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Linux touch command - Why do we need to change Timestamp?
We may use touch command in Linux to create an empty file. Of course we can use another command such as vi, nano or any editing tools to do it. But you may need more steps do it. But do know it that the main function of touch command is not create an empty file?
What is touch command
As we know, every file has timestamp attached to it. This time stamp is access time and modification time. Touch command is a command for file access and modification time.
Why we need touch
Since touch command description is to change timestamp, we may wonder why do we need to change timestamp? This question also comes to our mind. But then, we think there is a reason why we need it. If you are participating in open source projects over sea, then timestamp may become important. Compiling a source code files may need a same region of timestamp. If not, the program may failed to compiled.
Run touch without options
To use touch command is quite simple. Just type :
$ touch file_name
Please take a look a screenshot below.
File_1.txt originally has timestamp 12:42. After we do touch command, it changed to 17:08. By default, touch will change the file timestamp into current time.
Change only Access time
As we mention before that every file has Access time and Modify time attached to the file. File_1.txt above has timestamp 17:08. We can see more detail about it.
We see that Access time and Modify time has the same value which is 17:08:35 and the timezone is GMT +7.
Now if we want to change only the Access time, we need to use -a option.
$ touch -a file_1.txt
As you see, the Access time is changed into 17:51:37 but the Change time is still 17:08:35
Change only Modify time
To do this, we can use -m option. Now we will use file_2.txt as an example.
$ touch -m file_2.txt
Now the Modify time is change from 12:42:20 to 17:57:20. Please note, that the Change field value will always keep record when the file is accessed or modified.
Change into custom timestamp
Both -a and -m option will change the file timestamp into current time. We can also possible to change it into custom timestamp. To do this, use -t option.
From file_2.txt above example, we knew that its time stamp are :
12:42:20 for Access time
17:57:20 for Modify time
2014-01-14 for the date
Let say we want to change it into 09:58:27 for both Access time and Modify time and 12 January 2014. To do this, we can use this command :
$ touch -t 201401120958.27 file_2.txt
-t option is consist of :
[[CC]YY]MMDDhhmm [.SS]
CC - The first two digits of the year
YY - The second two digits of the year
MM - The month of the year [01-12]
DD - The day of the month [01-31]
hh - The hour of the day [00-23]
mm - The minute of the hour [00-59]
SS - The second of the minute [00-61]
Another way to change date and time
If you feel that the [[CC]YY]MMDDhhmm [.SS] format is not comfortable for you, we can use -d option. Here’s an example how to use -d option.
Change the date to specific date
For example we have file named file_3.txt with attributes as shown in the picture below.
Now we want to change the date from 14 January 2014 to 10 December 2013. We can use with this command :
$ touch -d ‘10-December-2013’ file_3.txt
We see now that the date for Access and Modify entries is changed into 10 December 2013.
Change the timezone
If we want to change it into time in specific GMT, we can also use with -d option. Let’s return the file_3.txt into the current time.
$ touch file_3.txt
We can see that file_3.txt has GMT +0700 timezone. To change it into GMT3 timezone, we can use this command.
$ touch -d GMT3 file_3.txt
Now the time is change into 10:00:00 AM
Combining values using -d option
There is something cool using -d option. Please take a look at the picture below.
We can use the word ‘next Sunday’ and combine it with GMT 3 values, and touch command can still recognize it. The date is changed into 21 January 2014, where the current date is 14 January 2014.
Here’s another example of -d option. First, we reset file_3.txt into current date and time.
$ touch file_3.txt
Then we do this command :
$ touch -d ‘1 year ago 13:43:07’ file_3.txt
Touch even recognize a word ‘1 year ago’. The date and time is now changed into 14 January 2013 and 13:43:07 time.
Create an empty file
When you run touch command and the destination file is not exist, then touch will create an empty file with the same name.
$ touch file_10.txt
Create multiple files at the same time
To create multiple files, you can put the file names separated by space.
$ touch doc_10.txt doc_20.txt doc_30.txt
Conclusion
Touch will useful for you if you are dealing with a timestamp of files and directories. As usual you can always type man touch or touch --help to display its manual page to explore more detail.