Understanding umask

[victorvas]

$ umask 026
$ umask -S
u=rwx,g=rx,o=x
$ touch foo
$ ls -l foo
-rw-r-----

I thought it would be

-rwxr-x--x

How does it work?

[IdOp]

Files will often be created using the fopen(3) system call. Looking at that man page shows

Any created files will have mode "S_IRUSR | S_IWUSR | S_IRGRP | S_IWGRP |
S_IROTH | S_IWOTH" (0666), as modified by the process' umask value (see
umask(2)).

So files are created by this without any execute permissions (0666) requested, and the umask can only strip off more bits (IIRC you AND with the NOT of the umask). I think this probably explains what you're seeing, though I haven't looked at the source code of touch(1).

[J65nko]

Wikipedia has a quite good entry about it: https://en.wikipedia.org/wiki/Umask

[fvgit]

While we're on topic, is there a way to set the umask for a process? Say my default umask is 022 and I'm creating a file in ed for example or with echo and want it to be written to disk with 077 just for this process (incl. any temporary files that might be created)?

[IdOp]

This may not be exactly what you want, but you could run it in a sub-shell; e.g.,

% ( umask 0777 ; echo FOO > foooo )

will take away all the usual permissions from the file, while leaving the outer umask unchanged.

[J65nko]

Shell scripts are usually run as a separate process.

You can set the umask in a shell script so it will only affect the file creating commands of that script.
After the script has ended the umask value of your interactive shell will still be the original.

#!/bin/sh

FILE=umask_test

[ -f $FILE ] && rm $FILE

cat <<END
Current value of umask: $( umask )
END

umask 066

cat <<END
Current value of umask: $( umask )
END

echo Creating file
touch $FILE
ls -l $FILE

echo End of script .....

Executing it gives:

j65nko@alix[~]./umask.tst    
Current value of umask: 022
Current value of umask: 066
Creating file
-rw-------  1 j65nko  j65nko  0 Oct 30 22:11 umask_test
End of script .....
j65nko@alix[~]umask
022
j65nko@alix[~]

[fvgit]

Originally Posted by IdOp

This may not be exactly what you want, but you could run it in a sub-shell; e.g.,

% ( umask 0777 ; echo FOO > foooo )

will take away all the usual permissions from the file, while leaving the outer umask unchanged.

Nice! It even works with a scripted ed session:

$ cat test.sh
( umask u-w,g-rx,o-rx ; ed <<- EOF
H
a
# By order of the Peaky fookin' Blinders!
.
w foooo
q
EOF
)
touch baaaar

$ umask                                                            
022
$ ls -al foooo baaaar
ls: baaaar: No such file or directory
ls: foooo: No such file or directory
$ ./test.sh
42
$ ls -al foooo baaaar                                               
-rw-r--r--  1 fvgit  fvgit   0 Oct 30 22:12 baaaar
-r--------  1 fvgit  fvgit  42 Oct 30 22:12 foooo

I also played around with install(1) but it wouldn't take input from stdin.

[fvgit]

@J65enko: My use case was actually a single command from within a larger script. So that that single cmd would have a different umask applied than the remaining commands in the same script. I thought about setting the umask, then running the command and setting the umask again, originally. But I didn't find it really elegant.

Originally Posted by J65enko

Shell scripts are usually run as a separate process.

You can set the umask in a shell script so it will only affect the file creating commands of that script.
After the script has ended the umask value of your interactive shell will still be the original.

This hadn't crossed my mind, though.

I'm adding both your and IdOp's examples to my toolchest.

[sabrina]

This script will show permissions based on current value of umask:

#!/bin/ksh
typeset -i2 defa
typeset -i onescomp
(( onescomp=~(8#$(umask)) ))
(( defa=(8#666)&onescomp ))
defaultPerm=${defa#*#}
print "\nDefault regular file permissions with current umask ($(umask))"
print " u  g  o"
print "_________"
print "rwxrwxrwx"
print $defaultPerm
print "\nDefault directory file permissions with current umask ($(umask))"
typeset -i2 ddefa
(( ddefa=(8#777)&onescomp ))
defaultPerm=${ddefa#*#}
print " u  g  o"
print '_________'
print 'rwxrwxrwx'
print $defaultPerm
print "\n1s are on, 0s are off\n"

Usage: put this content into ~/permissions

$ chmod u+x ~/permissions
~/permissions
umask 027
~/permissions

[victorvas]

If a file has permissions rw--------- und I create a link to it with ln -s, the link has permissions of lrwxr-xr-x. How can I change link's permissions to be lrw--------- ?

[victorvas]

Originally Posted by victorvas

If a file has permissions rw--------- und I create a link to it with ln -s, the link has permissions of lrwxr-xr-x. How can I change link's permissions to be lrw--------- ?

I found how to do it. chmod -h 600 link