if [ $ans == "yes" ]; then
echo "OK, let's get going"
else
exit
fi
There are, however, quite a few tests that you can run – whether strings are the same or not and even how they relate alphabetically. The script below compares two strings in many ways.
#!/bin/bash echo -n "Enter first string: " read str1 echo -n "Enter second string: " read str2 echo # run tests to see how strings relate if [ "$str1" = "$str2" ]; then echo strings are the same; fi if [ "$str1" == "$str2" ]; then echo strings are the same; fi if [ "$str1" != "$str2" ]; then echo "$str1" and "$str2" differ; fi if [ "$str1" < "$str2" ]; then echo "$str1" comes first alphabetically; fi if [ "$str1" > "$str2" ]; then echo "$str2" comes first alphabetically; fi if [ -z "$str1" ]; then echo string1 is null; fi if [ -z "$str2" ]; then echo string2 is null; fi if [ -n "$str1" ]; then echo string1 is not null; fi if [ -n "$str2" ]; then echo string2 is not null; fi
In some of the tests above, characters are inserted before quotes to ensure that the strings are displayed within quote marks.
Here’s a script that uses this test.
#!/bin/bash
echo -n "enter a year> "
read year
this_year=`date +%Y` # get current year using the date command
if [ "$year" -lt $this_year ]; then
diff=`expr $this_year - $year`
echo "Wow, that was $diff year{s} ago"
elif [ "$year" -gt $this_year ]; then
diff=`expr $year - $this_year`
echo "Only $diff year(s) to go"
else
echo "$year is current year"
fi
Notice that the script above uses if, else if and else logic instead of several independent if tests.
You can also try to collect mis-entered data with commands like these that will loop until a proper year is entered.
#!/bin/bash
echo -n "Enter year> "
read year
re='^[0-9]+$'
while ! [[ $year =~ $re ]]
do
echo "error: Year must be numeric"
echo -n "year> "
read year
done
The examples above are fairly advanced. They set up a numeric expression that will match any string of digits. If it doesn’t match (the “!” in the script means “not”), an error will be displayed and the script will continue looping until it does.
