What is the difference between umask and chmod

unix

I am completely confused between umask and chmod. Both are used to give permissions to the files. But where exactly is the difference and when to use them.

I have read the online documentation but both are looking same to me.

umask: umask is used to set default file permissions. These permissions will be used to all subsequent files during their creation.
chmod : used to change file and directory permissions.

As per my understanding if for example file test.doc is created.

By default unix gives the file 022 umask code.

Now when I change it to chmod 666 test.doc I can change the permission level of this file.

Now what if i use umask 666 for the same file.

What difference it happens when I use chmod 666 and umask 666

Best Answer

The difference is that umask entails only new files. As you stated, umask sets the default permissions that a file/directory will have on creation time, but afterwards umask doesn't affect them anymore.

chmod, however, needs the file be created prior to be run.

Therefore, if you run umask, it will have no effect at all on existing files.

Related Solutions

Linux – How to make new file permission inherit from the parent directory

You do not want to change your system's default umask, that is a security risk. The sticky bit option will work to some extent, but using ACL's is the best way to go. This is easier than you think. The problem with basic ACL's is that they are not recursive by default. If you set an ACL on a directory, only the files inside that directory inherit the ACL. If you create a subdirectory, it does not get the parent ACL unless the ACL is set to recurse.

First, make sure ACLs are enabled for the volume the directory is on. If you have tune2fs, you can perform the following:

# tune2fs -l /dev/sda1 | grep acl
Default mount options:    user_xattr acl

If you don't have tune2fs, then examine fstabs:

# cat /etc/fstab 
/dev/system/root        /                       ext3    defaults        1 1
/dev/system/home        /home                   ext3    defaults        1 2
/dev/storage/data       /data                   ext3    defaults        1 2
LABEL=/boot             /boot                   ext3    defaults        1 2

The 4th column that says "defaults" means on my system (CentOS 5.5), ACL's are on. When in doubt, leave it as defaults. If you try to set the ACL and it errors out, go back and add the acl option to /etc/fstab right after defaults: defaults,acl.

From what I understand, you want everyone in the users group to have write access to the data directory. That's accomplished by the following:

setfacl -Rm g:users:rwX,d:g:users:rwX data/

Whats the difference between chmod 777 and chmod 7777

In 7777, the first three bits are the setuid, setgid, and sticky flags. These should only be set under very special circumstances. You're correct that 777 is the more appropriate setting (if you want to make the file both world-writable and world-executable).

And unless the file is an executable program or script, or a directory, you usually shouldn't set the x bits. There's not much harm in doing so, though.

Best Answer

Related Solutions

Linux – How to make new file permission inherit from the parent directory

Whats the difference between chmod 777 and chmod 7777

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