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Full Discussion: Permission Question
Top Forums UNIX for Dummies Questions & Answers Permission Question Post 302269218 by Tiger75 on Wednesday 17th of December 2008 05:56:33 AM
Old 12-17-2008
s stands for setting the User ID/Group ID.
t stands for setting the Sticky bit
  • SUID or setuid: change user ID on execution. If setuid bit is set, when the file will be executed by a user, the process will have the same rights as the owner of the file being executed.
  • SGID or setgid: change group ID on execution. Same as above, but inherits rights of the group of the owner of the file. For directories it also may mean that when a new file is created in the directory it will inherit the group of the directory (and not of the user who created the file).
  • Sticky bit. It was used to trigger process to "stick" in memory after it is finished, now this usage is obsolete. Currently its use is system dependant and it is mostly used to suppress deletion of the files that belong to other users in the folder where you have "write" access to.
The most common use of the sticky bit today is on directories, where, when set, items inside the directory can only be renamed or deleted by the item's owner, the directory's owner, or the superuser. Generally this is set on the /tmp directory to prevent ordinary users from deleting or moving other users' files.
The sticky bit can only be set by superuser root. Using the chmod command, it can be set using its octal mode 1000 or by its symbol t (s is already used by the setuid bit).
SUID stands for "Set User-ID". If the suid bit is enable on an executable, then whenever that program is run the process will take on the privilege of whoever the file owner is. For example if I have a suid binary which is owned by "root" and I execute it as my normal (non-root) user name, it will execute with all the permissions of root.


SUIDIf set, then replaces "x" in the owner permissions to "s", if owner has execute permissions, or to "S" otherwise. Examples:
-rws------ both owner execute and SUID are set
-r-S------ SUID is set, but owner execute is not setSGIDIf set, then replaces "x" in the group permissions to "s", if group has execute permissions, or to "S" otherwise. Examples:
-rwxrws--- both group execute and SGID are set
-rwxr-S--- SGID is set, but group execute is not setStickyIf set, then replaces "x" in the others permissions to "t", if others have execute permissions, or to "T" otherwise. Examples:
-rwxrwxrwt both others execute and sticky bit are set
-rwxrwxr-T sticky bit is set, but others execute is not set
 

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STRMODE(3)						   BSD Library Functions Manual 						STRMODE(3)

NAME
strmode -- convert inode status information into a symbolic string LIBRARY
Standard C Library (libc, -lc) SYNOPSIS
#include <string.h> void strmode(mode_t mode, char *bp); DESCRIPTION
The strmode() function converts a file mode (the type and permission information associated with an inode, see stat(2)) into a symbolic string which is stored in the location referenced by bp. This stored string is eleven characters in length plus a trailing NUL. The first character is the inode type, and will be one of the following: - regular file b block special c character special d directory l symbolic link p fifo s socket w whiteout ? unknown inode type The next nine characters encode three sets of permissions, in three characters each. The first three characters are the permissions for the owner of the file, the second three for the group the file belongs to, and the third for the ``other'', or default, set of users. Permission checking is done as specifically as possible. If read permission is denied to the owner of a file in the first set of permis- sions, the owner of the file will not be able to read the file. This is true even if the owner is in the file's group and the group permis- sions allow reading or the ``other'' permissions allow reading. If the first character of the three character set is an ``r'', the file is readable for that set of users; if a dash ``-'', it is not read- able. If the second character of the three character set is a ``w'', the file is writable for that set of users; if a dash ``-'', it is not writable. The third character is the first of the following characters that apply: S If the character is part of the owner permissions and the file is not executable or the directory is not searchable by the owner, and the set-user-id bit is set. S If the character is part of the group permissions and the file is not executable or the directory is not searchable by the group, and the set-group-id bit is set. T If the character is part of the other permissions and the file is not executable or the directory is not searchable by others, and the ``sticky'' (S_ISVTX) bit is set. s If the character is part of the owner permissions and the file is executable or the directory searchable by the owner, and the set- user-id bit is set. s If the character is part of the group permissions and the file is executable or the directory searchable by the group, and the set- group-id bit is set. t If the character is part of the other permissions and the file is executable or the directory searchable by others, and the ``sticky'' (S_ISVTX) bit is set. x The file is executable or the directory is searchable. - None of the above apply. The last character will always be a space. SEE ALSO
chmod(1), find(1), stat(2), getmode(3), setmode(3) HISTORY
The strmode() function first appeared in 4.4BSD. BSD
July 28, 1994 BSD
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