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