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

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