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Byebug Cheatsheet

At a recent Three Rings developer event I found myself hunting for a good cheat sheet for the Ruby 2 debugger Byebug. I couldn’t find anything much, so I made my own :).

Acknowledgment

The design of this cheat sheet borrows heavily from my favourite GDB quick reference guide.

Contents

Starting Byebug
Stopping Again
Essential Commands
Breakpoints and Catchpoints
Program Stack
Execution Control
Display
Threads
Controlling Byebug
Source Files and Code

Starting Byebug

If you’re running Byebug on a Rails application in development mode, you no longer need to start the server with --debugger – the debugger is on by default.

To get going, simply type byebug (or debugger) into your source file at the line you’re interested in and run the program. If you’re running it on a Rails application, remember to switch to your terminal window to look at debugger output.

Note: byebug invocations are just method calls, so you can make them conditional:

byebug if foo == “bar”

Another Note: As is common with debuggers, hitting ‘Enter’ on an empty line in Byebug repeats the last command.

Stopping Again

q[uit] — a.k.a. “exit” unconditionally
Quit. It stops the thing running. Also exits your program. Note: To quit without an ‘are you sure?’ prompt, use quit unconditionally (shortened to q!)
kill
Really quit. This uses kill -9, for situations where quit just isn’t fierce enough.

Essential Commands

c[ontinue] <line-number>
Carry on running until program ends, hits a breakpoint or reaches line line-number (if specified).
n[ext] <number>
Go to next line, stepping over function calls. If number specified, go forward that number of lines.
s[tep] <number>
Go to next line, stepping into function calls. If numberis specified, make that many steps.
b[ack]t[race] — a.k.a. “w[here]”
Display stack trace.
h[elp] <command-name>
Get help. With no arguments, returns a list of all the commands Byebug accepts. When passed the name of a command, gives help on using that command.

Breakpoints and Catchpoints

b[reak]
Sets a breakpoint at the current line. These can be conditional: break if foo != bar. Keep reading for more ways to set breakpoints!
b[reak] <filename>:<line-number>
Puts a breakpoint at line-number in filename (or the current file if filename is blank). Again, can be conditional: b myfile.rb:15 unless my_var.nil?
b[reak] <class>(.|#)<method>
Puts a breakpoint at the start of the method method in class class. Accepts an optional condition: b MyClass#my_method if my_boolean
info breakpoints
List all breakpoints, with status.
cond[ition] <number> <expression>
Add condition expression to breakpoint <number<>. If no expression is given, removes any conditions from that breakpoint.
del[ete] <number>
Deletes breakpoint <number>. With no arguments, deletes all breakpoints.
disable breakpoints <number>
Disable (but don’t delete) breakpoint <number>. With no arguments, disables all breakpoints.
cat[ch] <exception> off
Enable or (with off argument) disable catchpoint on <exception>.
cat[ch]
Lists all catchpoints.
cat[ch] off
Deletes all catchpoints.
sk[ip]
Passes a caught exception back to the application, skipping the catchpoint.

Program Stack

b[ack]t[race] — a.k.a. “w[here]”
Display stack trace.
f[rame] <frame-number>
Moves to <frame-number> (frame numbers are shown by bt). With no argument, shows the current frame.
up <number>
Move up <number> frames (or 1, if no number specified).
down <number>
Move down <number> frames (or 1, if no number specified).
info args
Arguments of the current frame.
info locals
Local variables in the current stack frame.
info instance_variables
Instance variables in the current stack frame.
info global_variables
Current global variables.
info variables
Local and instance variables of the current frame.
m[ethod] <class|module>
Shows instance methods of the given class or module.
m[ethod] i[nstance] <object>
Shows methods of <object>.
m[ethod] iv <object>
Shows instance variables of <object>.
v[ar] cl[ass]
Shows class variables of self.
v[ar] co[nst] <object>
Shows constants of <object>.
v[ar] g[lobal]
Shows global variables (same as info global_variables).
v[ar] i[nstance] <object>
Shows instance variables of <object&gt (same as method iv <object>).
v[ar] l[ocal]
Shows local variables (same as info locals).

Execution Control

c[ontinue] <line-number>
Carry on running until program ends, hits a breakpoint or reaches line line-number (if specified).
n[ext] <number>
Go to next line, stepping over function calls. If number specified, go forward that number of lines.
s[tep] <number>
Go to next line, stepping into function calls. If numberis specified, make that many steps.
fin[ish] <num-frames>
With no argument, run until the current frame returns. Otherwise, run until <num-frames> frames have returned.
irb
Start an IRB session. This will have added commands cont, n and step, but these can’t take arguments (unlike the proper byebug commands of the same name).
restart
Restart the program. This also restarts byebug.

Threads

th[read]
Show current thread.
th[read] l[ist]
List all threads.
th[read] stop <number>
Stop thread number <number>.
th[read] resume <number>
Resume thread number <number>.
th[read] <number>
Switch context to thread <number>.

Display

e[val] — a.k.a. “p” <expression>
Evaluate <expression> and display result. By default, you can also just type the expression without any command and get the same thing (disabled by using set noautoeval
pp
Evaluate expression and pretty-print the result.
putl
Evaluate an expression with an array result and columnize the output.
ps
Evaluate an expression with an array result, sort and columnize the output.
disp[lay] <expression>
Automatically display <expression> every time the program halts. With no argument, lists the current display expressions.
info display
List all current display expressions.
undisp[lay] <number>
Remove display expression number <number> (as listed by info display). With no argument, cancel all current display expressions.
disable display <number>
Stop displaying expression number <number>. The display expression is kept in the list, though, and can be turned back on again using enable display .
enable display <number>
Re-enable previously disabled display expression <number>.

Controlling Byebug

hist[ory] <num-commands>
View last <num-commands> byebug commands (or all, if no argument given).
save <file>
Saves current byebug session options as a script file in <file>.
source <file>
Loads byebug options from a script file at <file>
set <option>
Change value of byebug option <option>.
show <option>
View current value of byebug option <option>.

Options are: autoeval, autoirb, autolist, autoreload, autosave, basename, callstyle, forcestep, fullpath, histfile, histsize, linetrace, tracing_plus, listsize, post_mortem, stack_on_error, testing, verbose and width.

Source Files and Code

reload
Reload source code.
info file
Information about the current source file.
info files
All currently loaded files.
info line
Shows the current line number and filename.
l[ist]
Shows source code after the current point. Keep reading for more list options.
l[ist] –
Shows source code before the current point.
l[ist] =
Shows source code centred around the current point.
l[ist] <first>-<last>
Shows all source code from <first> to <last> line numbers.
edit <file:lineno>
Edit <file>. With no arguments, edits the current file.

9 replies on “Byebug Cheatsheet”

At least on my version of byebug doing a plain b generates the following message …

(byebug) b

b[reak] [file:]line [if expr]
b[reak] [module::…]class(.|#)method [if expr]

They can be specified by line or method and an expression can be added
for conditionally enabled breakpoints.

Sets breakpoints in the source code

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