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Shell Variables

What is a Shell?

Strictly speaking there are several Shells (C, grain, under Unix Bash...). Each user can decide for its favourite shell muLinux used however only a Shell. The differences of the Shells are small (for normal users) and should not not be interested therefore for the time being.

What is a variable?

• System variables (vordefenierte variables)
• Special variables
• User-defined variables

System variables determine the characteristics system the user opposite and can by the user be adapted. System variables are written with uppercase letters.

The special variables can be only accessed reading. They determine general system properties.

User-defined variables can be produced, modified and deleted by the user. They can be used e.g. for shell Skripte. Those are small programs, which can simplify themselves repeating work.

Displays of available variables

/# set

The system variables with contents are displayed. Left are the variable names, then an equals sign and to it contents of the variables come. In order to see all variables, we must the sceen contents scrollen ( [ SHIFTS ] [ picture on ] , [ SHIFTS ] [ picture off ] ). for it there is nevertheless very many, as the following instruction displays us:

/# set | wc

We want to discuss only some system variables here. At the beginning we look ourselves times the system variable PS1 on. Who does not want to scrollen the display, can also grep leave only the line with PS1 to display:

/#  set | grep "PS1"
PS1=/#
/# _

The variable PS1 contained thus the two characters / # . What is that? That comes us nevertheless somehow admits forwards. Yes, that is our prompt! Times see whether we these to modify being able.

Assign contents

Variable=value

We want to modify the prompt in such a way that " instruction? " as prompt appears:

/# PS1=Command?
Command?_

More beautiful would it however be with a blank after "Command?". If we want to have blanks, we must include the value in stating lines:

/# PS1="Command? "
Command?_

This is also necessary, if we want to assign several words as value:

/# PS1="Please type a command: "
Please type a command: _

Who wants can also show the DOS-PROMPT PS1="C:\> "

Display to a variable

Please type a command:  echo PS1 
PS1
Please type a command: _

Thus unfortunately only the text "PS1" is displayed and not contents of PS1. In addition dollar character must $ before the variable are:

Please type a command: echo $PS1
Please type a command:
Please type a command: _

We can mix also text and variable:

Please type a command:  echo "That is the content of PS1: $PS1"
That is the content of PS1: Please type a command:
Please type a command: _

Make variables empty

/# PS1=
_

The variable remains existing thereby however. It is only empty.

set | grep "PS1"
PS1=
_

If we PS1, have we empty no more prompt. What actually occurs when changing into another directory?

cd /home
/home# _

The variable PS1 is thus overwritten here by the system.

Write protection for variables

/# readonly PS1

Each attempt PS1 to modify, is acknowledged with an error message. The write protection is preserved up to leaving the Shell. One calls the instruction readonly without parameters up, all write protected variables are displayed.

/# readonly
PS1

User-defined variables

• Variable names knows from small or uppercase letters, which underlined digits or exist. Umlauts (ae, oe, ue, ss) may not be used.
• A digit and underlined may not as the first character be.
• The length of the variable name is unlimited.
• It is recommended to write variable names only small. So they are to be differentiated more easily from the system variables.

Variable ones can be used for simplification or in repetition of instruction. So for example as instruction option:

/# a=/usr/bin
/# cd $a
/usr/bin # _

Or as instruction:

/# b="ls -l"
/# $b

long output de Verzeichnisinhaltes

We can store also the outputs of programs into a variable. In addition we must the program call in two rueckwertige inverted comma (franz. Accent grave) ` put in parentheses.

/# today="Today is `date`"
/# echo $heute
Today is November 16 15:54:16 1999
/# _

Extend the variables' content

/# gosh=Linux
/# gosh=mu$toll
/# echo $gosh 
muLinux
/# _

Delete from variables

/# unset a 
/# _