On Unix-like systems, the ls command lists files. GNU's version of ls also colors them according to their properties and the environment variable LS_COLORS. The main file property that ls bases its colors on is the file's mode. In this challenge, you will determine what keys from LS_COLORS ls should look for a given mode.
File modes
A file's mode determines what type of a file it is (is it a normal file? a socket? a pipe? a directory?) and what permissions it has (can the owner execute it? can everyone write to it?).
The mode is given by a 16-bit unsigned integer. The highest 4 bits determine the file's type, the next 3 bits determine certain special attributes, and the remaining 9 bits determine user, group, and world permissions (3 bits each).
The grouping-up by 3s makes it very common write these permissions in octal:
type special permissions | type sp perms
---- ------- ----------- | -- -- -----
(bin) 1111 111 111 111 111 | (oct) 17 7 7 7 7
The top 4 bits will take on one of these values. Any other values can produce undefined behavior:
Binary Octal File type
--------- ----- ----------------
0b1100... 014... socket
0b1010... 012... symbolic link
0b1000... 010... regular file
0b0110... 006... block device
0b0100... 004... directory
0b0010... 002... character device
0b0001... 001... FIFO (named pipe)
The remaining 4 octets can have any value. each bit is a flag which can be set independently from the others:
????------------- Filetype, see above
----1------------ SUID: Execute this file with the UID set to its owner
-----1----------- SGID: Execute this file with the GID set to its group
------1---------- Sticky: Users with write permissions to this dir cannot move or delete files within owned by others
-------1--------- User read: The owner of the file can read this
--------1-------- User write: The owner of the file can write to this
---------1------- User exec: The owner of the file can execute this
-----------111--- Group r/w/x: The group of this file can read/write/execute
--------------111 Other r/w/x: All users can read/write/execute
LS_COLORS
GNU's ls uses the environment variable LS_COLORS to add color to the files it lists. The variable LS_COLORS is a colon-delimited list of key=value rules. Some of those keys are for file names (e.g.: *.tar=01;31), but we care about the file type keys.
ln Symbolic link.
pi Named pipe
bd Block device
cd Character device
or Symbolic link pointing to a non-existent file
so Socket
tw Directory that is sticky and other-writable (+t,o+w)
ow Directory that is other-writable (o+w) and not sticky
st Directory with the sticky bit set (+t) and not other-writable
di Directory
su Normal file that is setuid (u+s)
sg Normal file that is setgid (g+s)
ex Executable normal file (i.e. has any 'x' bit set in permissions)
fi Normal file
Now when ls finds a match in the above list, it only applies that color if that key exists in LS_COLORS. If a directory is sticky and other-writeable, but no tw key is in LS_COLORS, it will fall back to ow, then fall back to st, then fall back to di. The list above is ordered to give the same result as ls.
Challenge
Take as an integer input any valid file mode, and output the fallback list of two-character codes.
Input: A file mode with a valid file type, in any convenient integer format that is at least 16 bits.
Output: The corresponding ordered list of two-character keys for a file with that mode, in fallback order.
Since all codes are two characters, the concatenated string (e.g.: "twowstdi") is also acceptable, as it is still unambiguous.
Additionally, printing the codes in reverse order (di st ow tw) is also fine, as long as it is consistent for all inputs.
Example:
- Input:
17389(in octal:0041755) - Output:
["st", "di"](the sticky bit0001000is set, and it is an directory0040000.)
Test cases (these are in octal, and a symbolic equivalent for your convenience):
Input Symbolic Output | Note:
------- ---------- ------------- | -----
0140000 s--------- so | socket
0147777 srwsrwsrwt so | suid/sgid/sticky/other-writeable does not apply
0120000 l--------- ln | symbolic link (you MAY choose to output "or" instead)
0127777 lrwsrwsrwt ln |
0060000 b--------- bd | block device
0067777 brwsrwsrwt bd |
0020000 c--------- cd | character device
0027777 crwsrwsrwt cd |
0010000 p--------- pi | pipe
0017777 prwsrwsrwt pi |
0040755 drwxr-xr-x di | directory
0041755 drwxrwxr-t st di | sticky bit set
0040002 d-------w- ow di | other-writable
0041777 drwxrwxrwt tw ow st di | sticky + other-writeable
0046000 d--S--S--- di | suid/sgid only apply to normal files
0100000 ---------- fi | normal file
0100000 ---------T fi | sticky bit only applies to directories
0100100 ---x------ ex fi | executable file
0100010 ------x--- ex fi |
0100001 ---------x ex fi |
0104000 ---S------ su fi | suid
0106777 -rwsrwsrwx su sg ex fi | suid has priority over sgid and executable
0102000 ------S--- sg fi | sgid
0102777 -rwxrwsrwx sg ex fi | sgid has priority over executable
0110000 ?--------- <undefined> | Unknown filetype, any output is valid
Standard loopholes are forbidden. This is code-golf, so shorter answers are best.
Similar Challenges: File Permissions, Do I have permission?
