commands and arguments
(Written by Paul Cobbaut, https://github.com/paulcobbaut/, with contributions by: Alex M. Schapelle, https://github.com/zero-pytagoras/)
This chapter introduces you to 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).
This chapter frequently uses the echo
command to
demonstrate shell features. The echo
command is very simple: it echoes
the input that it receives.
student@linux:~$ echo Burtonville
Burtonville
student@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
.
[student@linux ~]$ echo Hello World
Hello World
[student@linux ~]$ echo Hello World
Hello World
[student@linux ~]$ echo Hello World
Hello World
[student@linux ~]$ 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
.
[student@linux ~]$ echo 'A line with single quotes'
A line with single quotes
[student@linux ~]$
double quotes
You can also prevent the removal of white spaces by double quoting
the spaces. Same as above, echo
only receives one
argument
.
[student@linux ~]$ echo "A line with double quotes"
A line with double quotes
[student@linux ~]$
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).
[student@linux ~]$ echo -e "A line with \na newline"
A line with
a newline
[student@linux ~]$ echo -e 'A line with \na newline'
A line with
a newline
[student@linux ~]$ echo -e "A line with \ta tab"
A line with a tab
[student@linux ~]$ echo -e 'A line with \ta tab'
A line with a tab
[student@linux ~]$
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.
student@linux:~$ type cd
cd is a shell builtin
student@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.
student@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.
student@linux:~$ type -a echo
echo is a shell builtin
echo is /bin/echo
student@linux:~$ /bin/echo Running the external echo command...
Running the external echo command...
which
The which
command will search for binaries in the
$PATH
environment variable (variables will be explained
later). In the screenshot below, it is determined that cd
is
builtin
, and ls, cp, rm, mv, mkdir, pwd,
and which
are external
commands.
[root@linux ~]# which cp ls cd mkdir pwd
/bin/cp
/bin/ls
/usr/bin/which: no cd in (/usr/kerberos/sbin:/usr/kerberos/bin:...
/bin/mkdir
/bin/pwd
aliases
create an alias
The shell allows you to create aliases
. Aliases are
often used to create an easier to remember name for an existing command
or to easily supply parameters.
[student@linux ~]$ cat count.txt
one
two
three
[student@linux ~]$ alias dog=tac
[student@linux ~]$ dog count.txt
three
two
one
abbreviate commands
An alias
can also be useful to abbreviate an existing
command.
student@linux:~$ alias ll='ls -lh --color=auto'
student@linux:~$ alias c='clear'
student@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
.
[student@linux ~]$ rm -i winter.txt
rm: remove regular file `winter.txt'? no
[student@linux ~]$ rm winter.txt
[student@linux ~]$ ls winter.txt
ls: winter.txt: No such file or directory
[student@linux ~]$ touch winter.txt
[student@linux ~]$ alias rm='rm -i'
[student@linux ~]$ rm winter.txt
rm: remove regular empty file `winter.txt'? no
[student@linux ~]$
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.
student@linux:~$ alias c ll
alias c='clear'
alias ll='ls -lh --color=auto'
unalias
You can undo an alias with the unalias
command.
[student@linux ~]$ which rm
/bin/rm
[student@linux ~]$ alias rm='rm -i'
[student@linux ~]$ which rm
alias rm='rm -i'
/bin/rm
[student@linux ~]$ unalias rm
[student@linux ~]$ which rm
/bin/rm
[student@linux ~]$
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.
[student@linux ~]$ set -x
++ echo -ne '\033]0;student@linux:~\007'
[student@linux ~]$ echo $USER
+ echo paul
paul
++ echo -ne '\033]0;student@linux:~\007'
[student@linux ~]$ echo \$USER
+ echo '$USER'
$USER
++ echo -ne '\033]0;student@linux:~\007'
[student@linux ~]$ set +x
+ set +x
[student@linux ~]$ echo $USER
paul
practice: commands and arguments
-
How many
arguments
are in this line (not counting the command itself). -
Is
tac
a shell builtin command? -
Is there an existing alias for
rm
? -
Read the man page of
rm
, make sure you understand the-i
option ofrm
. Create and remove a file to test the-i
option. -
Execute:
alias rm='rm -i'
. Test your alias with a test file. Does this work as expected? -
List all current aliases.
-
Create an alias called 'city' that echoes your hometown.
Use your alias to test that it works.
-
Execute
set -x
to display shell expansion for every command. -
Test the functionality of
set -x
by executing yourcity
andrm
aliases. -
Execute
set +x
to stop displaying shell expansion. -
Remove your city alias.
-
What is the location of the
cat
and thepasswd
commands? -
Explain the difference between the following commands:
-
Explain the difference between the following commands:
-
Display
A B C
with two spaces between B and C. -
*(optional) Complete the following command (do not use spaces) to display exactly the following output:
-
Use
echo
to display the following exactly:Find two solutions with single quotes, two with double quotes and one without quotes (and say thank you to René and Darioush from Google for this extra).
-
Use one
echo
command to display three words on three lines.
solution: commands and arguments
-
How many
arguments
are in this line (not counting the command itself).answer: three
-
Is
tac
a shell builtin command? -
Is there an existing alias for
rm
? -
Read the man page of
rm
, make sure you understand the-i
option of rm. Create and remove a file to test the-i
option. -
Execute:
alias rm='rm -i'
. Test your alias with a test file. Does this work as expected? -
List all current aliases.
-
Create an alias called 'city' that echoes your hometown.
Use your alias to test that it works.
-
Execute
set -x
to display shell expansion for every command. -
Test the functionality of
set -x
by executing yourcity
andrm
aliases.shell should display the resolved aliases and then execute the command:
-
Execute
set +x
to stop displaying shell expansion. -
Remove your city alias.
-
What is the location of the
cat
and thepasswd
commands? -
Explain the difference between the following commands:
The
echo
command will be interpreted by the shell as thebuilt-in echo
command. The/bin/echo
command will make the shell execute theecho binary
located in the/bin
directory. -
Explain the difference between the following commands:
The -n option of the
echo
command will prevent echo from echoing a trailing newline.echo Hello
will echo six characters in total,echo -n hello
only echoes five characters. (The -n option might not work in the Korn shell.) -
Display
A B C
with two spaces between B and C. -
Complete the following command (do not use spaces) to display exactly the following output:
The solution is to use tabs with
\t
. -
Use
echo
to display the following exactly:Find two solutions with single quotes, two with double quotes and one without quotes (and say thank you to René and Darioush from Google for this extra).
-
Use one
echo
command to display three words on three lines.