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diff --git a/docs/busybox.sgml b/docs/busybox.sgml
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@@ -879,6 +879,58 @@
</para>
</sect1>
+ <sect1 id="expr">
+ <title>expr</title>
+
+ <para>
+ Usage: expr EXPRESSION
+ </para>
+
+ <para>
+ Prints the value of EXPRESSION to standard output.
+ </para>
+
+ <para>
+ EXPRESSION may be:
+ </para>
+
+ <para>
+ <screen>
+ ARG1 | ARG2 ARG1 if it is neither null nor 0, otherwise ARG2
+ ARG1 & ARG2 ARG1 if neither argument is null or 0, otherwise 0
+ ARG1 &lt ARG2 ARG1 is less than ARG2
+ ARG1 &lt= ARG2 ARG1 is less than or equal to ARG2
+ ARG1 = ARG2 ARG1 is equal to ARG2
+ ARG1 != ARG2 ARG1 is unequal to ARG2
+ ARG1 &gt= ARG2 ARG1 is greater than or equal to ARG2
+ ARG1 &gt ARG2 ARG1 is greater than ARG2
+ ARG1 + ARG2 arithmetic sum of ARG1 and ARG2
+ ARG1 - ARG2 arithmetic difference of ARG1 and ARG2
+ ARG1 * ARG2 arithmetic product of ARG1 and ARG2
+ ARG1 / ARG2 arithmetic quotient of ARG1 divided by ARG2
+ ARG1 % ARG2 arithmetic remainder of ARG1 divided by ARG2
+ STRING : REGEXP anchored pattern match of REGEXP in STRING
+ match STRING REGEXP same as STRING : REGEXP
+ substr STRING POS LENGTH substring of STRING, POS counted from 1
+ index STRING CHARS index in STRING where any CHARS is found, or 0
+ length STRING length of STRING
+ quote TOKEN interpret TOKEN as a string, even if it is a
+ keyword like `match' or an operator like `/'
+ ( EXPRESSION ) value of EXPRESSION
+ </screen>
+ </para>
+
+ <para>
+ Beware that many operators need to be escaped or quoted for shells.
+ Comparisons are arithmetic if both ARGs are numbers, else
+ lexicographical. Pattern matches return the string matched between
+ \( and \) or null; if \( and \) are not used, they return the number
+ of characters matched or 0.
+ </para>
+
+ </sect1>
+
+
<sect1 id="false">
<title>false</title>