Access time of a file

UNIX for Beginners Q & A UNIX for Dummies Questions & Answers
1

I thought that access time of a file is time when the file was run last time (or I read somewhere that it's time when system lookup the file -> but I'm not sure when it really is)

How is it exactly?

Thank you for help!

2

It's the time a process last accessed it to read/write contents.

3

i read somewhere the documentation for the c function stat, and describe what type of access changed the mod time.
im sure that tail (awk, head, and surely more) dont change it

4

2Broli: Well and what is the access time for then? I mean if behaviour of programs is different..

5

From the POSIX standards:

st_atime == atime
st_ctime == ctime
st_mtime == mtime

Plus if you search the forum for file time you will find a lot of good information.

Broli - the definitions of these have been fixed for years - different apps do not violate those definitions. Things that can change mtime are opening a file for write/append or a call to utime() (e.g., touch). chmod or chown changes ctime, and utime or opening a file for read changes the atime. These values are NOT part of the file, they are file metadata maintained by the filesystem, as accessed thru the kernel.

6

I made a simple script and copied the output:

file=example_1.sh;stat $file; cat $file >/dev/null; stat $file

  File: `example_1.sh'
  Size: 918             Blocks: 2          IO Block: 4096   regular file
Device: fh/15d  Inode: 2099578354  Links: 1
Access: (0744/-rwxr--r--)  Uid: (13654/vsetm7am)   Gid: (  200/ nofiles)
Access: 2008-09-02 21:47:11.000000000 +0200
Modify: 2008-09-02 21:47:11.000000000 +0200
Change: 2008-09-02 21:47:11.000000000 +0200
  File: `example_1.sh'
  Size: 918             Blocks: 2          IO Block: 4096   regular file
Device: fh/15d  Inode: 2099578354  Links: 1
Access: (0744/-rwxr--r--)  Uid: (13654/vsetm7am)   Gid: (  200/ nofiles)
Access: 2008-09-02 21:47:11.000000000 +0200
Modify: 2008-09-02 21:47:11.000000000 +0200
Change: 2008-09-02 21:47:11.000000000 +0200

How come that access time didn't change when data was accessed?

7

We would need to know the OS and how the file system was mounted. As one example: Sun's mount_ufs man page documents the noatime option:

Another example: the source files might be on cd-rom.

8

I am aware of some UNIX-like enviroments where st_atime is not updated after a read. Interix i.e. Microsoft SUA is one and I think Cygwin is another.

What happens if you touch the file using the touch(1) utility?

9

touch -a will modify the atime. touch access metadata like stat does. It doesn't open the file for read or write.

POSIX says that

In other words both filetimes are updated.

10

fpmurphy:

u-pl27:~/ZKOUSKOVE/TEZSI_PRIKLADY$ stat example_1.sh
  File: `example_1.sh'
  Size: 918             Blocks: 2          IO Block: 4096   regular file
Device: fh/15d  Inode: 2099578354  Links: 1
Access: (0744/-rwxr--r--)  Uid: (13654/vsetm7am)   Gid: (  200/ nofiles)
Access: 2008-09-02 21:47:11.000000000 +0200
Modify: 2008-09-02 21:47:11.000000000 +0200
Change: 2008-09-02 21:47:11.000000000 +0200
u-pl27:~/ZKOUSKOVE/TEZSI_PRIKLADY$ touch example_1.sh
u-pl27:~/ZKOUSKOVE/TEZSI_PRIKLADY$ stat example_1.sh
  File: `example_1.sh'
  Size: 918             Blocks: 2          IO Block: 4096   regular file
Device: fh/15d  Inode: 2099578354  Links: 1
Access: (0744/-rwxr--r--)  Uid: (13654/vsetm7am)   Gid: (  200/ nofiles)
Access: 2008-09-05 19:54:14.000000000 +0200
Modify: 2008-09-05 19:54:14.000000000 +0200
Change: 2008-09-05 19:54:14.000000000 +0200
11

MartyIX, are you on a Solaris 10 box with a ZFS filesystem?

I ask because the output of the stat utlity looks like that of Solaris and ZFS has an option to turn off atime updating.

12

@fpmurphy: uname command writes:

GNU/Linux 2.6.22-p4smp