There are 3 possible ways of adding breakpoints manually: break n: set breakpoint in line number n in the current source file. Break file:n if expression: set breakpoint in line number n inside file named file. If an expression is given it must evaluated to true to fire up the debugger. Method if expression: set breakpoint in method (. And for class and instance method respectively) defined in class. The expression works the same way as with file:n. For example, in the previous situation Use info breakpoints to list breakpoints. If you supply a number, it lists that breakpoint.
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But let's continue and move on with the report application execution. Use step (abbreviated s) to continue running your program until the next logical stopping point and return control to the debugger. Next is similar to step, but while step stops at the next line of code executed, doing just a single step, next moves to the next line without descending inside methods. For example, consider the following situation: If we use next, we won't go deep inside method calls. Instead, byebug will go to the next line within the same context. In this case, it is the last line of the current method, so byebug will return to the next line of the caller method. If we use step in the same situation, byebug will literally go to the next Ruby instruction to be executed - in this case, active support's week method. This is one of the best ways to find bugs in your code. 3.7 Breakpoints A breakpoint makes your application stop whenever a certain point in the program is reached. The debugger shell is invoked in that line. You can add breakpoints dynamically with the command break (or just b).
The var method is the most convenient way to show variables and their values. Let's have byebug help us with. This is a great way to inspect the values essays of the current context variables. For example, to check that we have no local variables currently defined: you can also inspect for an object method this way: you can also use display to start watching variables. This is a good way of tracking the values of a variable while the execution goes. The variables inside the displayed list will be printed with their values after you move in the stack. To stop displaying a variable use undisplay n where n is the variable number (1 in the last example). 3.6 Step by Step Now you should know where you are in the running trace and be able to print the available variables.
This command is very helpful when you are debugging concurrent threads and need to verify that there are no race conditions in your code. 3.5 Inspecting Variables Any expression can be evaluated in the current context. To evaluate an expression, just type it! This example shows how you can print the instance variables defined roles within the current context: As you may have figured out, all of the variables that you can access from a controller are displayed. This list is dynamically updated as you execute code. For example, run the next line using next (you'll learn more about this command later in this guide). And then ask again for the instance_variables: Now @articles is included in the instance variables, because the line defining it was executed.
N defaults to one. Up in this case is towards higher-numbered stack frames, and down is towards lower-numbered stack frames. 3.4 Threads The debugger can list, stop, resume and switch between running threads by using the thread command (or the abbreviated th). This command has a handful of options: thread: shows the current thread. Thread list: is used to list all threads and their statuses. The current thread is marked with a plus sign. Thread stop n: stops thread. Thread resume n: resumes thread. Thread switch n: switches the current thread context.
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The debugger creates a context when a stopping point or an event is reached. The context has information about the suspended program which enables the debugger to inspect the frame stack, evaluate variables from the perspective of the debugged program, and know the place where the debugged program is stopped. At any time you can call the backtrace command (or its alias where) to print the backtrace of the application. This can be very helpful to know how you got where you are. If you ever wondered about how you got somewhere in your code, then backtrace will supply the answer.
The current frame is marked with. You can move anywhere you want in this trace (thus changing the context) by using the frame n essay command, where n is the specified frame number. If you do that, byebug will display your new context. The available variables are the same as if you were running the code line by line. After all, that's what debugging. You can also use up n and down n commands in order to change the context n frames up or down the stack respectively.
3 Debugging with the byebug gem When your code is behaving in unexpected ways, you can try printing to logs or the console to diagnose the problem. Unfortunately, there are times when this sort of error tracking is not effective in finding the root cause of a problem. When you actually need to journey into your running source code, the debugger is your best companion. The debugger can also help you if you want to learn about the rails source code but don't know where to start. Just debug any request to your application and use this guide to learn how to move from the code you have written into the underlying rails code. 3.1 Setup you can use the byebug gem to set breakpoints and step through live code in rails.
To install it, just run: Inside any rails application you can then invoke the debugger by calling the byebug method. Here's an example:.2 The Shell As soon as your application calls the byebug method, the debugger will be started in a debugger shell inside the terminal window where you launched your application server, and you will be placed at the debugger's prompt (byebug). Before the prompt, the code around the line that is about to be run will be displayed and the current line will be marked by ' like this: If you got there by a browser request, the browser tab containing the request will be hung. For example: Now it's time to explore your application. A good place to start is by asking the debugger for help. Type: help to see the previous ten lines you should type list- (or l-). This way you can move inside the file and see the code above the line where you added the byebug call. Finally, to see where you are in the code again you can type list.3 The context When you start debugging your application, you will be placed in different contexts as you go through the different parts of the stack.
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Logging will always have a small impact on the performance of your rails app, particularly when logging to disk. Additionally, there are a few subtleties: Using the :debug level will have a greater performance penalty than :fatal, as a far greater number of strings are being evaluated and written to the log output (e.g. Another potential pitfall is too many calls to logger in father's your code: In the above example, there will be a performance impact even if the allowed output level doesn't include debug. The reason is that Ruby has to evaluate these strings, which includes instantiating the somewhat heavy string object and interpolating the variables. Therefore, it's recommended to pass blocks to the logger methods, as these are only evaluated if the output level is the same as — or included in — the allowed level (i.e. The same code rewritten would be: The contents of the block, and therefore the string interpolation, are only evaluated if debug is enabled. This performance savings are only really noticeable with large amounts of logging, but it's a good practice to employ.
or higher than the. If you want to know the current log level, you can call the rails. The available log levels are: :debug, :info, :warn, :error, :fatal, and :unknown, corresponding to the log level numbers from 0 up to 5, respectively. To change the default log level, use. This is useful when you want to log under development or staging without flooding your production log with unnecessary information. 2.3 Sending Messages, to write in the current log use the method from within a controller, model or mailer: Here's an example of a method instrumented with extra logging: Here's an example of the log generated when this controller action is executed: Adding extra logging. If you add extra logging, be sure to make sensible use of log levels to avoid filling your production logs with useless trivia. 2.4 Tagged Logging, when running multi-user, multi-account applications, it's often useful to be able to filter the logs using some custom rules. TaggedLogging in Active support helps you do exactly that by stamping log lines with subdomains, request ids, and anything else to aid debugging such applications. 2.5 Impact of Logs on Performance.
The above code will render something like this:.3 inspect, another useful method for displaying object values is inspect, especially when working with arrays or hashes. This will print the object value as a string. For example: Will render: 2 The logger, it can also be useful to save information to log files at runtime. Rails maintains a separate log file for each runtime environment. 2.1 What is the logger? Rails makes use of the Activesupport:Logger class to write log information. Other loggers, such as Log4r, may also be substituted. You can specify an plan alternative logger in config/application.
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1 view Helpers for Debugging, one common task is to remote inspect the contents of a variable. Rails provides three different ways to do this: debug to_yaml inspect.1 debug, the debug helper will return a pre tag that renders the object using the yaml format. This will generate human-readable data from any object. For example, if you have this code in a view: you'll see something like this:.2 to_yaml. Alternatively, calling to_yaml on any object converts it to yaml. You can pass this converted object into the simple_format helper method to format the output. This is how debug does its magic.