How Linux Works(Chapter Two)--Basic Commands(Part One)

All the summaries are from the book named How Linux Works.

The Bourne Shell

The shell is one of the most important parts of a Unix system. A shell is a program that runs commands, like the ones that users enter. The shell also serves as a small programming environment. Unix programmers often break common tasks into little components and use the shell to manage tasks and piece things together.

One of the best things about the shell is that if you make a mistake, you can easily see what you typed to find out what went wrong, and then try again.

Linux uses an enhanced version of the Bourne shell called bash or the “Bourne-again” shell. The bash shell is the default shell on most Linux distributions, and /bin/sh is normally a link to bash on a Linux system. You should use bash shell when running the examples I copy from the book.

Using the Shell

When you install Linux, you should create at least one regular user in addition to the root user; this will be your personal account.

cat

The cat command is one of the easiest Unix commands to understand, it simply outputs the contents of one or more files. The general syntax of the cat command is as follows:

1	cat file1 file2

When you run this command, cat prints the contents of file1, file2, and any other files that you specify, and then exits. The command is called cat because it performs concatenation when it prints the contents of more than one file.

Standard Input and Standard Output

We’ll use cat to briefly explore Unix input and output(I/O). Unix processes use I/O stream to read and write data. Processes read data from input streams and write data to output streams. Streams are very flexible. For example, the source of an input stream can be a file, a device, a terminal, or even the output stream from another process.

Standard output is similar. The kernel gives each process a standard output stream where it can write its output. The cat command always writes its output to the standard output. When you ran cat in the terminal, the standard output was connected to that terminal, so that’s where you saw the output.

Standard input and output are often abbreviated as stdin and stdout. Many commands operate as cat does; if you don’t specify an input file, the command reads from stdin. Output is a little different. Some commands (like cat) send output only to stdout, but others have the option to send output directly to files.

And there is a third standard I/O stream called standard error.

One of the best features of standard streams is that you can easily manipulate them read and write to places other than the terminal.

Basic Commands

Now let’s look at some more Unix commands.

`ls`

The ls command lists the contents of a directory. The default is the current directory. Use ls -l for a detailed (long) listing and ls -F to display file type information.

`cp`

In its simplest form cp copies files. For example, to copy file1 to file2, enter this:

1	cp file1 file2

To copy a number of files to a directory (folder) named dir, try this instead:

1	cp file1 ... fileN dir

`mv`

The mv (move) command is like cp. In its simplest form, it renames a file. For example, to rename file1 to file2, enter this:

1	mv file1 file2

You can use mv to move a number of files to a different directory:

1	mv file1 ... fileN dir

`touch`

The touch command creates a file. If the file already exists, touch does not change it, but it does update the file’s modification time stamp printed with the ls -l command. For example, to create empty file, enter this:

1	touch file

`rm`

To delete(remove) a file, use rm. After you remove a file, it’s gone from your system and generally cannot be undeleted.

rm file

`echo`

The echo command prints its arguments to the standard output:

1 2	echo Hello again. Hello again.

The echo command is very useful for finding expansions of shell globs and variables.

Navigating Directories

Unix has a directory hierarchy that starts at /, sometimes called the root directory. The directory separator is the slash (/), not the backslash (\). There are several standard subdirectories in the root directory, such as /usr.

When you refer to a file or directory, you specify a path or pathname. When a path starts with / (such as /usr/lib), it’s a full or absolute path.

A path component identified by two dot (..) specifies the parent of a directory. For example, if you’re working in /usr/lib, the path .. would refer to /usr. Similarly, ../bin would refer to /usr/bin.

One dot(.) refers to the current directory; for example, if you’re in /usr/lib, the path . is still /usr/lib, and ./X11 is /usr/lib/X11. You won’t have to use . very often because most commands default to the current directory if a path doesn’t start with / (you could just use X11 instead of ./X11 in the preceding example).

A path not beginning with / is called a relative path. Most of the time, you’ll work with relative pathnames, because you’ll already be in the directory you need to be in or somewhere close by.

`cd`

The current working directory is the directory that a process(such as the shell) is currently in. The cd command changes the shell’s current working directory:

cd dir

If you omit dir, the shell returns to your home directory, the directory you started in when you first logged in.

`mkdir`

The mkdir command creates a new directory dir.

mkdir dir

`rmdir`

The rmdir command removes the directory dir.

rmdir dir

If dir isn’t empty, this command fails. However, if you’re impatient, you probably don’t want to laboriously delete all the files and subdirectories inside dir first. You can use rm -rf dir to delete a directory and its contents, but be careful! Double-check your command before you run it.