while

while checks a condition and, as long as it is true, runs its block. The condition is evaluated again before every iteration, so the block may run zero times.

See also: while command help and Control Flow - while in the Book.

mut x = 0
while $x < 10 { $x = $x + 1 }
$x
# => 10

Language Notes

1. The condition is an expression that must evaluate to a bool. A pipeline or command call needs parentheses: while ($queue | is-empty) == false { ... }.

   mut queue = [a b c]
   while not ($queue | is-empty) {
     print $"processing ($queue | first)"
     $queue = ($queue | skip 1)
   }
   # => processing a
   # => processing b
   # => processing c

2. A non-boolean condition is a runtime error; Nushell does not convert values to "truthy" or "falsy" ones.

   while 1 { break }
   # => Error: nu::shell::type_mismatch
   # =>
   # =>   × Type mismatch.
   # =>    ╭─[repl_entry #1:1:7]
   # =>  1 │ while 1 { break }
   # =>    ·       ┬
   # =>    ·       ╰── expected bool
   # =>    ╰────

3. If the condition is false from the start, the block never runs:

   mut x = 10
   while $x < 3 { print "never runs" }
   $x
   # => 10

4. Like loop, while evaluates to nothing, the block's values are discarded, and the block can update mut variables from the enclosing scope because it is a block and not a closure. For the same reason, the condition must not be written as a closure:

   mut x = 0; while { $x < 3 } { $x += 1 }
   # => Error: nu::parser::expected_keyword
   # =>
   # =>   × Capture of mutable variable.
   # =>    ╭─[repl_entry #1:1:20]
   # =>  1 │ mut x = 0; while { $x < 3 } { $x += 1 }
   # =>    ·                    ─┬
   # =>    ·                     ╰── capture of mutable variable
   # =>    ╰────

5. break and continue work in while just as in loop and for. while true { ... } is equivalent to loop { ... }:

   mut i = 0
   while true {
     $i += 1
     if $i == 2 { continue }
     if $i > 4 { break }
     print $i
   }
   # => 1
   # => 3
   # => 4

6. while does not accept pipeline input. To iterate over a list or table, use for or a filter such as each. To consume a list only while a condition holds, take while is usually simpler than a while loop:

   [1 3 5 6 7] | take while {|n| $n mod 2 == 1 }
   # => ╭───┬───╮
   # => │ 0 │ 1 │
   # => │ 1 │ 3 │
   # => │ 2 │ 5 │
   # => ╰───┴───╯