do. Modern touch implementations don't do that. Since then, the utime() system call has been introduced, allowing changing modification and access time separately without having to mingle with the content of the files (which means it also works with non-regular files) and only needs write access for that.

GNU touch still doesn't fail if passed the -f option, but just ignores the flag. That way, scripts written for those old versions of BSD don't fail when ported to GNU systems. Not much relevant nowadays.

Updated Answer

I stumbled across this gem

touch -r filename -d '+8 days' filename

From the info coreutils touch invocation (Thank you @don_crissti ):

'-r FILE'

'--reference=FILE'

 Use the times of the reference FILE instead of the current time.
 If this option is combined with the '--date=TIME' ('-d TIME')
 option, the reference FILE's time is the origin for any relative
 TIMEs given, but is otherwise ignored.  For example, '-r foo -d
 '-5 seconds'' specifies a time stamp equal to five seconds before
 the corresponding time stamp for 'foo'.  If FILE is a symbolic
 link, the reference timestamp is taken from the target of the
 symlink, unless '-h' was also in effect.

If you want variable expansion you can use soft quotes around the -d argument like so.

DAYS=8
touch -r filename -d "+${DAYS} days" filename

Sample:

$ ls -l foo.bar
-rw-r--r-- 1 usr usr 69414810 Nov 10  2016 foo.bar
$ TEST=100
$ touch -r foo.bar -d "+${TEST} days" foo.bar
$ ls -l foo.bar 
-rw-r--r-- 1 usr usr 69414810 Feb 24  2017 foo.bar

Linux – Use touch command to set modification time of a file to the Unix epoch

-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

Best Answer

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Updated Answer

Linux – Use touch command to set modification time of a file to the Unix epoch

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