Linux Pocket Guide, 2nd Edition

Conditionals

The if statement chooses between alternatives, each of which may have a complex test. The simplest form is the if-then statement:

if command           If exit status of command is 0
then
  body
fi

For example:

if [ `whoami` = "root" ]
then
  echo "You are the superuser"
fi

Next is the if-then-else statement:

if command
then
  body1
else
  body2
fi

For example:

if [ `whoami` = "root" ]
then
  echo "You are the superuser"
else
  echo "You are an ordinary dude"
fi

Finally, we have the form if-then-elif-else, which may have as many tests as you like:

if command1
then
  body1
elif command2
then
  body2
elif ...
  ...
else
  bodyN
fi

For example:

if [ `whoami` = "root" ]
then
  echo "You are the superuser"
elif [ "$USER" = "root" ]
then
  echo "You might be the superuser"
elif [ "$bribe" -gt 10000 ]
then
  echo "You can pay to be the superuser"
else
  echo "You are still an ordinary dude"
fi

The case statement evaluates a single value and branches to an appropriate piece of code:

echo "What would you like to do?"
read answer
case "$answer" in
  eat)
    echo "OK, have a hamburger"
    ;;
  sleep)
    echo "Good night then"
    ;;
  *)
    echo "I'm not sure what you want to do"
    echo "I guess I'll see you tomorrow"
    ;;
esac

The general form is:

case string in
  expr1)
    body1
    ;;
  expr2)
    body2
    ;;
  ...
  exprN)
    bodyN
    ;;
  *)
    bodyelse
    ;;
esac

where string is any value, usually a variable value like $myvar, and expr1 through exprN are patterns (run the command info bash reserved case for details), with the final * like a final “else.” Each set of commands must be terminated ...