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<?xml version="1.0"?>
<guide self="tools-reference/cat/">
<chapter>
<title><c>cat</c> -- File Concatenation</title>
<body>
<p>
The <c>cat</c> command can be used to concatenate the contents of two or more
files. The usage is <c>cat firstfile secondfile ...</c>.
</p>
</body>
<section>
<title>Abuse of <c>cat</c></title>
<body>
<p>
If you find yourself about to use <c>cat</c> in an ebuild, stop and
reconsider. It is almost always unnecessary.
</p>
<p>
All usages in the form <c>cat somefile | somecommand</c> are silly and
should be eschewed. The form <c>somecommand < somefile</c> does the
same thing, and doesn't involve an extra fork and a pipe. With many
standard utilities the <c>somecommand somefile</c> form will work as well.
</p>
<p>
Using <c>foo=$(cat somefile)</c> to place the contents of a file into
the variable <c>foo</c> is also unnecessary. The
command <c>foo=$(<somefile)</c> works just as well and doesn't require
a fork. Similarly, <c>cat file | xargs cmd</c> and <c>xargs cmd <
file</c> can be replaced by <c>cmd $(<file)</c>.
</p>
<p>
Finally, <c>cat foo > bar</c>, where foo is a single file, can usually
be replaced by <c>cp -f foo bar</c>.
</p>
</body>
</section>
<section>
<title>Here Documents</title>
<body>
<p>
On the other hand, <c>cat</c> is exceptionally useful for so-called
"here" documents, such as the following example from the
<c>sendmail-8.13.3</c> ebuild:
</p>
<codesample lang="ebuild">
src_install() {
# ...
cat <<- EOF > "${D}/etc/mail/trusted-users" || die
# trusted-users - users that can send mail as others without a warning
# apache, mailman, majordomo, uucp are good candidates
EOF
# ...
}
</codesample>
<p>
In this example <c>cat</c> is used inside <c>src_install</c> to create
the <c>${D}/etc/mail/trusted-users</c> file. Specifically, the new
file will comprise the lines between the <c>cat</c> line and the line with
<c>EOF</c> in the ebuild.
</p>
<p>
The funny hyphen in <c><<-</c> is a >=bash-2.05 syntax that
tells <c>cat</c> to strip leading tabs (please note that when you want
to copy the example, you have to replace the leading spaces with
tabs), so that, as Azarah puts it, "we won't have the ebuilds looking
from space effect". For such small files as the example above, a set
of here documents is more elegant, and easier to maintain, than the
equivalent bunch of files floating about in <c>${FILESDIR}</c> would
be. If for some reason you need to preserve leading whitespace, then
simply use <c><<</c> instead of <c><<-</c> to get the
desired effect.
</p>
</body>
</section>
</chapter>
</guide>
|
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