Bourne Shell: Chapter 9a

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9. OPTIONAL CHAPTER - KORNSHELL PROGRAMMING.
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9.  OPTIONAL CHAPTER - KORNSHELL PROGRAMMING


The KornShell is a command-programming language that executes
commands read from a terminal or file.  The KornShell is close to
being upward compatible with the standard BourneShell.  For
example, scripts written for the BourneShell (sh) will also work
in the KornShell (ksh).  The major enhancements are command-line
reentry, in-line command editing, and aliasing.

This chapter will deal with some, not all, of the features that
differentiate it from the BourneShell.  For your information, here
is a list of features that have been enhanced from Bourne.
Improvements have been made as a command language, including
command-line editing, a command history mechanism, command-name
aliasing, job control, new capabilities for cd, and tilde
expansion.  

Improvements as a Programming Language include a more general I/O
mechanism, Menu selection primitive, Built-in integer arithmetic,
Substring operators, Array variables and attributes, More-general
function facility, Co-process facility, Easier to debug, Better
performance, and Better security.

The KornShell is a high level programming language and a command
line interpreter.

  Command Format: ksh [acefhikmnorstuvx] [ -o option]...[args] 
                                                               
      See the appendix for a complete list of options          
                                                               



9.1  KornShell Variables


The KornShell has variables that are initialized for you on login
such as:

PATH      Determine which directories are searched, sequentially
          for shell commands.

CDPATH    The directories that are searched for arguments to the
          "cd" command.

SHELL     The current interactive shell.

PWD       The present working directory.

IFS       Valid inter-field separators used to separate commands
          from arguments, arguments from each other and commands
          from each other.

EDITOR    Command line edit mode.

FCEDIT    Editor entered when using fc

TERM      Defines terminal capabilities

PS1       Primary shell prompt

PS2       Secondary shell prompt

PS3       Select command prompt

HOME      Home directory

ENV       File(s) to execute when entering this shell

LOGNAME   Login name of the user


The command line argument variables are also available:
$$, $?, $*, $#, and the positionals ($0, $1, $2...etc)

9.2  User Defined Variables

These variables are similar to the BourneShell.  The general form
is VARIABLE=value.

No spaces are allowed around the =.  You can enclose them in double
quotes "=" or single quotes '=' for clarity.  No spaces are allowed
in "value".  These can also be enclosed in double or single quotes.
"value" can be a string or an expression.  The value of a variable
can be accessed by preceding the name of the variable with a dollar
sign ($).

Examples:
 $MyString'='"This is a string"                                
 $MyStatic=47                                                  
 $readonly Mystatic                                            

The readonly command makes MyStatic read only (can't change the
contents).

Example:
 $typeset -i BIGINT=1492                                       


This will make the variable integer for faster arithmetics:

 $typeset -i8 OCTINT=9                                         


The output of OCTINT will be an octal integer; the assignment is
decimal.

Sample Session:


 $echo $OCTINT                                                 
 8#11                                                          
 $                                                             

9.3  Values of Variables Between Child and Parent Processes


Values in one shell are local only to that shell.  If a child
process or a subshell needs to have access to a value established
in its parent, the value must be exported from the parent.

     $typeset -i8 -x OCTINT=19


The -x option exports the variable OCTINT; subshells can read it,
but can't change the value in the parent.

     $MyString='Hi there'


At this point, the variable MyString is local to the current
process.  It is not available to a child process.

     $export MyString


Now the variable MyString is available to subshells.

To allow a subshell to change the content of an exported variable
and have that change be know to the parent, execute the child with
". program_name"

Sample Session:


 $cat my.vars                                                  
 echo Variable coming into script: $PARENT                     
 PARENT='child value'                                          
 echo Variable coming out of the script: $PARENT               
 $$PARENT='parent value'                                        
 $echo $PARENT                                                 
 parent value                                                  
 $my.vars                                                      
 Variable coming into script:                                 
 Variable coming out of the script: child value                
 $echo $PARENT                                                 
 parent value                                                  
 $                                                            



Sample session:
          
 $PARENT='parent value'                                        
 $echo $PARENT                                                
 parent value                                                  
 $. my.vars                                                    
 Variable coming into the script: parent value                 
 Variable coming out of the script: child value                
 $echo $PARENT                                                 
 child value                                                   
 $                                                             


9.4  ksh: Aliases

The first word of each command can be replaced by the text of an
alias if an alias for this word has been defined.  The first
character of an alias name can be any printable character, but the
remaining characters must be the same as any valid identifier.
Aliases can be created, listed, and exported with the alias
command.  They can be removed with the unalias command.

Aliasing is performed when scripts are read but not while they are
executed.  Therefore, for an alias to take effect the alias command
has to be executed before the command which references the alias
is read.

The format for the alias command is as shown below.

  Command Format:  alias [-tx] [name=value]                    
                                                               
  -t = to set and list tracked aliases                         
  -x = to set or display exported aliases                      
                                                               
  name = if specified, it must be a valid alias name           
                                                               
  value = value to equate with name                            


Example:
 $alias                                                        

The command alone will display all aliases.

Sample Session:


 $alias                                                        
 cd=c                                                          
 echo=print -                                                  
 false=let 0                                                   
 functions=typeset -f                                          
 hash=alias -t                                                 
 history=fc -l                                                 
 integer=typeset -i                                            
 monitor=/usr/lbin/top                                         
 nohup=nohup                                                   
 pwd=print - $PWD                                              
 r=fc -e -                                                     
 true=:                                                        
 type=whence -v                                                
 $                                                             

Suppose, instead of typing in the ls -la command to get a full,
long listing of the contents of the current directory, we want to
shorten the command to list.  Enter the following command to set
the alias.

Example:
 $alias list='ls -la'                                          

Now when you type in the command list, the alias will substitute
the command ls -la for list; and the long listing will be
displayed.


Sample Session:

 $list                                                         
 total 54   0                                                  
 drwx------  4 teacher  class   2590  May 1 09:39  .           
 drwxr-xr-x 63 teacher  class   1536  Sep 9 13:11  ..          
 -rw-r--r--  1 teacher  class     64  Jul 4 10:33  .assistrc   
 .                                                               
 .
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