12-09-2012
Quote:
Originally Posted by
Corona688
umask will never allow you to create executable files. The most permissions it will allow are 666.
Not quite. The
creat(const char *path, mode_t mode),
open(const char *path, int flag, mode_t mode), and
openat(int fd, const char *path, int flag, mode_t mode) system calls (when being used to create regular files) take the mode argument supplied by the programmer and mask off any permission bits specified by the umask. When an application (such as a shell output redirection or the vi :w command) creates a text file, it will usually have the low order bits of mode set to 0666 (RW by owner, RW by group, and RW by world). In this case if your umask is 122, the mode on the new file will be 0644 (RW by owner, R by group, and R by world). When an application (such as c99 or gcc) creates an executable file, it will usually have the low order bits of mode set to 0777 (RWX by owner, RWX, by group, and RWX by world). In this case if your umask is still 122, the mode on the new file will be 655 (RW but not X by owner, RX by group, and RX by world). Thus having a default umask of 122 is highly unusual. A much more common umask is 022 (block W access for group and world) or 027 (block W access for group and block all access for world).
Note that the umask also applies to mknod() when creating any file type and to mkdir() when creating a directory (but on directories, the X bit specifies being able to search the directory instead of being able to execute it (as it is on regular files) and write permission is used to control who is allowed to create and delete files in that directory.
This User Gave Thanks to Don Cragun For This Post:
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CHMOD(1) General Commands Manual CHMOD(1)
NAME
chmod - change access mode for files
SYNOPSIS
chmod [-R] mode file ...
OPTIONS
-R Change hierarchies recursively
EXAMPLES
chmod 755 file # Owner: rwx Group: r-x Others: r-x
chmod +x file1 file2
# Make file1 and file2 executable
chmod a-w file # Make file read only
chmod u+s file # Turn on SETUID for file
chmod -R o+w dir # Allow writing for all files in dir
DESCRIPTION
The given mode is applied to each file in the file list. If the -R flag is present, the files in a directory will be changed as well. The
mode can be either absolute or symbolic. Absolute modes are given as an octal number that represents the new file mode. The mode bits are
defined as follows:
4000 Set effective user id on execution to file's owner id
2000 Set effective group id on execution to file's group id
0400 file is readable by the owner of the file
0200 writeable by owner
0100 executable by owner
0070 same as above, for other users in the same group
0007 same as above, for all other users
Symbolic modes modify the current file mode in a specified way. The form is:
[who] op permissions { op permissions ...} {, [who] op ... }
The possibilities for who are u, g, o, and a, standing for user, group, other and all, respectively. If who is omitted, a is assumed, but
the current umask is used. The op can be +, -, or =; + turns on the given permissions, - turns them off; = sets the permissions exclu-
sively for the given who. For example g=x sets the group permissions to --x.
The possible permissions are r, w, x; which stand for read, write, and execute; s turns on the set effective user/group id bits. s only
makes sense with u and g; o+s is harmless.
SEE ALSO
ls(1), chmod(2).
CHMOD(1)