Why this error: "cannot create directory `foo': Permission denied"

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http://unix.stackexchange.com – I get this error when I try creating a directory: [rex <03:57 PM> /var/tmp/pb82]$ mkdir foo mkdir: cannot create directory `foo': Permission denied But doesn't the following output indicate that I should be able to create directories there since I am a member of the www-data group to which that directory belongs? [rex <03:57 PM> /var/tmp/pb82]$ ls -l .. total 8 drwxrwxr-x 5 root www-data 4096 Aug 7 15:32 jinfo drwxrwxr-x 3 root www-data 4096 Aug 7 20:43 pb82 [rex <03:58 PM> /var/tmp/pb82]$ whoami rex [rex <03:58 PM> /var/tmp/pb82]$ groups rex users www-data (HowTos)