Why does touch create new files

filestouch

I am aware that the touch command is used to update the date of last modification on a file. It is also used to create a new file if requested file does not exist on file system.

Since touch (as it's name implies), should just update last mod date, why does it also try to create a new file?

Is it just a check written in the touch's code, or is it something else that causes a file to be created?

Best Answer

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.

Related Solutions

Shell – delete old log files except the last one (alphanumerically sorted)

find /home/foo -maxdepth 1 -iname log\* -type f -mmin +1800 |
  sort | head -n -1 | xargs rm

Or if you want to use mtime instead of the filename:

find /home/foo -maxdepth 1 -iname log\* -type f -mmin +1800 -exec ls -t {} + |
  tail -n +2 | xargs rm

From @Stephane's comments, a more robust approach would be to do:

IFS=$'\n'
set -f
rm $(
  find /home/foo -maxdepth 1 -iname log\* ! -name $'*\n*' -type f -mmin +1800 |
  sort | head -n -1 )

Or for POSIX shell (still requires GNU tools):

IFS='
'
ex_newline='*
*'
set -f
rm $(
  find /home/foo -maxdepth 1 -iname log\* ! -name "$ex_newline" -type f -mmin +1800 |
  sort | head -n -1 )

A single (robust) pipeline can be used with a recent version of GNU sed/sort (and GNU find as with all of the above):

find /home/foo -maxdepth 1 -iname log\* -type f -mmin +1800 -print0 |
  sort -z | sed -z '$d' | xargs -0 rm

Linux – Change File’s ‘Date Created’ and ‘Last Modified’ Attributes

You can use the touch command along with the -r switch to apply another file's attributes to a file.

NOTE: There is no such thing as creation date in Unix, there are only access, modify, and change. See this U&L Q&A titled: get age of given file for further details.

$ touch -r goldenfile newfile

Example

For example purposes here's a goldenfile that was created with some arbitrary timestamp.

$ touch -d 20120101 goldenfile
$ ls -l goldenfile 
-rw-rw-r--. 1 saml saml 0 Jan  1  2012 goldenfile

Now I make some new file:

$ touch newfile
$ ls -l newfile 
-rw-rw-r--. 1 saml saml 0 Mar  7 09:06 newfile

Now apply goldenfile's attributes to newfile.

$ touch -r goldenfile newfile 
$ ls -l goldenfile newfile
-rw-rw-r--. 1 saml saml 0 Jan  1  2012 newfile
-rw-rw-r--. 1 saml saml 0 Jan  1  2012 goldenfile

Now newfile has the same attributes.

Modify via Samba

I just confirmed that I'm able to do this using my Fedora 19 laptop which includes version 1.16.3-2 connected to a Thecus N12000 NAS (uses a modified version of CentOS 5.x).

I was able to touch a file as I mentioned above and it worked as I described. Your issue is likely a problem with the either the mounting options being used, which may be omitting the tracking of certain time attributes, or perhaps it's related to one of these bugs: