Home » What are the legitimate uses of the `touch` command?

What are the legitimate uses of the `touch` command?

Solutons:


One advantage of touch is that it can specify arbitrary timestamps, while echo -n will always result in the current time.

An example of a legitimate use is to update a timestamp of a source code file so a program like make will consider the source file newer than its compiled object and rebuild it.

Other uses are to create files that function solely based on their existence e.g. /etc/nologin, which disallows logins if it exists. I’d also argue that touch myfile is a simpler construct than echo -n >> myfile, as well as being shorter to type.

The underlying system call (utime) is important for various utilities like tar to be able to set the timestamps of a newly copied (un-tarred) file. Some backup utilities can also be optioned to reset the last-accesstime of files that they have copied.

One legitimate use of touch is to create a file with a particular timestamp. Such a “reference” file can then be used by commands like find. For example, to find all files more recently modified than Nov 1, 2013 (ie 2013/11/01)”

touch -amt 201311010000 myref
find . -type f -newer myref -exec ls -ld {} +

The touch command’s primary purpose is manipulating the timestamps of files, and for creating files.

Examples

1. creating files

$ ls -l
total 0

$ touch file{1..3}

$ ls -l
total 0
-rw-rw-r--. 1 saml saml 0 Jan 12 13:33 file1
-rw-rw-r--. 1 saml saml 0 Jan 12 13:33 file2
-rw-rw-r--. 1 saml saml 0 Jan 12 13:33 file3

NOTE: The total 0 output from ls -l is correct. This implementation of ls shows the number of blocks being used by the files when they’re listed. You can assure yourself of this fact by adding the -s switch to ls. This will list the blocks in use by each file. Here I’ve added 2 characters to file1 & file2.

Example

$ ls -ls
total 8
4 -rw-rw-r--. 1 saml saml 3 Jan 13 12:07 file1
4 -rw-rw-r--. 1 saml saml 3 Jan 13 12:09 file2
0 -rw-rw-r--. 1 saml saml 0 Jan 13 12:05 file3

2. time/date info of a file – stat command

$ stat file1
  File: ‘file1’
  Size: 0           Blocks: 0          IO Block: 4096   regular empty file
Device: fd02h/64770d    Inode: 11403667    Links: 1
Access: (0664/-rw-rw-r--)  Uid: ( 1000/    saml)   Gid: ( 1000/    saml)
Context: unconfined_u:object_r:user_home_t:s0
Access: 2014-01-12 13:33:38.279456149 -0500
Modify: 2014-01-12 13:33:38.279456149 -0500
Change: 2014-01-12 13:33:38.279456149 -0500
 Birth: -

We can use touch to manipulate the various timestamps on a given file.

3. excerpt from touch man page

   -a     change only the access time
   -m     change only the modification time
   -t STAMP
          use [[CC]YY]MMDDhhmm[.ss] instead of current time

4. manipulating access time

$ touch -a -t200001010000 file1

$ stat file1
  File: ‘file1’
  Size: 0           Blocks: 0          IO Block: 4096   regular empty file
Device: fd02h/64770d    Inode: 11403667    Links: 1
Access: (0664/-rw-rw-r--)  Uid: ( 1000/    saml)   Gid: ( 1000/    saml)
Context: unconfined_u:object_r:user_home_t:s0
Access: 2000-01-01 00:00:00.000000000 -0500
Modify: 2014-01-12 13:33:38.279456149 -0500
Change: 2014-01-12 13:38:52.023434696 -0500
 Birth: -

5. manipulate modify time

$ touch -m -t200001010000 file1

$ stat file1
  File: ‘file1’
  Size: 0           Blocks: 0          IO Block: 4096   regular empty file
Device: fd02h/64770d    Inode: 11403667    Links: 1
Access: (0664/-rw-rw-r--)  Uid: ( 1000/    saml)   Gid: ( 1000/    saml)
Context: unconfined_u:object_r:user_home_t:s0
Access: 2000-01-01 00:00:00.000000000 -0500
Modify: 2000-01-01 00:00:00.000000000 -0500
Change: 2014-01-12 13:39:31.060432026 -0500
 Birth: -

You might be wondering about the change time (ctime). That cannot be manipulated using touch. That tracks the time any of the meta data was touched on the file. See this U&L Q&A for more details, titled: What can you do to a file without triggering the “Change” Timestamp?.

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