Unix stores three timestamps for each file:
atime- access time. It shows the last time the file has been read. Whenever you or some running process reads from a file itsatimetimestamp gets updated. Note thatatimedoesn't get updated when the file's contents or attributes are changed.mtime- modification time. It shows the last time the contents of a file has been changed. This means that whenever you or some process updates the contents of the file (write to it), itsmtimetimestamp gets updated. Note, thatmtimegets updated only when the contents of a file is changed, it doesn't get updated when attributes of a file (e.g. file permissions, owner) are changed.ctime- change time. It shows the last time the file (contents, attributes) has been changed. The difference betweenmtimeis that thectimetimestamp gets updated in case of any file changes including file attributes.
Let's create a new file called test.txt and check its timestamps:
$ echo "hello world" > test.txt
$ stat --printf '%n\nmtime: %y\nctime: %z\natime: %x\n' test.txt
test.txt
mtime: 2018-03-25 06:23:32.865110285 +0000
ctime: 2018-03-25 06:23:32.865110285 +0000
atime: 2018-03-25 06:23:32.865110285 +0000When a new file is created all the timestamps are set to the same value.
Let's read the file's contents and see how it affects the timestamps:
$ cat test.txt
hello world
$ stat --printf '%n\nmtime: %y\nctime: %z\natime: %x\n' test.txt
test.txt
mtime: 2018-03-25 06:23:32.865110285 +0000
ctime: 2018-03-25 06:23:32.865110285 +0000
atime: 2018-03-25 06:25:31.821110758 +0000Since we read the file, atime timestamp was updated. The ctime and mtime didn't change because we haven't changed anything about the file.
Let's write to a file and see what timestamps will change:
$ echo "unix rocks" >> test.txt
$ stat --printf '%n\nmtime: %y\nctime: %z\natime: %x\n' test.txt
test.txt
mtime: 2018-03-25 06:29:00.177111587 +0000
ctime: 2018-03-25 06:29:00.177111587 +0000
atime: 2018-03-25 06:25:31.821110758 +0000Since the file contents has been changed, the mtime timestamp was updated as well as ctime (which tracks all file changes including its contents).
Let's update files attributes. For example, let's change file permissions:
$ chmod 755 test.txt
$ stat --printf '%n\nmtime: %y\nctime: %z\natime: %x\n' test.txt
test.txt
mtime: 2018-03-25 06:29:00.177111587 +0000
ctime: 2018-03-25 06:42:44.093114865 +0000
atime: 2018-03-25 06:25:31.821110758 +0000The ctime attribute was updated since the file was changed. mtime didn't change since we didn't the file's contents didn't change, and atime didn't changed since we didn't read from the file.
stat command includes a lot of information about the file including its timestampes:
$ stat test.txt
File: ‘test.txt’
Size: 23 Blocks: 8 IO Block: 4096 regular file
Device: 801h/2049d Inode: 131082 Links: 1
Access: (0755/-rwxr-xr-x) Uid: ( 1000/ vagrant) Gid: ( 1000/ vagrant)
Access: 2018-03-25 06:25:31.821110758 +0000
Modify: 2018-03-25 06:29:00.177111587 +0000
Change: 2018-03-25 06:42:44.093114865 +0000ls command:
$ ls -l test.txt # check mtime
-rwxr-xr-x 1 vagrant vagrant 23 Mar 25 06:29 test.txt
$ ls -lc test.txt # check ctime
-rwxr-xr-x 1 vagrant vagrant 23 Mar 25 06:42 test.txt
$ ls -lu test.txt # check atime
-rwxr-xr-x 1 vagrant vagrant 23 Mar 25 06:25 test.txtThe useful option to ls command is t which allows you to sort files in a directory based on their timestamp. For example, to sort the output of ls command based on the files modification time (mtime), you can use the following command:
$ ls -l
total 4
-rw-rw-r-- 1 vagrant vagrant 0 Mar 25 06:56 file1.txt
-rw-rw-r-- 1 vagrant vagrant 0 Mar 25 06:57 file2.txt
-rwxr-xr-x 1 vagrant vagrant 23 Mar 25 06:29 test.txt
$ ls -lt
total 4
-rw-rw-r-- 1 vagrant vagrant 0 Mar 25 06:57 file2.txt
-rw-rw-r-- 1 vagrant vagrant 0 Mar 25 06:56 file1.txt
-rwxr-xr-x 1 vagrant vagrant 23 Mar 25 06:29 test.txtThe newest file will come first.