mjhjmtu

I understand that:

$ apt-cache search package_name

will search for matches in names and descriptions.

But how does this work?

$ apt-cache search package_name$

For example,

$ apt-cache search desktop

will produce a list of names and short descriptions of packages that have "desktop" anywhere in the name or description.

Then, if I understand correctly,

$ apt-cache search desktop$

should produce a subset in which "desktop" is the last word either in the name or in any line in the description.

But I found one example, xjig, which shows up with apt-cache search desktop but doesn't show up with apt-cache search desktop$ even though the description according to apt-cache show xjig has "desktop" in the second-last line of the description:

Description-en: An X11 jigsaw puzzle

xjig is a puzzle that tries to replicate a jigsaw puzzle on the screen

as closely as possible. Any GIF, JPEG or PPM image may be loaded and cut

into pieces. The goal (as with any jigsaw puzzle) is to reassemble the

original picture.

.

Tiles may be freely rotated by use of the mouse, flipped (for

double-sided puzzles) and even shown as shaped windows on the desktop

(fast machine/video card recommended for the latter!). An example image

is provided in /usr/share/games/xjig .

So why is xjig not included in the output of apt-cache search desktop$?

From the apt-cache man page:

search regex...
 search performs a full text search on all available package lists for the 
 POSIX regex pattern given, see regex(7). It searches the package names 
 and the descriptions for an occurrence of the regular expression and 
 prints out the package name and the short description, including virtual 
 package names. If --full is given then output identical to show is 
 produced for each matched package, and if --names-only is given then the 
 long description is not searched, only the package name is.

 Separate arguments can be used to specify multiple search patterns that 
 are and'ed together.

Notice it states regex(7). So when you tell apt-cache to search for something you're providing it a regular expression. The $ in a regex means that you want to match, anchored to the end of strings.

So if we're gave the value desktop$ we'd only match strings where desktop was at the end of strings such as:

• this is a desktop


• this is a very long string with desktop

But it wouldn't match strings such as:

• this is desktop in the window


• this is another desktop in the window

In your case the string desktop in xjig's description, though at the end of a line in the description, is not at the end of the description field's value.

Example

Here's xjig's description.

Description-en: An X11 jigsaw puzzle
 xjig is a puzzle that tries to replicate a jigsaw puzzle on the screen
 as closely as possible. Any GIF, JPEG or PPM image may be loaded and cut
 into pieces. The goal (as with any jigsaw puzzle) is to reassemble the
 original picture.
 .
 Tiles may be freely rotated by use of the mouse, flipped (for
 double-sided puzzles) and even shown as shaped windows on the desktop
 (fast machine/video card recommended for the latter!). An example image
 is provided in /usr/share/games/xjig .

If I search for all strings that end with xjig .$.

$ apt-cache search 'xjig .$'
xjig - An X11 jigsaw puzzle

Remember that the value for the description is just a long string that is wrapped at that point, so though "desktop" is at the end a line, it isn't actually at the end of the description's value.

Alternative

Given you're looking for the word desktop you could look for the string with either spaces on either side (s), or word boundaries (b).

$ apt-cache search 'sdesktops' | grep xjig
xjig - An X11 jigsaw puzzle

$ apt-cache search 'bdesktopb' | grep xjig
xjig - An X11 jigsaw puzzle

Understanding regex anchors

Perhaps using grep to see what an anchor does in a regex would help.

desktop not at the end

$ echo "this is a desktop string" | grep "desktop$"
$

desktop at the end

$ echo "this is a desktop" | grep "desktop$"
this is a desktop

The compliment to $ for anchoring matches to the end of strings, is the ^ for anchoring matches to the beginning.

desktop not at beginning

$ echo "this is a desktop" | grep "^desktop"
$

desktop at beginning

$ echo "desktop this is a desktop" | grep "^desktop"
desktop this is a desktop

The description field is a multi-line field but $ in the regexp, means the end of the field, not the end of each line of the field. So, if you want to search for desktop at the end of a line or at the end of the field, you need:

apt-cache search 'desktop(
|$)'

Or with the ksh93 $'...' quotes (also found in zsh and bash) allowing x escape sequences:

apt-cache search $'desktop(n|$)'

Also note that . does match a newline character. Written in perl, that would be:

print "$name - $summary" if 
 $name =~ qr$resi ||
 $description =~ qr$resi;

(except they are not perl regexps of course)