ls

ls lists the files in the current directory.

Flags:

-l lists files with metadata. It shows file size, permissions and ownership.
-A lists hidden files as well. Files starting with the . (period) are hidden, and don't appear with ls by default.

Arguments:

Arguments are not required, but if provided ls lists the files matching the argument. Passing a directory path lists the contents of that directory. Passing a filename just prints the filename, so without -l, it's not terribly useful.

cd

cd change directory.

Arguments:

An argument is not provided, then by default, it changes to the home directory: '~'. If an argument is provided, then it changes to that directory. Every folder (besides the root directory, '/') contains a symbolic link called '..', which points to the enclosing folder. To move up the directory tree (out of folders), use cd ..

mkdir

mkdir creates a new directory (make directory)

Flags:

-p created all intermediate directories if they don't exist (i.e. creates multiple nested directories)

Arguments:

The names of the directories to create

touch

touch creates a completely empty file (if the file does not already exist). If the file already exists, then the access information is updated ('last opened' is set to now)

Arguments:

The names of the files to create/update

rm

rm removes a file. This action cannot be undone. If git is in use, then it may be recoverable using git.

Flags:

-r delete recursively, this flag is required when deleting directories
-f force delete. Some files will ask for confirmation to delete, this flag ignores those confirmations and deletes the files anyway.

mv

mv moves a file or directory

It is also used to rename a file. Give the name of the file to rename as the first argument, and the desired new name as the second.

Arguments:

The first argiuments a re the files to move

The last argument is the destinaton for the files to be moved.

cp

cp copies a file or directory.

Flags:

-r recursive. This flag is required to copy directories

Arguments:

The first arguments are the names of the files to copy.

The final argument is the destination for the files to copy.

chmod

chmod change mode changed the permission mode on a file. Permissions on a file are visible using ls -l (or just ll), and appear like this

-rwxr-xr-x

The first character appears as a 'd' if the file is a directory It then contains three sets of three values.

Within each group, the first value appears as an 'r' if the file has permission to be read. The second appears as a 'w' if the file is writable, and the third appears as an 'x' if the file has permissions to be executed.

Each group refers to a different collection of users. The first group refers to the permissions granted to the owner, the second set refers to permissions granted to members of the same group, and the third represents permissions granted globally.

Arguments:

The first argument is a file mode or modifier. The modifiers are given in the table below. Thes modifiers grant/revoke permissions for all users

	Adds	Removes
Read	+r	-r
Write	+w	-w
Execute	+x	-x

The mode is a combination of three valued as described above: owner, group, and global permissions. Each is a 3 bit integer. Add 4 to make it readable, add 2 to make it writeable, add 1 to make it executable.

Examples:

   ls -l    mode
-rw-r--r--  644
-rwxr-xr-x  755
drwxrwxrwx  777
-rwxrw-r--  764
-r--------  400
          

The second (third, fourth, etc) argument(s) name the files whose permissions you want to change.

vim

vim is actually a text editor.

Arguments:

The name of the file to open. If the filedoesn't exist, then it is created.

Usage

vim has the steepest learning curve of any editor because it is so powerful. Here are some basic commands:

• i enters editing mode. once in editing mode, everything works as you might assume it should.
• [esc] exits the current mode.
• :q closes the file. It fails if there are unsaved changes
• :q! force closes the file
• :w saves
• :x saves and exits. It is equivalent to :wq
• :e loads changes from the disk (if the file has been changed by another user/program)

nano

nano is another text editor, easier to use than vim.

It's pretty straightforward to use. In some versions, crtl+s can be used to save, and in all versions, ctrl+x will exit (with the option to save ic ctrl-x isn't doing it).

Arguments:

the name of the files to open. It will be created if it doesn't exist

screen

screen is a terminal multiplexer. It allows you to create terminal sessions, then detach them from the display and continue running in the background. It is expecially useful for remote work that is expected to take a long time, expecially if you will need to log out in the middle of the session.

Flags:

r reattaches to a previous session. If there is more than one session to attach to, it will fail and print a list of possible session IDs. In this case, you must pass the listed ID of the session to attach to as an argument to -r

Arguments:

Usage:

man

man prints the manual pages for a command

Arguments:

The name of a command or tool. It opens the manual pages for the command or tool in a viewer. Press q to exit the viewer

top

top is your ctrl-alt-delete style process manager. It shows you the CPU and memory usage on the bar along the top, and the currently running processes, with their respective CPU and memory usage.

top opens an interactive monitor which updates every 3 seconds by default. For help with the commands within top, press h while it's open. To close it, press q.

kill

kill sends a termination signal to a process

Arguments:

The ID of the process to kill. This ID can be found rather easily using top

nice

nice modifies the priority of a process at start time.

When running calculations which will take a good deal of time, it's good to "nice" your commands—lower their priority— so as not to rob computing resources from other users.

Arguments:

The first argument is a priority between -20 and 19. Higher values correspond to lower priority, and vice versa. The remaining arguments constitute a regular terminal command.