Recently I am using Python module os, when I tried to change the permission of a file, I did not get the expected result. For example, I intended to change the permission to rw-rw-r--,
os.chmod("/tmp/test_file", 664)The ownership permission is actually -w--wx--- (230)
--w--wx--- 1 ag ag 0 Mar 25 05:45 test_fileHowever, if I change 664 to 0664 in the code, the result is just what I need, e.g.
os.chmod("/tmp/test_file", 0664)The result is:
-rw-rw-r-- 1 ag ag 0 Mar 25 05:55 test_fileCould anybody help explaining why does that leading 0 is so important to get the correct result?
27 Answers
Found this on a different forum
If you're wondering why that leading zero is important, it's because permissions are set as an octal integer, and Python automagically treats any integer with a leading zero as octal. So os.chmod("file", 484) (in decimal) would give the same result.
What you are doing is passing 664 which in octal is 1230
In your case you would need
os.chmod("/tmp/test_file", 436)[Update] Note, for Python 3 you have prefix with 0o (zero oh). E.G, 0o666
So for people who want semantics similar to:
$ chmod 755 somefileUse:
$ python -c "import os; os.chmod('somefile', 0o755)"If your Python is older than 2.6:
$ python -c "import os; os.chmod('somefile', 0755)"Use permission symbols (stat.S_I*) instead of raw octal numbers
Your problem would have been avoided if you had used the more semantically named permission symbols rather than raw magic numbers, e.g. for 664:
#!/usr/bin/env python3
import os
import stat
os.chmod( 'myfile', stat.S_IRUSR | stat.S_IWUSR | stat.S_IRGRP | stat.S_IWGRP | stat.S_IROTH
)This is documented at and the names are the same as the POSIX C API which is also present at man 2 stat and man 2 chmod:
S_IRUSR (00400) read by owner
S_IWUSR (00200) write by owner
S_IXUSR (00100) execute/search by owner
S_IRGRP (00040) read by group
S_IWGRP (00020) write by group
S_IXGRP (00010) execute/search by group
S_IROTH (00004) read by others
S_IWOTH (00002) write by others
S_IXOTH (00001) execute/search by othersAnother advantage is the greater portability as mentioned in the docs:
Note: Although Windows supports
chmod(), you can only set the file’s read-only flag with it (via thestat.S_IWRITEandstat.S_IREADconstants or a corresponding integer value). All other bits are ignored.
chmod +x is demonstrated at: How do you do a simple "chmod +x" from within python?
Tested in Ubuntu 16.04, Python 3.5.2.
2leading 0 means this is octal constant, not the decimal one. and you need an octal to change file mode.
permissions are a bit mask, for example, rwxrwx--- is 111111000 in binary, and it's very easy to group bits by 3 to convert to the octal, than calculate the decimal representation.
0644 (octal) is 0.110.100.100 in binary (i've added dots for readability), or, as you may calculate, 420 in decimal.
If you have desired permissions saved to string then do
s = '660'
os.chmod(file_path, int(s, base=8))@mc.dev's answer was the best answer here I ended up leveraging that to make the below function wrapper for reuse. Thanks for the share.
def chmod_digit(file_path, perms): """ Helper function to chmod like you would in unix without having to preface 0o or converting to octal yourself. Credits: """ os.chmod(file_path, int(str(perms), base=8))Using the stat.* bit masks does seem to me the most portable and explicit way of doing this. But on the other hand, I often forget how best to handle that. So, here's an example of masking out the 'group' and 'other' permissions and leaving 'owner' permissions untouched. Using bitmasks and subtraction is a useful pattern.
import os
import stat
def chmodme(pn): """Removes 'group' and 'other' perms. Doesn't touch 'owner' perms.""" mode = os.stat(pn).st_mode mode -= (mode & (stat.S_IRWXG | stat.S_IRWXO)) os.chmod(pn, mode)
