Cookbook: Shadow history

This is a 0.8.2 feature.

Both readline history and standard IPython history are fragile beasts - readline history disappears when you have multiple IPython sessions open, and standard IPython history (shown by %hist) disappears when you quit IPython.

In shadow history, however, all the commands (well, most - the ones starting at the beginning of input line) are stored forever. You can search through shadow history by running the command %hist -g (g is for grep):

 [d:ipython/dist]|38> hist -g change
 0720: svn revert doc/ChangeLog
 0721: svn ci -m "whitespace cleanup changelog"
 0766: hist -g change
 ===
 ^shadow history ends, fetch by %rep <number> (must start with 0)
 === start of normal history ===
 38: _ip.magic("hist -g change")
 [d:ipython/dist]|39>

Note how the command greps through the shadow and normal history. Shadow history entries have the 0 as prefix (e.g. 0721) to tell them apart from normal history entries.

To fetch the command for editing at command line, invoke %rep with history line number:

[d:ipython/dist]|39> %rep 0721
[d:ipython/dist]|40> svn ci -m "whitespace cleanup changelog"

The command 0721 is not executed directly; rather, the cursor is left blinking at the end of line 40 for editing.

Other uses of %rep

The most typical meaning of %rep is "fetch this history item for command line editing". More creative uses are:

%rep line1-line2 line3-line4...

[d:ipython/dist]|42> print "hello"
hello
[d:ipython/dist]|43> print "world"
world
[d:ipython/dist]|44> rep 42-43
lines [u'print "hello"\nprint "world"\n']
hello
world

And just calling %rep without arguments, which fetches the contents of _ (last computation result) for command line editing, allowing you to create elaborate command lines without manual copy-pasting with mouse:

[~/_ipython]|60> a = !ls *.pyo
 ==
['ipy_profile_sh.pyo', 'ipy_user_conf.pyo']
[~/_ipython]|61> a
            <61> SList (.p, .n, .l, .s available). Value:
0: ipy_profile_sh.pyo
1: ipy_user_conf.pyo
[~/_ipython]|62> 'rm ' + a.s
            <62> 'rm ipy_profile_sh.pyo ipy_user_conf.pyo'
[~/_ipython]|63> rep
[~/_ipython]|64> rm ipy_profile_sh.pyo ipy_user_conf.pyo

And the cursor is left blinking at the line 64.

Advanced topics

• Every command is only once in the shadow history. The history line number indicates when the command was last run.
• Shadow history can be cleared by '%clear shadow_nuke'
• If your IPython prompt becomes slow because there is too much stuff in shadow history (every command is stored immediately after pressing enter), enter '%clear shadow_compress'. This is yet to prove a problem, though.
• If you want to know how much space shadow history is taking, it's stored at ~/_ipython/db/shadowhist (on pickleshare hashed storage). %clear shadow_nuke is essentially the same is deleting that directory.