1 files changed, 52 insertions, 0 deletions

diff --git a/docs/busybox.sgml b/docs/busybox.sgml
index ec6f5041d..a25aa0189 100644
--- a/docs/busybox.sgml
+++ b/docs/busybox.sgml
@@ -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>