====== Test for existence ======

===== Synopsis =====

    isset <value>

===== Examples =====

=== Boot if a filename is specified ===

    isset ${filename} && chain ${filename} || goto no_filename

=== Set a default DNS server if none is specified ===

    isset ${dns} || set dns 192.168.0.1

=== Retry DHCP indefinitely until a filename is obtained ===

    #!ipxe
    :retry_dhcp
    dhcp && isset ${filename} || goto retry_dhcp

===== Description =====

Test existence of the specified value, which will normally be a configuration [[:settings|setting]].  Existence is indicated via the command status; the conditional operators ''&&'' and ''||'' can be used to decide what action to take as a result.

===== Command status =====

^ Success   | The value exists          |
^ Failure   | The value does not exist  |

===== See also =====

  * ''[[:cmd:iseq]]''
  * ''[[:cmd:set]]''
  * ''[[:cmd:show]]''
  * ''[[:cmd:goto]]''
  * iPXE [[:scripting|scripting]] guide
  * [[:cmd|List of all iPXE commands]]

===== Notes =====

You should always consider what will happen if the value does not exist.  For example:

    isset ${filename} && chain ${filename}

If no ''[[:cfg:filename]]'' is specified then the ''isset'' command will fail and the script will terminate, as described in the [[:scripting#error_handling|scripting guide]].  To avoid this problem, you can use the ''||'' operator:

    isset ${filename} && chain ${filename} ||