Every file in Linux is associated with timestamps, which specifies the last access time, last modification time and last change time.
Whenever we create a new file, or modify an existing file or its attributes, these timestamps will be updated automatically.
Touch command is used to change these timestamps (access time, modification time, and change time of a file).
1. Create an Empty File using touch
You can create an empty file using touch command. The following example will create a zero byte new file named tgs.txt.
$ touch tgs.txt
You can also use -c option to avoid creating new files. If you use -c option, and if a file doesn’t exists, touch will not create the file.
$ touch -c a.txt
You can also create more than 1 files from a single touch command. The following example will create 4 files named a, b, c, and d.
$ touch a b c d
2. Change File’s Access Time using -a
We can change the access time of a file using -a option. By default it will take the current system time and update the atime field.
Before touch command is executed:
$ stat tgs.txt File: `tgs.txt' Size: 0 Blocks: 0 IO Block: 4096 regular empty file Device: 801h/2049d Inode: 394283 Links: 1 Access: (0644/-rw-r--r--) Uid: ( 1000/lakshmanan) Gid: ( 1000/lakshmanan) Access: 2012-10-18 23:58:21.663514407 +0530 Modify: 2012-10-18 23:58:21.663514407 +0530 Change: 2012-10-18 23:58:21.663514407 +0530
$ touch -a tgs.txt
After the above touch command (Please note that the access time is changed):
$ stat tgs.txt File: `tgs.txt' Size: 0 Blocks: 0 IO Block: 4096 regular empty file Device: 801h/2049d Inode: 394283 Links: 1 Access: (0644/-rw-r--r--) Uid: ( 1000/lakshmanan) Gid: ( 1000/lakshmanan) Access: 2012-10-19 00:08:23.559514525 +0530 Modify: 2012-10-18 23:58:21.663514407 +0530 Change: 2012-10-19 00:08:23.559514525 +0530
3. Change File’s Modification Time using -m
You can change the modification time of a file using -m option.
$ touch -m *.o
The above method can be used to change the mtime of all obj files, when using make utility.
NOTE: It is not possible to change the ctime using touch command
4. Explicitly Setting Access and Modification time using -t and -d
Instead of taking the current time-stamp, you can explicitly specify the time using -t and -d options.
The format for specifying -t is [[CC]YY]MMDDhhmm[.SS]
$ touch -t [[CC]YY]MMDDhhmm[.SS]
The following explains the above format:
- CC – Specifies the first two digits of the year
- YY – Specifies the last two digits of the year. If the value of the YY is between 70 and 99, the value of the CC digits is assumed to be 19. If the value of the YY is between 00 and 37, the value of the CC digits is assumed to be 20. It is not possible to set the date beyond January 18, 2038.
- MM – Specifies the month
- DD – Specifies the date
- hh – Specifies the hour
- mm – Specifies the minute
- SS – Specifies the seconds
$ touch -a -m -t 203801181205.09 tgs.txt
Verify the above change using stat command:
$ stat tgs.txt File: `tgs.txt' Size: 3 Blocks: 8 IO Block: 4096 regular file Device: 801h/2049d Inode: 394283 Links: 1 Access: (0644/-rw-r--r--) Uid: ( 1000/lakshmanan) Gid: ( 1000/lakshmanan) Access: 2038-01-18 12:05:09.000000000 +0530 Modify: 2038-01-18 12:05:09.000000000 +0530 Change: 2012-10-19 00:40:58.763514502 +0530
You can also use a string to change the time
$ touch -d "2012-10-19 12:12:12.000000000 +0530" tgs.txt
For developers, touch command will be really helpful when you are working with Makefiles
5. Copy the Time-stamp from Another File using -r
You can also take a file as a reference, and update the time for other files, so that both file will hold the same time.
The following touch command example will update the time-stamp of file a.txt with the time-stamp of tgs.txt file.
$ touch a.txt -r tgs.txt