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Shell Basics

A quick overview of the Unix shell

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