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09. Korn Shell

Korn Shell

Introduction

  • The Korn shell was developed by David Korn from AT&T Bell Lab
  • Like bash, the Korn shell is based on the older Bourne shell and therefore the Korn shell is similar to bash
  • In this note, we discuss the many similarities and few differences between the Korn and bash shells

Running the Korn Shell
  • The Korn shell can be the default login shell, just like bash is our default login shell on voyager
  • The default Korn shell prompt (stored in PS1) is:  $
  • From any shell, to start the Korn shell:  ksh
  • To check where the Korn shell is in the system:  whereis  ksh
    • Normally the Korn shell is at:  /bin/ksh
  • If the Korn shell is started as a child process of another shell, to get back to the original shell, use:  exit
  • To find the current shell that is running:  echo $0
  • If the command line editing is not set for ksh as it is for bash, here are some general tips to re-run previous commands with ksh:
    • history will show the last commands that are run, along with their number in the history file
    • r       re-run the last command
    • r number re-run the command with the matching number in the history file


Input / Output
  • As in bash, use read for reading in user input
  • To print to screen, we can use echo as with bash, but most Korn shell scripts use print
    • For example:   print  current value is $var
  • Just like with any other shell, the Korn shell script also accepts command line arguments and options
  • Command line arguments are accessed through the positional parameters  and getopts is also available with ksh
  • We can also use redirection of both input and output of the shell
  • We can also use command substitutions and pipes for input and output


Variables and Expressions
  • User defined variables are created, initialized, unset, and accessed the same way as in bash
  • Likewise, system variables such as PATH, PS1, HOME, etc. are the same as in bash
  • Shell variables (such as $0, $1, $2, $*, $@, $?) are the same
  • Arrays are defined and accessed the same way as in bash
  • Strings and arrays can be manipulated with the same operators, such as ${#array[@]} or ${str/expr/newStr}, and by the same built-in functions such as shift and set

  • Regular expressions can be used for string matching by the shell with the operator =~, just as with bash
  • However, the regex metacharacters are limited to: 
    • Anchors:  ^     $  
    • Single character:    .
  • Most of the time, regex in ksh are used in conjunction with egrep, sed, or awk
  • There is a special set of regex that are used with the string operators
  • They are similar to regex used by bash and other utilities, but are in different format
Korn :         bash:
*(expr)         expr*
+(expr)         expr+
?(expr)         expr?
@(expr1 | expr2) expr1 | expr2
{n,m}(expr) expr{n,m}
  • For example, using regex with string substitution:
str="cis33a   cis33b"
print ${str/+(3)/4}            print:  cis4a cis33b
Or
 classes=(cis33a   cis33b   cis18a  cis18b  cis18c)
 print ${classes[@]/*([a-z])/}    print: 33a  33b  18a  18b  18c



Evaluating Expressions
  • Logical operators: !, &&, ||  are the same
  • String relational operators: =, !=, <, >  are the same
  • Numeric relational operators: -ne, -eq, -lt, -gt, -ge, -le are the same
    • As in bash, to evaluate logical and relational expression, use [[   ]]
  • Arithmetic operators are the same: +, /, *=, ++ …
    • Evaluate arithmetic expression the same way:    (( expression ))
    • where the variable names do not need a preceding $
  • As with bash, arithmetic compares can be written as     (( expression )) where the compare operators are < , >=, == …
  • File tests are the same way, except for the file exist test
    • -a or  -e   file exists -s    exists and is not empty
    • -d file is a directory -nt   file1 is newer than file2
    • -f file is a regular file -ot   file1 is older than file2
    • -h file is a link
    • -r have read permission
    • -w have write permission
    • -x have execute permission


Flow Control
  • The if and case statements have the same syntax as in bash
  • The for, while, until loops have the same syntax as in bash
  • Functions can be defined the same way as in bash
  • Functions can also be defined this way:
    function  fA # start with keyword function, no  ( )
    {
    }
  • The return or exit statement works as in bash
  • As in bash, to check for return or exit status, use:  $?
  • As in bash, variables are global by default
  • To make a variable local, use:  typeset  varName


Signals
  • Signals and signal handling are the same way as in bash
  • The trap command is used to catch signals
  • The difference  from bash is when signal handling are reset to default:   
    • trap   -   signalName


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