1. Lecture notes, week 4, point 3 (grep):

On some systems you will need to use double brackets when refering to the character classes.
E.g. : grep [[:digit:]]

2. Lecture notes, week 3, point 3 (standard inputs, outputs, pipelines):

The command line appearing between back quotation mark: this is does not refer to the return value of the command line (as stated before), rather to the output of the given command line (as corrected on the site).

E.g. try:

echo The current date is `date`
echo `ls`
echo `expr 3 + 4`
3.    At the same point: [[:xdigit:]] refers to hexadecimal digits, i.e. 0-9, A-F and a-f, of course (and not A-Z and a-z).

4.  A summary of the syntax of the condition- and loop-commands in shell scripts has been added to the notes of week 7.