File and directory timestamps in Unix
Three times tracked for each file in Unix are these:
access time - atime
change time – ctime
modify time – mtime
atime – File Access Time:
Access time shows the last time the data from a file was accessed – read by one of the Unix processes directly or through commands and scripts.
ctime – File Change Time:
ctime also changes when you change file's ownership or access permissions. It will also naturally highlight the last time file had its contents updated.
mtime – File Modify Time:
Last modification time shows time of the last change to file's contents. It does not change with owner or permission changes, and is therefore used for tracking the actual changes to data of the file itself.
The simplest way to confirm the times associated with a file is to use ls command.
Timestamps are shown when using the long-format output of ls command, ls -l:
ximena@anakin: ls -l /tmp/file1
-rw-r--r-- 1 greys root 9 2008-04-05 07:10 /tmp/file1
This is the default output of ls -l, which shows you the time of the last file modification – mtime. In our example, file /tmp/file1 was last changed around 7:10am.
If we want to see the last access time for this file, atime – you need to use -lu options for ls. The output will probably show some later time:
ximena@anakin: ls -lu /tmp/file1
-rw-r--r-- 1 greys root 9 2008-04-05 07:27 /tmp/file1
In the example, it's 7:27am.
Lastly, ls -lc will show you the last time our file was changed, ctime:
ximena@anakin: ls -lc /tmp/file1
-rw-r--r-- 1 greys root 9 2008-04-05 07:31 /tmp/file1
Show atime, ctime and mtime with stat command
In Linux distributions, you will probably find a stat command, which can be used to show all of the times in a more convenient way, and among plenty of other useful information about your file:
ximena@anakin: stat /tmp/file1
Size: 9 Blocks: 8 IO Block: 4096 regular file
Device: 811h/2065d Inode: 179420 Links: 1
Access: (0644/-rw-r--r--) Uid: ( 0/ root) Gid: ( 0/ root)
Access: 2008-04-05 07:27:51.000000000 +0100
Modify: 2008-04-05 07:10:14.000000000 +0100
Change: 2008-04-05 07:35:22.000000000 +0100