Shell Scripting

Prof. Brian D. Davison

Computer Science & Engineering, Lehigh University

Announcements

• New project online (scripting!), due Thursday, March 24
• Today's schedule
  • Short quiz #7
  • Return Project #4, Range 83-100, average=98.7
  • Continue Shell Scripting

Ridirecting Input

• Sometimes you want to be able to pass multiline data into a program.
• If that data is in a file, you redirect stdin
• If that data is the output of another program, you use a pipe to connect them
• But what if you want to dynamically generate that data?
  • You could use something like "echo line-n >> file"
  • and then run as "program < file"
  • A better answer is the "here document"

The here document

• The here document provides the ability to send inline text to a program until a user-defined terminator is reached.
• To use the here document, after the command receiving the input we place a << symbol and the name of the terminating string. The text to be used follows, terminated by the same user-defined string on a line by itself.

  $ cat << FINISH
  > Hello $LOGNAME
  > Time is $(date +%T).
  > FINISH
  Hello brian
  Time is 13:21:33.
  

Positional Parameters

• The scriptname and arguments can be referenced using positional parameters.
• $0 is the script, and $1 - $9 are the first 9 arguments.
• If you need access to more, but the number in braces, e.g., ${10}
  
• Positional parameters can be reset using the set command

  #!/bin/bash
  echo Script was called as $0 $*
  oldargs=$*
  set new1 new2 new3
  echo New params are $0 $*
  set $oldargs
  echo Restored $0 $*
  

More on Functions

• The example of a function we saw previously was not much more than a macro.
• In fact, bash functions can take parameters, use local variables, and return (integer) values.
  
• Arguments are interpreted inside the function as positional parameters.

  #!/bin/bash
  
  function Usage { echo "error: $*" 2>&1; exit 1; }
  if (( $# != 2 ))
  then
     Usage "$0: requires two arguments"
  fi
  

Function variables and return values

• The local keyword declares a local variable
• return allows you to return a single value between 0 and 255 (just like an exit code)
• After executing a function or a program, the exit status is stored in $?

  #!/bin/bash
  
  increment() {
     local sum;
     let "sum=$1 + 1"
     return $sum
  }
  
  increment 5
  echo $?
  

Shell Scripts All Around

• It turns out that you run shell scripts all the time without knowing it.
• When you login, the system runs multiple scripts, including the one in /etc/profile
• The startx program (which starts X-windows if not already running) is a script.
• Many others, such as netscape, ps2epsi, and a number in /usr/bin/ are scripts, too.
• Let's look at a few of them.

bash versus tcsh

• I "grew up" with tcsh
• Both tcsh and bash are modern, powerful shells.
• Both support
  • Aliases
  • Command-line editing
  • Directory stacks
  • Filename completion
  • History
  • Job control
  • Key binding
  • Prompt formatting
• In addition, bash supports functions!
• For some details of the differences in syntax, see Quigley Appendix B and this quick online chart