Shell Basics

A quick overview of the Unix shell

• Introduction
• Commands
• Paths
• I/O redirection
• Shell scripts
• The <Ctrl> + z “problem”
• See also

Introduction

The Unix shell (or terminal, or command line) is a program that allows us to run other programs via a text interface.

Running a program

To run a program, type its name and hit <Enter>. For example, the pwd program is for printing the path of the current directory.

$ pwd
/home/y/yuanqing

(In all examples shown, a command is indicated with a $ prefix. Do not type the $.)

Passing arguments to a program

Some programs accept arguments, which come after the name of the program. For example, the cd program is for changing the current directory:

$ cd foo
$ pwd
/home/y/yuanqing/foo

Commands

Listed here are the more important command line operations that you should know.

Operation	Command
Show the path of the current directory	pwd
List all files and directories in the current directory	ls -a
List all files and directories in the current directory, excluding hidden files	ls
Change the current directory to the specified directory	cd foo
Change the current directory to the home directory	cd
Change the current directory to the parent directory	cd ..
Change the current directory to the previous directory	cd -
Show the contents of a file	cat foo
Print a string	echo 'foo'
Create an empty file	touch foo
Create a file containing the specified string	echo 'foo' > bar
Create an empty directory	mkdir foo
Copy a file	cp foo bar
Copy a file at the specified path to the current directory	cp ~/foo/bar .
Copy a directory	cp -r foo bar/
Delete a file	rm foo
Delete a directory	rm -rf foo
Move/rename a file/directory	mv foo bar
Compare the contents of two files	diff foo bar
Clear the terminal screen	clear
Show your command history	history
Open the user manual for a command	man foo
Write the output of a program to a file	java HelloWorld > foo
Write the output of a program to a file, appending to the end of the specified file	java HelloWorld >> foo
Use the contents of a file as the input to a program	java Hello < input
Use the contents of a file as the input to a program, and write its output to a file	java Hello < input > output
Run the shell commands listed in the specified text file	bash foo
Bring the most recent “backgrounded” program back to the foreground	fg

Before hitting <Enter>, you can:

1. Press <Ctrl> + c at any time to discard the command.
2. Use the ↑ and ↓ arrow keys to toggle through your command history.
3. Use the <Tab> key to autocomplete commands.

To terminate a running program, press <Ctrl> + d. (See the <Ctrl> + z “problem”.)

Paths

Many programs take files/directories as arguments. A file/directory path can be specified as follows:

File/directory	Path
The current directory	.
The parent directory	..
The directory two levels up	../..
The home directory	~
The root directory	/
A file/directory in the current directory	foo or ./foo
A file/directory in the parent directory	../foo
A file/directory in the directory two levels up	../../foo
A file/directory in the home directory	~/foo
A file/directory in the root directory	/foo

I/O redirection

Redirect output

The output of a program is typically displayed on the terminal (ie. stdout):

$ javac HelloWorld.java
$ java HelloWorld
hello world

Use the > operator to write the program’s output to a file:

$ java HelloWorld > foo
$ cat foo
hello world

If you use the > operator, the specified file will be overwritten, so be careful! To merely append the output to the end of the specified file, use the >> operator instead:

$ java HelloWorld >> bar
$ cat bar
hello world
$ java HelloWorld >> bar
$ cat bar
hello world
hello world

Redirect input

Many programs accept input from the keyboard (ie. stdin):

$ javac Hello.java
$ java Hello
foo
hello foo

(Here, Hello is a toy Java program that accepts a string, then outputs hello  followed by the string that was entered.)

We can use the contents of a file as the input to a program via the < operator:

$ cat input
foo
$ java Hello < input
hello foo

Redirect input and output

We can redirect both input and output in a single command:

$ javac Hello.java
$ cat input
foo
$ java Hello < input > output
$ cat output
hello foo

Shell scripts

A shell script is simply a sequence of commands listed in a text file.

Suppose we have a text file named commands containing the following:

javac Hello.java
cat input
java Hello < input > output
cat output

We can run the commands listed in the commands text file using the bash program:

$ bash commands
foo
hello foo

The <Ctrl> + z “problem”

If you’d pressed <Ctrl> + z while a program was still running, you would see something like the following:

$ javac Hello.java
$ java Hello
^Z
[1]+  Stopped                 java Hello

A quick fix is to run the fg program:

$ fg

This brings us back to our running program, and all is well with the world.

(Explanation: Pressing <Ctrl> + z places the currently-running program in the background. The fg program simply brings the most recent “backgrounded” program back to the foreground.)

See also

• Vim Basics
• Unix Workshop
• The Unix Philosophy