The touch command changes these times and is used in the following manner:
# touch options expressions filename(s)
When touch is used without options or an expression, both times are set to the current time. The file is created if it doesn't exist.., but not overwritten if it does:
#touch note #Creates file if it doesn't exist
Various Options of Touch Command:
-a (touch -a) Update only the access time to current time
-c (touch -c) Don't create if the file doesn't exist
-d (touch -d) Set an arbitrary time for both times using free format human readable data string
-m (touch -m) Update only the modification time to current time
-r (touch -r) Update both file's times from a reference file
-t (touch -t) Set an arbitrary time for both time using stamp
If you want to check the file timestamp, you can do so with this command:
#stat file
[root@localhost indrajit]# stat abcd
File: `abcd'
Size: 0 Blocks: 0 IO Block: 4096 regular empty file
Device: 802h/2050d Inode: 281784 Links: 1
Access: (0644/-rw-r--r--) Uid: ( 0/ root) Gid: ( 0/ root)
Access: 2013-04-15 22:40:00.000000000 -0700
Modify: 2013-04-15 22:40:00.000000000 -0700
Change: 2013-12-12 20:08:43.506978374 -0800
Now let's use the command on file abcd, but only after seen its initial time stamps:
[root@localhost indrajit]# ls -l abcd Modification Time
-rw-r--r--. 1 root root 0 Apr 15 2013 abcd
[root@localhost indrajit]# ls -lu abcd Access Time
-rw-r--r--. 1 root root 0 Apr 15 2013 abcd
With the -m (modification) option, you can alter the modificaiton time alone:
#touch abcd
#ls -l abcd
-rw-r--r-- 1 root sys 0 Dec 13 05:33 abcd
#touch -m 02281030 abcd;ls -l abcd
-rw-r--r-- 1 root sys 0 Feb 28 2013 abcd
The -a (access) option changes the access time:
#ls -lu abcd
-rw-r--r-- 1 root sys 0 Dec 13 05:33 abcd
#touch -a 01261650 abcd;ls -lu abcd
-rw-r--r-- 1 root sys 0 Jan 26 2013 abcd
-c option
If you use this option, touch won’t do anything at all if the file specified doesn't exist. Look:
#ls -l xyz
xyz not found
#touch -c xyz;ls -l xyz
xyz not found
-r option
This option might come in handy if you want to copy a timestamp from a file to another file. Like so:
#ls -l abcd abcd1
-rw-r--r-- 1 root sys 0 Feb 28 2013 abcd
-rw-r--r-- 1 root sys 0 Dec 13 05:41 abcd1
#touch -r abcd1 abcd;ls -l abcd
-rw-r--r-- 1 root sys 0 Dec 13 05:41 abcd
-d and –t options
Both (-d) and (-t) options do the same thing, which is setting the same arbitrary timestamps for access and modification times. The difference is that (-d) uses free format human readable date, this means that you can use “Sun, 29 Feb 2004 16:21:42? or “2004-02-29 16:21:42” or even “next Thursday”. This option is complex to fully describe it here. On the other hand (-t) uses a simple stamp that you are confined to use. The stamp is [[CC]YY]MMDDhhmm[.ss]. [CC] is for century and you may ignore it and ignore the seconds as well. If you ignore [CC] the command will substitute it depending on what you enter as year. If you specify the year with only two digits, then CC is 20 for years in the range (0~68) and 19 for years in (69~99).
#ls -l abcd
-rw-r--r-- 1 root sys 0 Dec 13 05:41 abcd
#touch -t 3404152240 abcd;ls -l abcd
-rw-r--r-- 1 root sys 0 Apr 15 2034 abcd
#touch -t 8804152240 abcd;ls -l abcd
-rw-r--r-- 1 root sys 0 Apr 15 1988 abcd
In the first command the file timestamps will be set to: 15th April 2034 10:40 PM. While the second command will set it to: 15th April 1988 which is in a different century. If no year is specified it will be set to the current year. Example:
#touch -t 04152240 abcd;ls -l abcd
-rw-r--r-- 1 root sys 0 Apr 15 2013 abcd
