commands and arguments(shell expansion)
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.
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.
double quotes
You can also prevent the removal of white spaces by double quoting the spaces. Same as above, echo only receives one argument.
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).
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.
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 @tryit-ethical:~$ 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.
which
The which command will search for binaries in the $PATH environment variable (variables will be explained later). In the screenshot below, it is determined that cd is builtin, and ls, cp, rm, mv, mkdir, pwd, and which are external commands.
aliases
create an alias
The shell allows you to create aliases. Aliases are often used to create an easier to remember name for an existing command or to easily supply parameters.
[root@tryit-ethical~]$ cat count.txt
one
two
three
[root@tryit-ethical ~]$ alias dog=tac
[root@tryit-ethical ~]$ dog count.txt
three
two
one
abbreviate commands
An alias can also be useful to abbreviate an existing command.
root@tryit-ethical:~$ alias ll=’ls -lh –color=auto’
root@tryit-ethical:~$ alias c=’clear’
root@tryit-ethical:~$
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.
[root@tryit-ethical ~]$ rm -i winter.txt
rm: remove regular file `winter.txt’? no
[root@tryit-ethical ~]$ rm winter.txt
[root@tryit-ethical ~]$ ls winter.txt
ls: winter.txt: No such file or directory
[root@tryit-ethical ~]$ touch winter.txt
[root@tryit-ethical ~]$ alias rm=’rm -i’
[root@tryit-ethical ~]$ rm winter.txt
rm: remove regular empty file `winter.txt’? no
[root@tryit-ethical ~]$
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@tryit-ethical:~$ alias c ll
alias c=’clear’
alias ll=’ls -lh –color=auto’
unalias
You can undo an alias with the unalias command.
[root@tryit-ethical ~]$ which rm
/bin/rm
[root@tryit-ethical ~]$ alias rm=’rm -i’
[root@tryit-ethical ~]$ which rm
alias rm=’rm -i’
/bin/rm
[root@tryit-ethical ~]$ unalias rm
[root@tryit-ethical ~]$ which rm
/bin/rm
[root@tryit-ethical ~]$
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.
[root@tryit-ethical ~]$ set -x
++ echo -ne ‘\033]0;root@tryit-ethical:~\007’
[root@tryit-ethical ~]$ echo $USER
+ echo root
root
++ echo -ne ‘\033]0;root@tryit-ethical:~\007’
[root@tryit-ethical ~]$ echo \$USER
+ echo ‘$USER’
$USER
++ echo -ne ‘\033]0;root@tryit-ethical:~\007’
[root@tryit-ethical ~]$ set +x
+ set +x
[root@tryit-ethical ~]$ echo $USER
root
