Shell Scripting - Part 3

Purpose of Shell Scripting - Part 3

Bash Shell Scripting Tips

• The Here Document
  
  A neat little trick involving a special type of redirection of stdin ( << ) that allows input to be redirected to a command from within the command. The name relates to where the stdin is contained: not in a file, but "here in the command itself". A character (like +) is used to mark the boundary of stdin. It is important that the ending boundary only contains a line with that matching character (eg +); otherwise the stdin will continue to be read! This command is a convenience way to display multiple lines on that screen, but this command can be used with any Linux command that accept stdin.
  
  Examples (try at the shell prompt)
  
  cat <<+
  This is a test message
  This is the second line
  +
  
  mail -s "test message" youremailaddr <<+
  This is a test message
  I hope you like it.
  +
  
  tr [a-z] [A-Z] <<+
  i like ops235
  i love scripting.
  +
  
  
• Using sed to Manipulate Text
  
  The Linux command sed stands for Streaming Editor which is an effective way to manipulate a text file, output sent from a command, or from within a "here document". This command can manipulate matching text on a variety of criteria (such as line number, regular expression match, etc). Commands can then be used for manipulation such as omitting, printing, substituting, adding, inserting, etc. The sed option -n suppresses display of text so the print (p) command can be used; otherwise, the text will be displayed (with edits via the sed command instructions). Results of text manipulation with sed can be stored in a variable using command substitution, or redirected to a file. NEVER redirect the stdout from a sed command to the same input file (or the input file will be destroyed)!
  
  
• Examples (try at the shell prompt)
  
  sed 's/|/ /g' <<+
  I|like|weekends!
  +
  
  sed 's/$/\n/g' <<+
  This text
  should be
  double-spaced!
  +
  
  

Online Scripting Resources

If you require additional practice in creating shell scripts and using the vi text editor, run the commands in your Matrix account: