For Developers

The cmd module from the Python standard library includes readline history.

cmd2.Cmd offers the same readline capabilities, but also maintains its own data structures for the history of all commands entered by the user. When the class is initialized, it creates an instance of the cmd2.history.History class (which is a subclass of list) as cmd2.Cmd.history.

Each time a command is executed (this gets complex, see Command Processing Loop for exactly when) the parsed cmd2.Statement is appended to cmd2.Cmd.history.

cmd2 adds the option of making this history persistent via optional arguments to cmd2.Cmd.__init__(). If you pass a filename in the persistent_history_file argument, the contents of cmd2.Cmd.history will be written as compressed JSON to that history file. We chose this format instead of plain text to preserve the complete cmd2.Statement object for each command.


readline saves everything you type, whether it is a valid command or not. cmd2 only saves input to internal history if the command parses successfully and is a valid command. This design choice was intentional, because the contents of history can be saved to a file as a script, or can be re-run. Not saving invalid input reduces unintentional errors when doing so.

However, this design choice causes an inconsistency between the readline history and the cmd2 history when you enter an invalid command: it is saved to the readline history, but not to the cmd2 history.

The cmd2.Cmd.history attribute, the cmd2.history.History class, and the cmd2.history.HistoryItem class are all part of the public API for cmd2.Cmd. You could use these classes to implement write your own history command (see below for documentation on how the included history command works).

For Users

You can use the up and down arrow keys to move through the history of previously entered commands.

If the readline module is installed, you can press Control-p to move to the previously entered command, and Control-n to move to the next command. You can also search through the command history using Control-r.

Eric Johnson hosts a nice readline cheat sheet, or you can dig into the GNU Readline User Manual for all the details, including instructions for customizing the key bindings.

cmd2 makes a third type of history access available with the history command. Each time the user enters a command, cmd2 saves the input. The history command lets you do interesting things with that saved input. The examples to follow all assume that you have entered the following commands:

(Cmd) alias create one !echo one
Alias 'one' created
(Cmd) alias create two !echo two
Alias 'two' created
(Cmd) alias create three !echo three
Alias 'three' created
(Cmd) alias create four !echo four
Alias 'four' created

In it’s simplest form, the history command displays previously entered commands. With no additional arguments, it displays all previously entered commands:

(Cmd) history
    1  alias create one !echo one
    2  alias create two !echo two
    3  alias create three !echo three
    4  alias create four !echo four

If you give a positive integer as an argument, then it only displays the specified command:

(Cmd) history 4
    4  alias create four !echo four

If you give a negative integer N as an argument, then it display the Nth last command. For example, if you give -1 it will display the last command you entered. If you give -2 it will display the next to last command you entered, and so forth:

(Cmd) history -2
    3  alias create three !echo three

You can use a similar mechanism to display a range of commands. Simply give two command numbers separated by .. or :, and you will see all commands between, and including, those two numbers:

(Cmd) history 1:3
    1  alias create one !echo one
    2  alias create two !echo two
    3  alias create three !echo three

If you omit the first number, it will start at the beginning. If you omit the last number, it will continue to the end:

(Cmd) history :2
    1  alias create one !echo one
    2  alias create two !echo two
(Cmd) history 2:
    2  alias create two !echo two
    3  alias create three !echo three
    4  alias create four !echo four

If you want to display the last three commands entered:

(Cmd) history -- -3:
    2  alias create two !echo two
    3  alias create three !echo three
    4  alias create four !echo four

Notice the double dashes. These are required because the history command uses argparse to parse the command line arguments. As described in the argparse documentation , -3: is an option, not an argument:

If you have positional arguments that must begin with - and don’t look like negative numbers, you can insert the pseudo-argument ‘–’ which tells parse_args() that everything after that is a positional argument:

There is no zeroth command, so don’t ask for it. If you are a python programmer, you’ve probably noticed this looks a lot like the slice syntax for lists and arrays. It is, with the exception that the first history command is 1, where the first element in a python array is 0.

Besides selecting previous commands by number, you can also search for them. You can use a simple string search:

(Cmd) history two
    2  alias create two !echo two

Or a regular expression search by enclosing your regex in slashes:

(Cmd) history '/te\ +th/'
    3  alias create three !echo three

If your regular expression contains any characters that argparse finds interesting, like dash or plus, you also need to enclose your regular expression in quotation marks.

This all sounds great, but doesn’t it seem like a bit of overkill to have all these ways to select commands if all we can do is display them? Turns out, displaying history commands is just the beginning. The history command can perform many other actions:

  • running previously entered commands
  • saving previously entered commands to a text file
  • opening previously entered commands in your favorite text editor
  • running previously entered commands, saving the commands and their output to a text file
  • clearing the history of entered commands

Each of these actions is invoked using a command line option. The -r or --run option runs one or more previously entered commands. To run command number 1:

(Cmd) history --run 1

To rerun the last two commands (there’s that double dash again to make argparse stop looking for options):

(Cmd) history -r -- -2:

Say you want to re-run some previously entered commands, but you would really like to make a few changes to them before doing so. When you use the -e or --edit option, history will write the selected commands out to a text file, and open that file with a text editor. You make whatever changes, additions, or deletions, you want. When you leave the text editor, all the commands in the file are executed. To edit and then re-run commands 2-4 you would:

(Cmd) history --edit 2:4

If you want to save the commands to a text file, but not edit and re-run them, use the -o or --output-file option. This is a great way to create Scripts, which can be executed using the run_script command. To save the first 5 commands entered in this session to a text file:

(Cmd) history :5 -o history.txt

The history command can also save both the commands and their output to a text file. This is called a transcript. See Transcripts for more information on how transcripts work, and what you can use them for. To create a transcript use the -t or --transcription option:

(Cmd) history 2:3 --transcript transcript.txt

The --transcript option implies --run: the commands must be re-run in order to capture their output to the transcript file.

The last action the history command can perform is to clear the command history using -c or --clear:

(Cmd) history -c

In addition to these five actions, the history command also has some options to control how the output is formatted. With no arguments, the history command displays the command number before each command. This is great when displaying history to the screen because it gives you an easy reference to identify previously entered commands. However, when creating a script or a transcript, the command numbers would prevent the script from loading properly. The -s or --script option instructs the history command to suppress the line numbers. This option is automatically set by the --output_file, --transcript, and --edit options. If you want to output the history commands with line numbers to a file, you can do it with output redirection:

(Cmd) history 1:4 > history.txt

You might use -s or --script on it’s own if you want to display history commands to the screen without line numbers, so you can copy them to the clipboard:

(Cmd) history -s 1:3

cmd2 supports both aliases and macros, which allow you to substitute a short, more convenient input string with a longer replacement string. Say we create an alias like this, and then use it:

(Cmd) alias create ls shell ls -aF
Alias 'ls' created
(Cmd) ls -d h*
history.txt     htmlcov/

By default, the history command shows exactly what we typed:

(Cmd) history
    1  alias create ls shell ls -aF
    2  ls -d h*

There are two ways to modify that display so you can see what aliases and macros were expanded to. The first is to use -x or --expanded. These options show the expanded command instead of the entered command:

(Cmd) history -x
    1  alias create ls shell ls -aF
    2  shell ls -aF -d h*

If you want to see both the entered command and the expanded command, use the -v or --verbose option:

(Cmd) history -v
    1  alias create ls shell ls -aF
    2  ls -d h*
    2x shell ls -aF -d h*

If the entered command had no expansion, it is displayed as usual. However, if there is some change as the result of expanding macros and aliases, then the entered command is displayed with the number, and the expanded command is displayed with the number followed by an x.