Shell Variables

• The ideas are somewhat universal, but this is bash.
• The shell maintains a set of variables
• You can see the variables set with printenv
• You can look at the value of a variable with echo $VARNAME
• You can set variables for the current shell
  • BOB="hello"
  • echo $BOB
  • bash
  • echo $BOB
  • exit
• or for all invocations
  • export FRED="hello"
  • echo $FRED
  • bash
  • echo $FRED
  • exit
• Environment variables are used all over the place
  • To control shells
    • $PATH tells the shell where to find executable.
    • adding . to $PATH tells it to look in the local directory
    • PATH=$PATH:.
    • export PATH
    • Careful when executing programs in others directories.
  • To control programs
    • All programs have a third argument to main
    • char * envp[]
    • This is a null terminated array of strings.
    • Write a quick program.
  • Shell related tasks
    • $? is the exit status of the last program run
    • $# the number of arguments to the shell
    • $n - the n^th argument to the shell
• A list of standard variables is here
• On startup bash will execute the code in a number of files.
  • Normally a system wide file.
  • And one or two files in the user's directory.
  • Be careful modifying these, you can make it so you can't log in.