Which of the following commands can be used to update the last modified timestamp on a file or if the file specified does not exist will create the file?

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

Commands like ls command and find command uses these timestamp information for listing and finding files.

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

For example:

$ 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

Another example:

$ 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

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The touch command is a standard command used in UNIX/Linux operating system which is used to create, change and modify timestamps of a file. Basically, there are two different commands to create a file in the Linux system which is as follows:

• cat command: It is used to create the file with content.
• touch command: It is used to create a file without any content. The file created using touch command is empty. This command can be used when the user doesn’t have data to store at the time of file creation.

Using touch Command

Initially, we are in home directory and this can be checked using the pwd command. Checking the existing files using command ls and then long listing command[ll] is used to gather more details about existing files. As you can see in the below figure there is no existing files.

Touch command Syntax to create a new file: You can create a single file at a time using touch command.

Syntax:

touch file_name

The file which is created can be viewed by ls command and to get more details about the file you can use long listing command ll or ls -l command . Here file with name ‘File1‘ is created using touch command.

Touch command to create multiple files: Touch command can be used to create the multiple numbers of files at the same time. These files would be empty while creation.

Syntax:

touch File1_name File2_name File3_name

Multiple files with name Doc1, Doc2, Doc3 are created at the same time using touch command here.

touch Command Options

Like all other command Touch command have various options. These options are very useful for various purpose.

touch -a: This command is used to change access time only. To change or update the last access or modification times of a file touch -a command is used.

Syntax:

touch -a fileName

Here touch -a command changes access time of the file named Doc1.

touch -c : This command is used to check whether a file is created or not. If not created then don’t create it. This command avoids creating files.

Syntax:

touch -c fileName

touch -c-d : This is used to update access and modification time.

Syntax:

touch -c-d fileName

touch -m : This is used to change the modification time only. It only updates last modification time.

Syntax:

touch -m fileName

touch -r : This command is used to use the timestamp of another file. Here Doc2 file is updated with the time stamp of File 1.

Syntax:

touch -r second_file_name first_file_name

touch -t : This is used to create a file using a specified time.

Syntax:

touch -t YYMMDDHHMM fileName

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