bash: string operators

• ${varname:-word}
• echo ${count:-0} : prints '0' if count is null.
  
• ${varname:=word}
• echo ${count:=0} : prints '0' and assign 0 to count if count is null.
  
• ${varname:?message}
• echo ${count:?NULL} : prints 'bash: count: NULL' if count is null.
  
• ${varname:+word}
• echo ${count:+0} : prints '0' if count it NOT null.
  
• ${varname:offset:length}
• var1='sample'
  
• ehco ${var1:1:2} : prints 'am'.
  

• ${varname#pattern}
• echo ${PWD} is '/home/wonkim'.
• echo ${PWD#*/} is 'home/wonkim'.
  
• ${varname##pattern}
• echo ${PWD##*/} is 'wonkim'.
  
• ${varname%pattern}
• echo {PWD%/*} is '/home'.
  
• ${varname%%pattern}
• ehco {PWD%%/*} is null.
  
• ${varname/pattern/string}
• echo {PWD/\//:} is ':home/wk157575'
  
• ${varname//pattern/string}
• ehco {PWD//\//:} is ':home:wk157575'

Refer 'page 91' and 'pag 96' of Learning the bash