LJ TECH TIP: Debugging Shell Scripts
This tip comes courtesy of Linux Journal columnist Dave Taylor and No
Starch Press. This is (truly) the last in a series Tech Tips on shell
scripting from Dave where he explains the "how it works" factor behind
debugging shell scripts.
Although this section does not contain a true script per se, it's a
good place to spend a few pages talking about some of the basics of
debugging and developing shell scripts, because it's a sure bet that
bugs are going to creep in!
The best debugging strategy I have found is to build scripts
incrementally. Some script programmers have a high degree of optimism
that everything will work right the first time, but I find that
starting small, on a modest scale, can really help move things along.
Additionally, liberal use of echo statements to track variables, and
using the -x flag to the shell for displaying debugging output, are
quite useful. To see these in action, let's debug a simple
number-guessing game.
The Code
#!/bin/sh
# hilow -- A simple number-guessing game
biggest=100 # maximum number possible
guess=0 # guessed by player
guesses=0 # number of guesses made
number=$(( $$ % $biggest ) # random number, b/w 1 & $biggest
while [ $guess -ne $number ] ; do
echo -n "Guess? " ; read answer
if [ "$guess" -lt $number ] ; then
echo "... bigger!"
elif [ "$guess" -gt $number ] ; then
echo "... smaller!
fi
guesses=$(( $guesses + 1 ))
done
echo "Right!! Guessed $number in $guesses guesses."
exit 0
Running the Script
The first step in debugging this game is to test and ensure that the
number generated will be sufficiently random. To do this, we take the
process ID of the shell in which the script is run, using the $$
notation, and reduce it to a usable range using the % mod function.
To test the function, enter the commands into the shell directly:
$ echo $(( $$ % 100 ))
5
$ echo $(( $$ % 100 ))
5
$ echo $(( $$ % 100 ))
5
It worked, but it's not very random. A moment's thought reveals why
that is: When the command is run directly on the command line, the PID
is always the same. When run in a script, the command is in a
different subshell each time, so the PID varies.
The next step is to add the basic logic of the game. A random number
between 1 and 100 is generated, the player makes guesses at the
number, and after each guess the player is told whether the guess is
too high or too low until he or she figures out what number it is.
After entering all the basic code, it's time to run the script and see
how it goes, using exactly the code just shown, warts and all:
$ hilow
./013-hilow.sh: line 19: unexpected EOF while looking for matching ' " '
./013-hilow.sh: line 22: syntax error: unexpected end of file
Ugh; the bane of shell script developers: an unexpected EOF. To
understand what this message means, recall that quoted passages can
contain newlines, so just because the error is flagged on line 19
doesn't mean that it's actually there. It simply means that the shell
read merrily along, matching quotes (incorrectly) until it hit the
very last quote, at which point it realized something was amiss. In
fact, line 19 is perfectly fine:
$ sed -n 19p hilow
echo "Right!! Guessed $number in $guesses guesses."
The problem, therefore, must be earlier in the script. The only
really good thing about the error message from the shell is that it
tells you which character is mismatched, so I'll use grep to try to
extract all lines that have a quote and then screen out those that
have two quotes:
$ grep '"' 013-hilow.sh | egrep -v '.*".*".*'
echo "... smaller!
That's it: The close quote is missing. It's easily fixed, and we're
ready to go:
$ hilow
./013-hilow.sh: line 7: unexpected EOF while looking for matching ' ) '
./013-hilow.sh: line 22: syntax error: unexpected end of file
Nope. Another problem. Because there are so few parenthesized
expressions in the script, I can eyeball this problem and ascertain
that somehow the closing parenthesis of the instantiation of the
random number was mistakenly truncated, as the following line shows:
number=$(( $$ % $biggest ) #random number, between 1 and $biggest
This is fixed by adding the closing parenthesis. Now are we ready to
try this game? Let's find out:
$ hilow
Guess? 33
... bigger!
Guess? 66
... bigger!
Guess? 99
... bigger!
Guess? 100
... bigger!
Guess? ^C
Because 100 is the maximum possible value, there seems to be a logic
error in the code. These errors are particularly tricky because
there's no fancy grep or sed invocation to identify the problem.
Look back at the code and see if you can identify what's going wrong.
To try and debug this, I'm going to add a few echo statements in the
code to output the number chosen and verify that what I entered is
what's being tested. The relevant section of the code is
echo -n "Guess? " ; read answer
if [ "$guess" -lt $number ] ; then
In fact, as I modified the echo statement and looked at these two
lines, I realized the error: The variable being read is answer , but
the variable being tested is called guess. A bonehead error, but not
an uncommon one (particularly if you have oddly spelled variable
names). To fix this, I change read answer to read guess.
The Results:
Finally, it works as expected.
$ hilow
Guess? 50
... bigger!
Guess? 75
... bigger!
Guess? 88
... smaller!
Guess? 83
... smaller!
Guess? 80
... smaller!
Guess? 77
... bigger!
Guess? 79
Right!! Guessed 79 in 7 guesses.
Hacking the Script
The most grievous bug lurking in this little script is that there's no
checking of input. Enter anything at all other than an integer and
the script spews up bits and fails. Including a rudimentary test
could be as easy as adding the following lines of code:
if [ -z "$guess" ] ; then
echo "Please enter a number. Use ^C to quit"; continue;
fi
However, a call to the validint function shown in Script #5 is what's
really needed.
Excerpted with permission from the book Wicked Cool Shell Scripts: 101
Scripts for Linux, Mac OS X, and UNIX Systems by Dave Taylor.
Published by No Starch Press. http://www.nostarch.com/wcss.htm.
Dave Taylor is a long-time Unix and Linux geek and runs the popular
http://www.AskDaveTaylor.com/ tech support blog. His book Wicked Cool
Shell Scripts can be found at: http://www.intuitive.com/wicked/ and
the entire library of scripts at:
http://www.intuitive.com/wicked/wicked-cool-shell-script-library.shtml.
HTH
--
Bharathi S
