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Full Discussion: Umask question
Top Forums UNIX for Dummies Questions & Answers Umask question Post 302741653 by Don Cragun on Sunday 9th of December 2012 05:57:07 PM
Old 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.
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CHMOD(1)						      General Commands Manual							  CHMOD(1)

NAME
chmod - change mode SYNOPSIS
chmod mode file ... DESCRIPTION
The mode of each named file is changed according to mode, which may be absolute or symbolic. An absolute mode is an octal number con- structed from the OR of the following modes: 4000 set user ID on execution 2000 set group ID on execution 1000 sticky bit, see chmod(2) 0400 read by owner 0200 write by owner 0100 execute (search in directory) by owner 0070 read, write, execute (search) by group 0007 read, write, execute (search) by others A symbolic mode has the form: [who] op permission [op permission] ... The who part is a combination of the letters u (for user's permissions), g (group) and o (other). The letter a stands for ugo. If who is omitted, the default is a but the setting of the file creation mask (see umask(2)) is taken into account. Op can be + to add permission to the file's mode, - to take away permission and = to assign permission absolutely (all other bits will be reset). Permission is any combination of the letters r (read), w (write), x (execute), s (set owner or group id) and t (save text - sticky). Let- ters u, g or o indicate that permission is to be taken from the current mode. Omitting permission is only useful with = to take away all permissions. The first example denies write permission to others, the second makes a file executable: chmod o-w file chmod +x file Multiple symbolic modes separated by commas may be given. Operations are performed in the order specified. The letter s is only useful with u or g. Only the owner of a file (or the super-user) may change its mode. SEE ALSO
ls(1), chmod(2), chown (1), stat(2), umask(2) CHMOD(1)
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