commands and arguments

shell expansion by taking a close look at commands and arguments. Knowing shell expansion is important because many commands on your Linux system are processed and most likely changed by the shell before they are executed.
The command line interface or shell used on most Linux systems is called bash, which stands for Bourne again shell. The bash shell incorporates features from sh (the original Bourne shell), csh (the C shell), and ksh (the Korn shell).

echo

This chapter frequently uses the echo command to demonstrate shell features. The

echo command is very simple: it echoes the input that it receives.

root@linux:~$ echo Burtonville

Burtonville

root@linux:~$ echo Smurfs are blue

Smurfs are blue

arguments

One of the primary features of a shell is to perform a command line scan. When

you enter a command at the shell's command prompt and press the enter key, then

the shell will start scanning that line, cutting it up in arguments. While scanning the

line, the shell may make many changes to the arguments you typed. This process

is called shell expansion. When the shell has finished scanning and modifying that

line, then it will be executed.

white space removal

Parts that are separated by one or more consecutive white spaces (or tabs) are

considered separate arguments, any white space is removed. The first argument is

the command to be executed, the other arguments are given to the command. The

shell effectively cuts your command into one or more arguments.

This explains why the following four different command lines are the same after shell

expansion.

[paul@RHELv4u3 ~]$ echo Hello World

Hello World

[paul@RHELv4u3 ~]$ echo Hello World

Hello World

[paul@RHELv4u3 ~]$ echo Hello World

Hello World

[paul@RHELv4u3 ~]$ echo Hello World

Hello World

The echo command will display each argument it receives from the shell. The echo

command will also add a new white space between the arguments it received.

single quotes

You can prevent the removal of white spaces by quoting the spaces. The contents of

the quoted string are considered as one argument. In the screenshot below the echo

receives only one argument.

[paul@RHEL4b ~]$ echo 'A line with single quotes'

A line with single quotes

[paul@RHEL4b ~]$

double quotes

You can also prevent the removal of white spaces by double quoting the spaces.

Same as above, echo only receives one argument.

[paul@RHEL4b ~]$ echo "A line with double quotes"

A line with double quotes

[paul@RHEL4b ~]$

Later in this book, when discussing variables we will see important differences

between single and double quotes.

echo and quotes

Quoted lines can include special escaped characters recognised by the echo command

(when using echo -e). The screenshot below shows how to use \n for a newline and

\t for a tab (usually eight white spaces).

[paul@RHEL4b ~]$ echo -e "A line with \na newline"

A line with

a newline

[paul@RHEL4b ~]$ echo -e 'A line with \na newline'

A line with

a newline

[paul@RHEL4b ~]$ echo -e "A line with \ta tab"

A line with a tab

[paul@RHEL4b ~]$ echo -e 'A line with \ta tab'

A line with a tab

[paul@RHEL4b ~]$

The echo command can generate more than white spaces, tabs and newlines. Look

in the man page for a list of options.

Commands

external or builtin commands ?

Not all commands are external to the shell, some are builtin. External commands

are programs that have their own binary and reside somewhere in the file system.

Many external commands are located in /bin or /sbin. Builtin commands are an

integral part of the shell program itself.

Type

To find out whether a command given to the shell will be executed as an external

command or as a builtin command, use the type command.

root@linux:~$ type cd

cd is a shell builtin

root@linux:~$ type cat

cat is /bin/cat

As you can see, the cd command is builtin and the cat command is external.

You can also use this command to show you whether the command is aliased or not.

root@linux:~$ type ls

ls is aliased to `ls --color=auto'

running external commands

Some commands have both builtin and external versions. When one of these

commands is executed, the builtin version takes priority. To run the external version,

you must enter the full path to the command.

root@linux:~$ type -a echo

echo is a shell builtin

echo is /bin/echo

root@linux:~$ /bin/echo Running the external echo command...

Running the external echo command...

Which

The which command will search for binaries in the $PATH environment variable

(variables will be explained later). In the screenshot below, it is determined that cd

is builtin, and ls, cp, rm, mv, mkdir, pwd, and which are external commands.

[root@RHEL4b ~]# which cp ls cd mkdir pwd

/bin/cp

/bin/ls

/usr/bin/which: no cd in (/usr/kerberos/sbin:/usr/kerberos/bin:...

/bin/mkdir

/bin/pwd

Aliases

create an alias

The shell allows you to create aliases. Aliases are often used to create an easier to

remember name for an existing command or to easily supply parameters.

[paul@RHELv4u3 ~]$ cat count.txt

one

two

three

[paul@RHELv4u3 ~]$ alias dog=tac

[paul@RHELv4u3 ~]$ dog count.txt

three

two

one

abbreviate commands

An alias can also be useful to abbreviate an existing command.

root@linux:~$ alias ll='ls -lh --color=auto'

root@linux:~$ alias c='clear'

root@linux:~$

default options

Aliases can be used to supply commands with default options. The example below

shows how to set the -i option default when typing rm.

[paul@RHELv4u3 ~]$ rm -i winter.txt

rm: remove regular file `winter.txt'? no

[paul@RHELv4u3 ~]$ rm winter.txt

[paul@RHELv4u3 ~]$ ls winter.txt

ls: winter.txt: No such file or directory

[paul@RHELv4u3 ~]$ touch winter.txt

[paul@RHELv4u3 ~]$ alias rm='rm -i'

[paul@RHELv4u3 ~]$ rm winter.txt

rm: remove regular empty file `winter.txt'? no

[paul@RHELv4u3 ~]$

Some distributions enable default aliases to protect users from accidentally erasing

files ('rm -i', 'mv -i', 'cp -i')

viewing aliases

You can provide one or more aliases as arguments to the alias command to get their

definitions. Providing no arguments gives a complete list of current aliases.

root@linux:~$ alias c ll

alias c='clear'

alias ll='ls -lh --color=auto'

unalias

You can undo an alias with the unalias command.

[paul@RHEL4b ~]$ which rm

/bin/rm

[paul@RHEL4b ~]$ alias rm='rm -i'

[paul@RHEL4b ~]$ which rm

alias rm='rm -i'

/bin/rm

[paul@RHEL4b ~]$ unalias rm

[paul@RHEL4b ~]$ which rm

/bin/rm

[paul@RHEL4b ~]$

displaying shell expansion

You can display shell expansion with set -x, and stop displaying it with set +x. You

might want to use this further on in this course, or when in doubt about exactly what

the shell is doing with your command.

[paul@RHELv4u3 ~]$ set -x

++ echo -ne '\033]0;paul@RHELv4u3:~\007'

[paul@RHELv4u3 ~]$ echo $USER

+ echo paul

paul

++ echo -ne '\033]0;paul@RHELv4u3:~\007'

[paul@RHELv4u3 ~]$ echo \$USER

+ echo '$USER'

$USER

++ echo -ne '\033]0;paul@RHELv4u3:~\007'

[paul@RHELv4u3 ~]$ set +x

+ set +x

[paul@RHELv4u3 ~]$ echo $USER

Paul