CS246-F20-01-UnixShell
Lecture 1.17
• Writing bash scripts
– Two examples
CS246
An example
• Payday is on the last Friday of the month; print the
date of this month’s payday
– We’ll use the UNIX command cal as a helper
• There are 2 tasks:
1. Compute the date
2. Present the answer
#!/bin/bash
report() {
# Inside a fcn, $1, etc. denote the *fcn’s* params
if [ $1 -eq 31 ]; then
echo “This month: the 31st” # not perfect, just fun
else
echo “This month: the ${1}th”
fi
}
# Compute the date; pass as argument 1 to the “answer” routine
# cal with no args prints calendar of current month
# The call to awk picks the sixth token in each line
# egrep filters out non-numbers
# tail -1 gives us the last number
report $(cal | awk ‘{print $6}’ | egrep “[0-9]” | tail -1)
Another example
• Now, let’s generalize this script to work for any
month of any year
• We note that cal May 2016 gives the calendar
for May 2016 …
– … so we can let ./payday May 2016 give May 2016’s
payday
#!/bin/bash
report() {
if [ $2 ]; then
# $2 (2nd argument) is the month, if supplied
# -n suppresses the trailing newline
echo -n $2
else
echo -n “This month”
fi
if [ $1 -eq 31 ]; then
echo “: the 31st” # not perfect, just fun
else
echo “: the ${1}th”
fi
}
report $(cal $1 $2|awk ‘{print $6}’|egrep “[0-9]”|tail -1) $1
# Note that arg $2 in report is the same as arg$1 in the main
# script, as $1 was passed as the second argument to report.
End
CS246
1. Some notes on OSs,
UNIX, and the UNIX shell
CS246 Fall 2020
Prof.
University of Waterloo