#!/bin/bash
number=1
if [ $number = "1" ]; then
echo "Number equals 1"
else
echo "Number does not equal 1"
fi
Missing quotes
Edit line 6 to remove the trailing quote from the end of the line: echo "Number equals 1
[me@linuxbox me]$ ./trouble.bash
./trouble.bash: line 8: unexpected EOF while looking for matching "
./trouble.bash: line 10 systax error: unexpected end of file
Here we have another case of a mistake in one
line causing a problem later in the script. What
happens is the shell keeps looking for the closing
quotation mark to tell it where the end of the
string is, but runs into the end of the file before
it finds it../trouble.bash: line 8: unexpected EOF while looking for matching "
./trouble.bash: line 10 systax error: unexpected end of file
These errors can be a real pain to find in a long script. This is one reason you should test your scripts frequently when you are writing them so there is less new code to test. I also find that text editors with syntax highlighting (like nedit or kate) make these kinds of bugs easier to find.
Isolating problems
Finding bugs in your programs can sometimes be very difficult and frustrating. Here are a couple of techniques that you will find useful:Isolate blocks of code by "commenting them out." This trick involves putting comment characters at the beginning of lines of code to stop the shell from reading them. Frequently, you will do this to a block of code to see if a particular problem goes away. By doing this, you can isolate which part of a program is causing (or not causing) a problem.
For example, when we were looking for our missing quotation we could have done this:
#!/bin/bash
number=1
if [ $number = "1" ]; then
echo "Number equals 1
#else
# echo "Number does not equal 1"
fi
Use echo commands to verify your assumptions. As you gain experience tracking down bugs, you will discover that bugs are often not where you first expect to find them. A common problem will be that you will make a false assumption about the performance of your program. You will see a problem develop at a certain point in your program and assume that the problem is there. This is often incorrect, as we have seen. To combat this, you should place echo commands in your code while you are debugging, to produce messages that confirm the program is doing what is expected. There are two kinds of messages that you should insert.
The first type simply announces that you have reached a certain point in the program. We saw this in our earlier discussion on stubbing. It is useful to know that program flow is happening the way we expect.
The second type displays the value of a variable (or variables) used in a calculation or test. You will often find that a portion of your program will fail because something that you assumed was correct earlier in your program is, in fact, incorrect and is causing your program to fail later on.
Empty variables
Edit the script to change line 3 from:number=1
number=
[me@linuxbox me]$ ./trouble.bash
/trouble.bash: [: =: unary operator expected.
Number does not equal 1
As you can see, bash
displayed an error message when we ran the script.
You probably think that by removing the "1" on line
3 it created a syntax error on line 3, but it
didn't. Let's look at the error message again:/trouble.bash: [: =: unary operator expected.
Number does not equal 1
./trouble.bash: [: =: unary
operator expected
We can see that ./trouble.bash is reporting the error
and the error has to do with "[". Remember that "[" is an abbreviation for the test shell builtin. From this we
can determine that the error is occurring on line 5
not line 3.First, let me say there is nothing wrong with line 3. number= is perfectly good syntax. You will sometimes want to set a variable's value to nothing. You can confirm the validity of this by trying it on the command line:
[me@linuxbox me]$ number=
[me@linuxbox me]$
See, no error message. So what's wrong with line
5? It worked before.[me@linuxbox me]$
To understand this error, we have to see what the shell sees. Remember that the shell spends a lot of its life substituting text. In line 5, the shell substitutes the value of number where it sees $number. In our first try (when number=1), the shell substituted 1 for $number like so:
if [ 1 = "1" ]; then
if [ = "1" ]; then
To fix this problem, change line 5 to read:
if [ "$number" = "1" ]; then
if [ "" = "1" ]; then
This brings up an important thing to remember when you are writing your scripts. Consider what happens if a variable is set to equal nothing.
Watching your script run
It is possible to have bash show you what it is doing when you run your script. To do this, add a "-x" to the first line of your script, like this:#!/bin/bash -x
[me@linuxbox me]$ ./trouble.bash
+ number=1
+ '[' 1 = 1 ']'
+ echo 'Number equals 1'
Number equals 1
Alternately, you can use the set command within your script to turn
tracing on and off. Use set
-x to turn tracing on and set
+x to turn tracing off. For example.:+ number=1
+ '[' 1 = 1 ']'
+ echo 'Number equals 1'
Number equals 1
#!/bin/bash
number=1
set -x
if [ $number = "1" ]; then
echo "Number equals 1"
else
echo "Number does not equal 1"
fi
set +x
by William Shotts, Jr.
