String Operators

Curly-Bracket Syntax

${varname} returns the value of variable varname, null if it is not set.

Other string handling operators allow you to

• Ensure variables exist (i.e., are not null)
• Set default values for variables
• Catch errors resulting from variables not being set
• Remove parts of variable values matching patterns

Substitution Operators

Test for the existence of variables, and allow substitutions

• ${varname:-word} return value of varname if it exists and isn't null; otherwise return word
• ${varname:=word} return value of varname if it exists and isn't null; otherwise set it to word and then return its value
• ${varname:?message} return value of varname if it exists and isn't null; otherwise print varname: message then abort the current command or script
• ${varname:+word} return word if varname exists and isn't null; otherwise return null

Pattern-matching Operators

Delete patterns from a variable's contents

• ${varname#pattern} if pattern matches the beginning of the value of varname then delete the shortest part that matches and return the rest
• ${varname##pattern} if pattern matches the beginning of the value of varname then delete the longest part that matches and return the rest
• ${varname%pattern} if pattern matches the end of the value of varname then delete the shortest part that matches and return the rest
• ${varname%%pattern} if pattern matches the end of the value of varname then delete the longest part that matches and return the rest

Regular Expressions

Regular expressions extend the string-matching power of the Korn shell beyond *, ?, and [].

• *(exp) matches 0 or more occurrences of exp
• +(exp) matches 1 or more occurrences of exp
• ?(exp) matches 0 or 1 occurrences of exp
• @(exp1|exp2|…) matches exp1 or exp2 or …
• !(exp) matches anything but exp

Command Substitution

Command substitution allows you to use the standard output of a command as if it were the value of a variable:

$(command)

Examples:

• $(ls)
• $(< filename) or $(cat filename)
• mail $(who | cut -d' ' -f1)

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More Control-Flow Commands

case

Similar to the switch in C and case in Pascal

• Compares a string to a sequence of patterns
• Each pattern may actually be several patterns separated by |
• ;; breaks out of each case
• The * pattern matches all patterns, used as default

Syntax:

case expression in
  pattern1 )
    statements ;;
  pattern2 )
    statements ;;
esac

case example

for fname in $* ; do
  case $fname in
    *.c )
      objname=${fname%.c}.o
      gcc -c $fname -o $objname ;;
    *.s )
      objname=${fname%.s}.o
      as $fname $objname ;;
    *.o ) ;;
    * )
      print "error: $fname is wrong type"
      return 1 ;;
  esac
done

select

Syntax:

select name [ in list ]
do
  statements that can use $name
done

This flow-control construct

• Generates a menu of each item in list (which defaults to "$@" if omitted)
• Prompts the user for a number using the prompt string in shell variable PS3
• Stores the selected choice in the variable name and the selected number in shell variable REPLY
• Executes the statements in the body
• Repeats forever (until break encountered)

select example
(from page 147 in "Learning the Korn Shell")

The code

print 'Select your terminal type: '
PS3='terminal? '
select term in gl35a t2000 s531 vt99 ; do
  if [[ -n $term ]] ; then
    TERM=$term
    print TERM is $TERM
    break
  else
    print 'invalid'
  fi
done

will produce the following menu:

Select your terminal type:
1) gl35a
2) t2000
3) s531
4) vt99
terminal?

---

Here Document

Syntax:

command << label
input line 1
…
input line n
label

This allows you to redirect input to a shell script from within the shell script itself.

• << label indicates that label marks the end of the here document
• label must appear on a line by itself to end the here document
• << 'label' prevents the shell from doing parameter and command substitution in the here document
• <<- label deletes leading tabs (but not spaces) from the here document

Here Document Example
(from page 189 in "Learning the Korn Shell")

pgmname=$1
for user in $(ypcat passwd | cut -f1 -d:)
do
  mail $user <<- EOF
  Dear user,
  A new version of $pgmname has been installed
  in $(whence $pgmname).
  Regards,
  Your friendly sysadmin
EOF
done