Bashing some more Commands

• In strings.sh
• If variable names contain things other than letters, it is best to include them in brackets.
• echo $10
  • Will really echo -n $1; echo 0
• some string operators
  • ${varname:-word}
    • if varname is defined, return it, otherwise return word.
    • echo ${undefined:-not defined}
    • echo ${undefined:-$0}
    • This is used to return a default value for a variable.
  • ${varname:=word}
    • If varname is defined return it, otherwise set it to word and return that
  • ${varname:?message}
    • If varname exists return it, otherwise print an error message and abort.
  • ${varname:offset:length}
    • Substring of varname from offset for length characters.
    • If length is omitted, go to the end of the string
    • Special case for $@
      • return length parameters starting at offset
  • There are others, but these will do.
• He gives a nice example in the book.
  • Write a script that takes a file name and an optional number
  • The file contains a number of lines in the format number word
  • Display the top n lines of the file.
  • Look at howmany.sh
• Patterns and pattern matching.
  • Primitive, much better tools are available.
  • ${varname#pattern}
    • Find the shortest pattern at the beginning of the value in varname and return the rest of the string.
  • ${varname##pattern}
    • Find the longest pattern at the beginning of the value in varname and return the rest of the string.
  • ${varname%pattern}
    • Find the shortest pattern at the end of the value in varname and return the rest of the string.
  • ${varname%%pattern}
    • Find the longest pattern at the end of the value in varname and return the rest of the string.
  • He gives the example outifle=${filename%.pcx}.jpg
• The length of a string is ${#varname}
• Saving the output of a command: command substitution
• val=`ls`