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man: extend documentation on the unit name escaping logic

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Lennart Poettering 2017-10-26 12:22:38 +02:00
parent 40b1a32ca8
commit 2651d03751
2 changed files with 38 additions and 23 deletions
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15
man/systemd-escape.xml
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@ -73,6 +73,9 @@
special mode of escaping is applied instead, which assumes the
string is already escaped but will escape everything that
appears obviously non-escaped.</para>
<para>For details on the escaping and unescaping algorithms see the relevant section in
<citerefentry><refentrytitle>systemd.unit</refentrytitle><manvolnum>5</manvolnum></citerefentry>.</para>
</refsect1>
<refsect1>
@ -107,11 +110,12 @@
<term><option>--path</option></term>
<term><option>-p</option></term>
<listitem><para>When escaping or unescaping a string, assume
it refers to a file system path. This eliminates leading,
trailing or duplicate <literal>/</literal> characters
and rejects <literal>.</literal> and <literal>..</literal>
path components.</para></listitem>
<listitem><para>When escaping or unescaping a string, assume it refers to a file system path. This eliminates
leading, trailing or duplicate <literal>/</literal> characters and rejects <literal>.</literal> and
<literal>..</literal> path components. This is particularly useful for generating strings suitable for
unescaping with the <literal>%f</literal> specifier in unit files, see
<citerefentry><refentrytitle>systemd.unit</refentrytitle><manvolnum>5</manvolnum></citerefentry>.
</para></listitem>
</varlistentry>
<varlistentry>
@ -172,6 +176,7 @@ systemd-nspawn@My\x20Container\x201.service systemd-nspawn@containerb.service sy
<title>See Also</title>
<para>
<citerefentry><refentrytitle>systemd</refentrytitle><manvolnum>1</manvolnum></citerefentry>,
<citerefentry><refentrytitle>systemd.unit</refentrytitle><manvolnum>5</manvolnum></citerefentry>,
<citerefentry><refentrytitle>systemctl</refentrytitle><manvolnum>1</manvolnum></citerefentry>
</para>
</refsect1>

46
man/systemd.unit.xml
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@ -213,22 +213,6 @@
socket-based activation which make dependencies implicit,
resulting in a both simpler and more flexible system.</para>
<para>Some unit names reflect paths existing in the file system
namespace. Example: a device unit
<filename>dev-sda.device</filename> refers to a device with the
device node <filename noindex='true'>/dev/sda</filename> in the
file system namespace. If this applies, a special way to escape
the path name is used, so that the result is usable as part of a
filename. Basically, given a path, "/" is replaced by "-", and all
other characters which are not ASCII alphanumerics are replaced by
C-style "\x2d" escapes (except that "_" is never replaced and "."
is only replaced when it would be the first character in the
escaped path). The root directory "/" is encoded as single dash,
while otherwise the initial and ending "/" are removed from all
paths during transformation. This escaping is reversible. Properly
escaped paths can be generated using the
<citerefentry><refentrytitle>systemd-escape</refentrytitle><manvolnum>1</manvolnum></citerefentry>
command.</para>
<para>Optionally, units may be instantiated from a
template file at runtime. This allows creation of
@ -265,6 +249,32 @@
</refsect1>
<refsect1>
<title>String Escaping for Inclusion in Unit Names</title>
<para>Sometimes it is useful to convert arbitrary strings into unit names. To facilitate this, a method of string
escaping is used, in order to map strings containing arbitrary byte values (except NUL) into valid unit names and
their restricted character set. A common special case are unit names that reflect paths to objects in the file
system hierarchy. Example: a device unit <filename>dev-sda.device</filename> refers to a device with the device
node <filename noindex='true'>/dev/sda</filename> in the file system.</para>
<para>The escaping algorithm operates as follows: given a string, any <literal>/</literal> character is replaced by
<literal>-</literal>, and all other characters which are not ASCII alphanumerics or <literal>_</literal> are
replaced by C-style <literal>\x2d</literal> escapes. In addition, <literal>.</literal> is replaced with such a
C-style escape when it would appear as the first character in the escaped string.</para>
<para>When the input qualifies as absolute file system path, this algorithm is extended slightly: the path to the
root directory <literal>/</literal> is encoded as single dash <literal>-</literal>. In addition, any leading,
trailing or duplicate <literal>/</literal> characters are removed from the string before transformation. Example:
<filename>/foo//bar/baz/</filename> becomes <literal>foo-bar-baz</literal>.</para>
<para>This escaping is fully reversible, as long as it is known whether the escaped string was a path (the
unescaping results are different for paths and non-path strings). The
<citerefentry><refentrytitle>systemd-escape</refentrytitle><manvolnum>1</manvolnum></citerefentry> command may be
used to apply and reverse escaping on arbitrary strings. Use <command>systemd-escape --path</command> to escape
path strings, and <command>systemd-escape</command> without <option>--path</option> otherwise.</para>
</refsect1>
<refsect1>
<title>Implicit Dependencies</title>
@ -1241,7 +1251,7 @@
<row>
<entry><literal>%N</literal></entry>
<entry>Unescaped full unit name</entry>
<entry>Same as <literal>%n</literal>, but with escaping undone</entry>
<entry>Same as <literal>%n</literal>, but with escaping undone. This undoes the escaping used when generating unit names from arbitrary strings (see above). </entry>
</row>
<row>
<entry><literal>%p</literal></entry>
@ -1266,7 +1276,7 @@
<row>
<entry><literal>%f</literal></entry>
<entry>Unescaped filename</entry>
<entry>This is either the unescaped instance name (if applicable) with <filename>/</filename> prepended (if applicable), or the unescaped prefix name prepended with <filename>/</filename>.</entry>
<entry>This is either the unescaped instance name (if applicable) with <filename>/</filename> prepended (if applicable), or the unescaped prefix name prepended with <filename>/</filename>. This implements unescaping according to the rules for escaping absolute file system paths discussed above.</entry>
</row>
<row>
<entry><literal>%t</literal></entry>