Does chown or chmod generate write reques when they need not change anything?

Question

Do utilities chown and chmod perform actual writing to the file when the settings they were instructed to set are already what file has? For example:

> ls -l 

... gooduser gods ... state_secrets

> chown gooduser:gods state_secrets

Should I use find to find files that need changes, or would just chmod -R be more effective ?

Answer 1

A nice way to find out whether chown actually changes ownership of files even if they stay the same (e.g. from pduck to pduck) is to run the command under strace. This way we can see the APIs that actually get called.

I created a test file which is owned by me and my group. Then I did chown pduck:pduck test.txt but with strace prepended:

$ touch test.txt
$ ls -l
total 0
-rw-r--r-- 1 pduck pduck 0 Jun 16 12:10 test.txt

$ strace chown pduck:pduck test.txt
execve("/bin/chown", ["chown", "pduck:pduck", "test.txt"], 0x7ffd0309c990 /* 74 vars */) = 0
brk(NULL)                               = 0x5589645df000
access("/etc/ld.so.nohwcap", F_OK)      = -1 ENOENT (No such file or directory)
access("/etc/ld.so.preload", R_OK)      = -1 ENOENT (No such file or directory)
...
newfstatat(AT_FDCWD, "test.txt", {st_mode=S_IFREG|0644, st_size=0, ...}, AT_SYMLINK_NOFOLLOW) = 0
fchownat(AT_FDCWD, "test.txt", 1000, 1000, 0) = 0
close(1)                                = 0
close(2)                                = 0
exit_group(0)                           = ?
+++ exited with 0 +++

As we can see, the ownership is indeed unconditionally changed from pduck:pduck to 1000:1000 (which are the numerical IDs of pduck:pduck).

---

You have multiple options:

• chown --from=pduck:pduck pgoose:pgoose text.txt Will only change the ownership to pgoose:pgoose if it currently is pduck:pduck. Files with other ownerships remain untouched. Add -R to make the changes recursively.

• find . -type f -user pduck -group pduck -exec chown pgoose:pgoose {} + This will (recursively) find all files owned by pduck:pduck and only for them run chown pgoose:pgoose ….

• find . -type f -not -\( -user pduck -or -group pduck -\) -exec chown pduck:pduck {} + Changes ownership of files that do not belong to user pduck or group pduck.

As long as you don't have hundreds of thousands of files I don't think there is much difference in the performance.

When I do chown -R ... then usually because a whole directory has "wrong" ownership (for example on thumbdrives). But I usually never do chmod -R because files and directories need different permissions and the x has a different meaning for them. We have many questions here on how to fix that terrible chmod -R 777 … thing. Instead, I do it in two separate commands (find -type d … and find -type f …).