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

1. How many arguments are in this line (not counting the command itself).

touch '/etc/cron/cron.allow' 'file 42.txt' "file 33.txt"

2. Is tac a shell builtin command ?

3. Is there an existing alias for rm ?

4. 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.

5. Execute: alias rm='rm -i' . Test your alias with a test file. Does this work as expected ?

6. List all current aliases.

7a. Create an alias called \'city\' that echoes your hometown.

7b. Use your alias to test that it works.

8. Execute set -x to display shell expansion for every command.

9. Test the functionality of set -x by executing your city and rm aliases.

10 Execute set +x to stop displaying shell expansion.

11. Remove your city alias.

12. What is the location of the cat and the passwd commands ?

13. Explain the difference between the following commands:

echo

/bin/echo

14. Explain the difference between the following commands:

echo Hello

echo -n Hello

15. Display A B C with two spaces between B and C.

(optional)16. Complete the following command (do not use spaces) to display exactly the following output:

4+4     =8
10+14   =24

17. 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).

18. Use one echo command to display three words on three lines.

solution: commands and arguments

1. How many arguments are in this line (not counting the command itself).

touch '/etc/cron/cron.allow' 'file 42.txt' "file 33.txt"

answer: three

2. Is tac a shell builtin command ?

type tac

3. Is there an existing alias for rm ?

alias rm

4. 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.

man rm

touch testfile

rm -i testfile

5. Execute: alias rm='rm -i' . Test your alias with a test file. Does this work as expected ?

touch testfile

rm testfile (should ask for confirmation)

6. List all current aliases.

alias

7a. Create an alias called \'city\' that echoes your hometown.

alias city='echo Antwerp'

7b. Use your alias to test that it works.

city (it should display Antwerp)

8. Execute set -x to display shell expansion for every command.

set -x

9. Test the functionality of set -x by executing your city and rm aliases.

shell should display the resolved aliases and then execute the command:
student@linux:~$ set -x
student@linux:~$ city
+ echo antwerp
antwerp

10 Execute set +x to stop displaying shell expansion.

set +x

11. Remove your city alias.

unalias city

12. What is the location of the cat and the passwd commands ?

which cat (probably /bin/cat)

which passwd (probably /usr/bin/passwd)

13. Explain the difference between the following commands:

echo

/bin/echo

The echo command will be interpreted by the shell as the built-in echo command. The /bin/echo command will make the shell execute the echo binary located in the /bin directory.

14. Explain the difference between the following commands:

echo Hello

echo -n Hello

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.)

15. Display A B C with two spaces between B and C.

echo "A B  C"

16. Complete the following command (do not use spaces) to display exactly the following output:

4+4     =8
10+14   =24

The solution is to use tabs with \t.

echo -e "4+4\t=8" ; echo -e "10+14\t=24"

17. Use echo to display the following exactly:

??\\
echo '??\\'
echo -e '??\\\\'
echo "??\\\\"
echo -e "??\\\\\\"
echo ??\\\\

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).

18. Use one echo command to display three words on three lines.

echo -e "one \ntwo \nthree"