Setting the Bash option histexpand allows some convenient typing shortcuts
using Bash history expansion. The option can be set with either of these:
$ set -H
$ set -o histexpand
It’s likely that this option is already set for all interactive shells, as it’s
on by default. The manual, man bash, describes these features as follows:
-H Enable ! style history substitution. This option is on
by default when the shell is interactive.
You may have come across this before, perhaps to your annoyance, in the
following error message that comes up whenever ! is used in a double-quoted
string, or without being escaped with a backslash:
$ echo "Hi, this is Tom!"
bash: !": event not found
If you don’t want the feature and thereby make ! into a normal character, it
can be disabled with either of these:
$ set +H
$ set +o histexpand
History expansion is actually a very old feature of shells, having been
available in csh before Bash usage became common.
This article is a good followup to Better Bash history, which among other
things explains how to include dates and times in history output, as these
examples do.
Basic history expansion
Perhaps the best known and most useful of these expansions is using !! to
refer to the previous command. This allows repeating commands quickly, perhaps
to monitor the progress of a long process, such as disk space being freed while
deleting a large file:
$ rm big_file &
[1] 23608
$ du -sh .
3.9G .
$ !!
du -sh .
3.3G .
It can also be useful to specify the full filesystem path to programs that
aren’t in your $PATH:
$ hdparm
-bash: hdparm: command not found
$ /sbin/!!
/sbin/hdparm
In each case, note that the command itself is printed as expanded, and then run
to print the output on the following line.
History by absolute index
However, !! is actually a specific example of a more general form of history
expansion. For example, you can supply the history item number of a specific
command to repeat it, after looking it up with history:
$ history | grep expand
3951 2012-08-16 15:58:53 set -o histexpand
$ !3951
set -o histexpand
You needn’t enter the !3951 on a line by itself; it can be included as any
part of the command, for example to add a prefix like sudo:
$ sudo !3850
If you include the escape string \! as part of your Bash prompt, you can
include the current command number in the prompt before the command, making
repeating commands by index a lot easier as long as they’re still visible on
the screen.
History by relative index
It’s also possible to refer to commands relative to the current command. To
subtitute the second-to-last command, we can type !-2. For example, to check
whether truncating a file with sed worked correctly:
$ wc -l bigfile.txt
267 bigfile.txt
$ sed -i '11,$d' bigfile.txt
$ !-2
wc -l bigfile.txt
10 bigfile.txt
This works further back into history, with !-3, !-4, and so on.
Expanding for historical arguments
In each of the above cases, we’re substituting for the whole command line.
There are also ways to get specific tokens, or words, from the command if we
want that. To get the first argument of a particular command in the history,
use the !^ token:
$ touch a.txt b.txt c.txt
$ ls !^
ls a.txt
a.txt
To get the last argument, add !$:
$ touch a.txt b.txt c.txt
$ ls !$
ls c.txt
c.txt
To get all arguments (but not the command itself), use !*:
$ touch a.txt b.txt c.txt
$ ls !*
ls a.txt b.txt c.txt
a.txt b.txt c.txt
This last one is particularly handy when performing several operations on
a group of files; we could run du and wc over them to get their size and
character count, and then perhaps decide to delete them based on the output:
$ du a.txt b.txt c.txt
4164 a.txt
5184 b.txt
8356 c.txt
$ wc !*
wc a.txt b.txt c.txt
16689 94038 4250112 a.txt
20749 117100 5294592 b.txt
33190 188557 8539136 c.txt
70628 399695 18083840 total
$ rm !*
rm a.txt b.txt c.txt
These work not just for the preceding command in history, but also absolute and
relative command numbers:
$ history 3
3989 2012-08-16 16:30:59 wc -l b.txt
3990 2012-08-16 16:31:05 du -sh c.txt
3991 2012-08-16 16:31:12 history 3
$ echo !3989^
echo -l
-l
$ echo !3990$
echo c.txt
c.txt
$ echo !-1*
echo c.txt
c.txt
More generally, you can use the syntax !n:w to refer to any specific
argument in a history item by number. In this case, the first word, usually
a command or builtin, is word 0:
$ history | grep bash
4073 2012-08-16 20:24:53 man bash
$ !4073:0
man
What manual page do you want?
