Ls
In computing, ls is a command to list computer files in Unix and Unix-like operating systems. ls is specified by POSIX and the Single UNIX Specification. When invoked without any arguments, ls lists the files in the current working directory. The command is also available in the EFI shell. In other environments, such as DOS, OS/2, and Microsoft Windows, similar functionality is provided by the dir command. The numerical computing environments MATLAB and GNU Octave include an ls 
function with similar functionality.

History 
An ls utility appeared in the first version of AT&T UNIX, the name inherited from a similar command in Multics also named 'ls', short for the word "list".  is part of the X/Open Portability Guide since issue 2 of 1987. It was inherited into the first version of POSIX.1 and the Single Unix Specification.

An ls command is also part of ASCII's MSX-DOS2 Tools for MSX-DOS version 2.

Today, the two popular versions of ls are the one provided with the GNU coreutils package, and that released by various BSD variants. Both are free software and open source, and have only minor syntax differences. The version of ls bundled in GNU coreutils was written by Richard Stallman and David MacKenzie.

The command is available as a separate package for Microsoft Windows as part of the UnxUtils collection of native Win32 ports of common GNU Unix-like utilities.

Behavior 
Unix and Unix-like operating systems maintain the idea of a current working directory, that is, where one is currently positioned in the hierarchy of directories. When invoked without any arguments, ls lists the files in the current working directory. If another directory is specified, then ls will list the files there, and in fact the user may specify any list of files and directories to be listed.

Files names starting with "." are not listed unless -a (show all) is specified, -A (show all except "." and "..") is specified, or the files are specified explicitly.

Without options, ls displays files names only. The most common options to display additional information are:
 -l long format, displaying Unix file types, permissions, number of hard links, owner, group, size, last-modified date and filename
 -F append a "/" to directory names and a "*" to executable files.
 -g display group but not owner
 -o display owner but not group (when combined with -g both group and owner are suppressed)
 -d shows information about a symbolic link or directory, rather than about the link's target or listing the contents of a directory.
 -h output sizes in human readable format. (e.g., 1K, 234M, 2G, etc.) This option is not part of the POSIX standard, although implemented in several systems, e.g., GNU coreutils in 1997, FreeBSD 4.5 in 2002, and Solaris 9 in 2002.

Additional options controlling how files are displayed include:
 -f do not sort. Useful for directories containing large numbers of files.
 -t sort the list of files by modification time. (default is alphabetically)
 -1 (one) force output to be one entry per line.
 -R recursively list files in subdirectories and their subdirectories …
 --full-time to show times with seconds and milliseconds instead of down to the minute.

It is frequently possible to highlight different types of files with different colors, instead of with characters as -F would. This is an area where the two main ls versions differ:

 GNU ls uses the --color option; checks the Unix file type, the file permissions and the file extension and uses its own database to control colors maintained using dircolors.
 FreeBSD ls uses the -G option; checks only the Unix file type and file permissions. and uses the termcap database
When the option to use color to indicate file types is selected, the output might look like:
-rw-r--r--    1 tsmitt nregion   26650 Dec 20 11:16 audio.ogg
brw-r--r--    1 tsmitt nregion      64 Jan 27 05:52 bd-block-device
crw-r--r--    1 tsmitt nregion     255 Jan 26 13:57 cd-character-device
-rw-r--r--    1 tsmitt nregion     290 Jan 26 14:08 image.png
drwxrwxr-x    2 tsmitt nregion      48 Jan 26 11:28 di-directory
-rwxrwxr-x    1 tsmitt nregion      29 Jan 26 14:03 ex-executable
-rw-r--r--    1 tsmitt nregion       0 Dec 20 09:39 fi-regular-file
lrwxrwxrwx    1 tsmitt nregion       3 Jan 26 11:44 ln-soft-link -> dir
lrwxrwxrwx    1 tsmitt nregion      15 Dec 20 10:57 or-orphan-link -> mi-missing-link
drwxr-xrwx    2 tsmitt nregion    4096 Dec 20 10:58 ow-other-writeable-dir
prw-r--r--    1 tsmitt nregion       0 Jan 26 11:50 pi-pipe
-rwxr-sr-x    1 tsmitt nregion       0 Dec 20 11:05 sg-setgid
srw-rw-rw-    1 tsmitt nregion       0 Jan 26 12:00 so-socket
drwxr-xr-t    2 tsmitt nregion    4096 Dec 20 10:58 st-sticky-dir
-rwsr-xr-x    1 tsmitt nregion       0 Dec 20 11:09 su-setuid
-rw-r--r--    1 tsmitt nregion   10240 Dec 20 11:12 compressed.gz
drwxrwxrwt    2 tsmitt nregion    4096 Dec 20 11:10 tw-sticky-other-writeable-dir

Sample usage 
The following example demonstrates the output of the ls command given two different arguments:
$ ls -l
drwxr--r--   1 fjones editors    4096  drafts
-rw-r--r--   1 fjones editors   30405  edition-32
-r-xr-xr-x   1 fjones bookeepers 8460  edit.sh
$ ls -F
drafts/
edition-32
edit.sh*
In the above example, the user fjones has a directory named drafts, a regular file named edition-32, and an executable named edit.sh in his home directory. ls uses Unix file permission notation to define the access for the user(i.e. himself), group members for various groups and other users. 
drwxr--r--   1 fred  editors   4096  Mar 1  2007 drafts

In this example, drafts is a directory (denoted by the file descriptor d), and the characters after this indicate the permissions:

 rwx: the owner (fjones) has the right to read (r), write (w) and execute (x)
 r--: users who are members of the editors group have read-only permissions; write and execute are not permitted, as denoted by the hyphen characters (-)
 r--: others (users aside from the owner or members of editors) have read-only permissions; write and execute are not permitted

See also 
 chown
 chgrp
 du (Unix)
 mdls
 User identifier (Unix)
 Group identifier (Unix)
 List of Unix commands
 Unix directory structure

References

External links 

 
 
 
 
 
 
 
 
 GNU ls source code (as part of coreutils)
 ls at the LinuxQuestions.org wiki

Multics commands
Standard Unix programs
Unix SUS2008 utilities
Plan 9 commands
Inferno (operating system) commands