ls = List of objects in the current working directory.
will bring up a list of files and subdirectories. It may also be flagged to provide some different ways of looking at the list of files, such as
which will provide a long directory listing (similar to a DOS dir), and
which will show all of the files in the directory, including hidden ones and
ls | more
which will provide a directory listing one screen at a time. The ‘|’ is called a ‘pipe’, and passes data from one command to another.
gives you a list of files with special notation of their type:
* means “executable”
/ means “directory”
= means “socket”
| means “pipe”
man ls dir
Columns in the ls –la listing:
number of hard links
size (in bytes)
The permissions column:
directory, link or file?
r = read
w = write
x = execute
The hard links column:
What the heck is a hard link? You can always:
man linkinfo coreutils link man link info coreutils link
But we’ll go in-depth below in More About File Systems
The Owner column:
The owner and the author are the same in most *nix, but not in all *nix. The owner is referenced in the owner triad of the permissions. Generally, files under your ownership are in your home directory, but not necessarily (see /var/mail/username).
The Group column:
When you are created as a user, a group with the same name is created, with you as its only member. You may be assigned to other groups, for instance, the wheel group. Your file permissions are set upon file creation. These defaults can be changed.
The Size column:
Size is displayed in bytes, though there are options for showing it in blocks (often 4096 byte blocks).
The Last-Modified column:
Month, Day, Year (if not current year)
Time in “military” format
The File Name column:
Regular files can be named almost anything within this keyspace:
a-z A-Z 0-9 _ – ~ .
You will run into certain special characters that you can’t use.
Unix does NOT use file extensions to identify file type; Gnome and KDE however DO.
Hidden files start with a dot: .
Backup files end with a tilde: ~
Some programs like emacs create backups automatically, creating near-duplicate file names.
Soft links (symlinks) are designated by an l. (That’s the letter l, not the number 1.) Both these and hard links are discussed below in More About File Systems
Start with a dot
Contain system configuration
Usually text files
Can be directly modified
Now is a good time to become familiar with another useful web site: LinuxCommand.org.
http://linuxcommand.org/man_pages/ls1.html. Take a look at some other useful options. Do you notice if any of them are set by default?
|Some Common Options to ls|
|List all files, even hidden||la -a|
|List all files, except the . and .. usually listed at the top||ls -A|
|-C||List in columns||ls -C|
|–color=always||Controls whether color is used to distinguish file types: ‘never’, ‘always’, or ‘auto’.||ls –color=never|
|List directory names, rather than their contents||ls -d|
|-f||List without sorting||ls -f|
|List filenames with markers to indicate type: executable* directory/ socket= or pipe|||ls -F|
|–full-time||Gets a long listing with full modification time||ls –full-time|
|-l||Long listing||ls -l|
|Long listing with human-readable (simple) file sizes||ls -lh|
|Long listing omitting group information||ls -lG|
|Lists files in reverse ASCII-betic order||ls -r|
|List files in directory, and in subdirectories||ls -R|
|-s||List files and size in kilobytes (KB)||ls -s|
|-S||List files sorted by size||ls -S|
|-t||List files sorted by modification time||ls -t|
|-U||Lists files without sorting||ls -U|
|-x||List files in rows rather than columns||ls -x|
Mix and match! Any of the above may be used together.
|You can use an initial capital letter to name your directories. Because files are listed “ASCII-betically,” capital letters come before lower-case letters.|
|The command ll is an alias, on many systems, for ls -l.|
Research the file command. What does it do? How does it compare to ls?
More About File Systems: Inodes, hard links and symlinks
Creating a Hard Link
You create a hard link with a command like this:
ln /bin/bash ~/mybash
where the syntax is:
ln target_of_link link_file_name
So you choose a target file (like the bash binary above) and then a location for the hard link (in this case, my home directory, with the name “mybash”).
At the very root of the file system is the superblock. The superblock is the physical location of the inode table. And the inode table in turn is a tabular list of inodes (information nodes), each of which is uniquely numbered, and holds the master information for a file: file size, data block location, last modification time and date; ownership; and permissions. The inode more-or-less “is” the file; when a file is deleted (unlinked), what is actually deleted is its inode. The file’s data blocks are only overwritten when they are needed. And we do need them; in most systems there is a limit of 64K inodes. Yes, we can run out of “file space” even when there is still space on the hard disk. (Want more explanation? Search for “inode” when you get to this page.)
Nothing Prevents A File From Being Listed In Two Different Directories
(or even twice in one directory)
With the above concept clearly in mind, now think of a file that appears in two different directories. What the directory listing is really referring to is an inode. There are not two copies of the file; there is only one. It just shows up in two different places. You can verify this by creating a link, then using the -i option to the ls command:
touch myfile ln ./myfile ./myfile2 ln -li
In this listing, note the inode number on the left. Both myfile and myfile2 have the same inode.
Can You Hard Link Directories?
Normal users can’t. Root can. Generally it’s a bad idea.
There Are Also Soft Links (Symlinks)
Symlinks are somewhat similar to Windows shortcuts. Most actions are passed through the link to the target, i.e. opening a program. Only creation and deletion actually act on the link file itself.
How Do You Create A Symlink?
With a command like this:
ln -s /bin/bash ~/mybash
where the syntax is:
ls -s target_of_link link_file_name
Note the -s option, which makes ln create a soft link (symlink) rather than a hard link.
Can You Soft Link Directories?
Absolutely. This is commonly done to simplify navigation:
ln -s /var/tmp /usr/tmp