$ !4073:1
bash
You can even select ranges of words by separating their indices with a hyphen:
$ history | grep apt-get
3663 2012-08-15 17:01:30 sudo apt-get install gnome
$ !3663:0-1 purge !3663:3
sudo apt-get purge gnome
You can include ^ and $ as start and endpoints for these ranges, too. 3*
is a shorthand for 3-$, meaning “all arguments from the third to the last.”
Expanding history by string
You can also refer to a previous command in the history that starts with
a specific string with the syntax !string:
$ !echo
echo c.txt
c.txt
$ !history
history 3
4011 2012-08-16 16:38:28 rm a.txt b.txt c.txt
4012 2012-08-16 16:42:48 echo c.txt
4013 2012-08-16 16:42:51 history 3
If you want to match any part of the command line, not just the start, you can
use !?string?:
$ !?bash?
man bash
Be careful when using these, if you use them at all. By default it will run the
most recent command matching the string immediately, with no prompting, so it
might be a problem if it doesn’t match the command you expect.
Checking history expansions before running
If you’re paranoid about this, Bash allows you to audit the command as expanded
before you enter it, with the histverify option:
$ shopt -s histverify
$ !rm
$ rm a.txt b.txt c.txt
This option works for any history expansion, and may be a good choice for more
cautious administrators. It’s a good thing to add to one’s .bashrc if so.
If you don’t need this set all the time, but you do have reservations at some
point about running a history command, you can arrange to print the command
without running it by adding a :p suffix:
$ !rm:p
rm important-file
In this instance, the command was expanded, but thankfully not actually run.
Substituting strings in history expansions
To get really in-depth, you can also perform substitutions on arbitrary
commands from the history with !!:gs/pattern/replacement/. This is getting
pretty baroque even for Bash, but it’s possible you may find it useful at some
point:
$ !!:gs/txt/mp3/
rm a.mp3 b.mp3 c.mp3
If you only want to replace the first occurrence, you can omit the g:
$ !!:s/txt/mp3/
rm a.mp3 b.txt c.txt
Stripping leading directories or trailing files
If you want to chop a filename off a long argument to work with the directory,
you can do this by adding an :h suffix, kind of like a dirname call in
Perl:
$ du -sh /home/tom/work/doc.txt
$ cd !$:h
cd /home/tom/work
To do the opposite, like a basename call in Perl, use :t:
$ ls /home/tom/work/doc.txt
$ DOCUMENT=!$:t
DOCUMENT=doc.txt
Stripping extensions or base names
A bit more esoteric, but still possibly useful; to strip a file’s extension,
use :r:
$ vi /home/tom/work/doc.txt
$ STRIPEXT=!$:r
STRIPEXT=/home/tom/work/doc
To do the opposite, to get only the extension, use :e:
$ vi /home/tom/work/doc.txt
$ EXTONLY=!$:e
EXTONLY=.txt
Quoting history
If you’re performing substitution not to execute a command or fragment but to
use it as a string, it’s likely you’ll want to quote it. For example, if
you’ve just found through experiment and trial and error an ideal ffmpeg
command line to accomplish some task, you might want to save it for later use
by writing it to a script:
$ ffmpeg -f alsa -ac 2 -i hw:0,0 -f x11grab -r 30 -s 1600x900 \
> -i :0.0+1600,0 -acodec pcm_s16le -vcodec libx264 -preset ultrafast \
> -crf 0 -threads 0 $(date +%Y%m%d%H%M%S).mkv
To make sure all the escaping is done correctly, you can write the command
into the file with the :q modifier:
$ echo '#!/usr/bin/env bash' >ffmpeg.sh
$ echo !ffmpeg:q >>ffmpeg.sh
In this case, this will prevent Bash from executing the command expansion
$(date ... ), instead writing it literally to the file as desired. If you
build a lot of complex commands interactively that you later write to scripts
once completed, this feature is really helpful and saves a lot of cutting and
pasting.
Thanks to commenter Mihai Maruseac for pointing out a bug in the examples.
