09. Korn Shell

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