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Linux Commands

Linux Commands and arguments

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Introduction

In this session, we have covered the 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.

[email protected]:~$ echo Burtonville
Burtonville
[email protected]:~$ 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.

[email protected]:~$ echo My  first  webpage
My first webpage
[email protected]:~$ echo My   first    webpage
My first webpage
[email protected]:~$ echo    My    first    webpage
My first webpage
[email protected]:~$      echo      My      first       webpage
My first webpage

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.

[email protected]:~$ echo 'A line with   single   quotes'
A line with   single   quotes
[email protected]:~$

double quotes

You can also prevent the removal of white spaces by double quoting the spaces. Same as above, echo only receives one argument.

[email protected]:~$ echo "A line with          double         quotes"
A line with          double         quotes
[email protected]:~$

Later in this tutorial, when discussing variables we will see important differences between single and double quotes.

echo and quotes

Quoted lines can include special escaped characters recognized 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).

[email protected]:~$ echo -e "A line with \na newline"
A line with 
a newline
[email protected]:~$ echo -e 'A line with \na newline'
A line with 
a newline
[email protected]:~$ echo -e "A line with \ta tab"
A line with 	a tab
[email protected]:~$ echo -e 'A line with \ta tab'
A line with 	a tab
[email protected]:~$

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

[email protected]:~$ type cd
cd is a shell builtin
[email protected]:~$ 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.

[email protected]:~$ 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.

[email protected]:~$ type -a echo
echo is a shell builtin
echo is /bin/echo
[email protected]:~$ /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.

[email protected]:/$ which cp ls cd mkdir pwd
/bin/cp
/bin/ls
/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 the name for an existing command or to easily supply parameters.

[email protected]:~$ cat text2
apple
mango
banana
[email protected]:~$ alias xyz=tac
[email protected]:~$ xyz text2
banana
mango
apple

abbreviate commands

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

[email protected]:~$ alias ll='ls -lh --color=auto'
[email protected]:~$ alias c='clear'
[email protected]:~$

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.

[email protected]:~$ rm -i xyz.txt
rm: remove regular file ?xyz.txt?? no
[email protected]:~$ rm xyz.txt
[email protected]:~$ ls xyz.txt
ls: cannot access xyz.txt: No such file or directory
[email protected]:~$ touch xyz.txt
[email protected]:~$ alias rm='rm -i'
[email protected]:~$ rm xyz.txt
rm: remove regular empty file ?xyz.txt?? no
[email protected]:~$

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.

[email protected]:~$ alias c ll
alias c='clear'
alias ll='ls -lh --color=auto'

unalias

You can undo an alias with the unalias command.

[email protected]:~$ which rm
/bin/rm
[email protected]:~$ alias rm='rm -i'
[email protected]:~$ which rm
/bin/rm
[email protected]:~$ unalias rm
[email protected]:~$ which rm
/bin/rm
[email protected]:~$

displaying shell expansion

You can display shell expansion with the set -x, and stop displaying it with set +x. You might want to use this further on in this tutorial, or when in doubt about exactly what the shell is doing with your command.

[email protected]:~$ set -x pran
[email protected]:~$ echo $user
+ echo

[email protected]:~$ echo \$user
+ echo '$user'
$user
[email protected]:~$ set -x \pran
+ set -x pran
[email protected]:~$ echo \$user
+ echo '$user'
$user
[email protected]:~$ set +x
+ set +x
[email protected]:~$ echo \$user
$user
[email protected]:~$

Exercise, Practice and Solution :

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

touch '/etc/cron/cron.allow' 'abc123.txt' "abc456.txt"

Code:

touch '/etc/cron/cron.allow' 'abc123.txt' "abc456.txt"
answer: three

2. Is tac a shell builtin command ?

Code:

Yes.
type tac

3. Is there an existing alias for rm ?

Code:

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.

Code:

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 ?

Code:

touch testfile
rm testfile (should ask for confirmation)

6. List all current aliases.

Code:

alias

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

Code:

 alias city='echo Antwerp'
 

8. Use your alias to test that it works.

Code:

city (it should display Antwerp

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

Code:

set -x

10. Test the functionality of set +x by executing your city and rm aliases.

Code:

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

11. Execute set +x to stop displaying shell expansion.

Code:

set +x

12. Remove your city alias.

Code:

unalias city

13. What are the location of the cat and the passwd commands?

which cat (probably /bin/cat)
which passwd (probably /usr/bin/passwd)

14. Explain the difference between the following commands :
echo
/bin/echo

Code:

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.

15. Explain the difference between the following commands :
echo Hello
echo -n Hello

Code:

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.)

16. Display A B C with two spaces between B and C.
(optional)

Code:

echo "A B C"

17. 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.

Code:

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

18. Use echo to display the following exactly:

Code:

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

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

Code:

echo -e "one \ntwo \nthree"


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