index range doc

doc_src/index.hdr.in

@@ -80,7 +80,7 @@ while '-f' will turn it off.
 
 \subsection quotes Quotes
 
-Sometimes features such as <a href="#globbing">parameter expansion</a>
+Sometimes features such as <a href="#expand">parameter expansion</a>
 and <a href="#escapes">character escapes</a> get in the way. When that
 happens, the user can write a parameter within quotes, either '
 (single quote) or " (double quote). There is one important difference
@@ -581,6 +581,9 @@ A command substitution will not change the value of the <a
 href='#variables-status'>status</a> variable outside of the command
 substitution.
 
+Only part of the output can be used, see <a href='#expand-index-range'>index
+range expansion</a> for details.
+
 Example:
 
 The command <code>echo (basename image.jpg .jpg).png</code> will
@@ -674,6 +677,50 @@ element of the foo variable should be dereferenced and never that the fifth
 element of the doubly dereferenced variable foo. The latter can
 instead be expressed as $$foo[1][5].
 
+\subsection expand-index-range Index range expansion
+
+Both command substitution and environment variables support accessing only 
+specific items by providing a set of indices in square brackets. It's 
+often needed to access a sequence of elements. To do this, one can use
+range operator '..' for this. A range 'a..b', where range limits 'a' and 'b' 
+are integer numbers, is expanded into a sequence of indices 
+'a a+1 a+2 ... b' or 'a a-1 a-2 ... b' depending on which of 'a' or 'b' 
+is higher. The negative range limits are calculated from the end of the array
+or command substitution.
+
+Some examples:
+<pre>
+# Limit the command substitution output
+echo (seq 10)[2..5] # will use elements from 2 to 5
+# Output is:
+# 2 3 4 5
+
+# Use overlapping ranges:
+echo (seq 10)[2..5 1..3] # will take elements from 2 to 5 and then elements from 1 to 3 
+# Output is:
+# 2 3 4 5 1 2 3
+
+# Reverse output
+echo (seq 10)[-1..1] # will use elements from the last output line to the first one in reverse direction
+# Output is:
+# 10 9 8 7 6 5 4 3 2 1
+</pre>
+
+The same works when setting or expanding variables:
+<pre>
+# Reverse path variable
+set PATH $PATH[-1..1]
+# or 
+set PATH[-1..1] $PATH
+
+# Use only n last items of the PATH
+set n -3
+echo $PATH[$n..-1]
+</pre>
+
+NOTE: Currently variables are allowed inside variables index expansion, but not in indices, 
+used for command substitution.
+
 \subsection expand-home Home directory expansion
 
 The ~ (tilde) character at the beginning of a parameter, followed by a
@@ -909,6 +956,9 @@ If you specify a negative index when expanding or assigning to an
 array variable, the index will be calculated from the end of the
 array. For example, the index -1 means the last index of an array.
 
+A range of indices can be specified, see <a href='#expand-index-range'>index
+range expansion</a> for details.
+
 \subsection variables-special Special variables
 
 The user can change the settings of \c fish by changing the values of