Advanced Programming in the UNIX® Environment

ptg10805159

Appendix C Chapter 4 Solutions 909

$umask 777

$date > temp.foo

$ls -l temp.foo

---------- 1 sar 29 Feb 5 14:06 temp.foo

4.3 The following shows what happens when user-read permission is turned off:

$date > foo

$chmod u-r foo turn off user-read permission

$ls -l foo verify the file’spermissions

--w-r--r-- 1 sar 29 Feb 5 14:21 foo

$cat foo and try to read it

cat: foo: Permission denied

4.4 If we try,using eitheropenorcreat, to create a file that already exists, the file’s

access permission bits arenot changed. We can verify this by running the

program from Figure4.9:

$rm foo bar delete the files in case they already exist

$date > foo create them with some data

$date > bar

$chmod a-r foo bar turn off all read permissions

$ls -l foo bar verify their permissions

--w------- 1 sar 29 Feb 5 14:25 bar

--w------- 1 sar 29 Feb 5 14:25 foo

$./a.out run program from Figure4.9

$ls -l foo bar check permissions and sizes

--w------- 1 sar 0 Feb 5 14:26 bar

--w------- 1 sar 0 Feb 5 14:26 foo

Note that the permissions didn’t change but that the files weretruncated.

4.5 The size of a directory should never be 0, since thereshould always be entries for

dot and dot-dot. The size of a symbolic link is the number of characters in the

pathname contained in the symbolic link, and this pathname must always contain

at least one character.

4.7 The kernel has a default setting for the file access permission bits when it creates a

newcorefile. In this example, it wasrw-r--r--.This default value may or

may not be modified by theumaskvalue. The shell also has a default setting for

the file access permission bits when it creates a new file for redirection. In this

example, it wasrw-rw-rw-,and this value is always modified by our current

umask.Inthis example, ourumaskwas 02.

4.8 We can’t usedu,because it requires either the name of the file, as in

du tempfile

or a directory name, as in

du.

But when theunlinkfunction returns, the directory entry fortempfileis gone.

Thedu .command just shown would not account for the space still taken by