Mfcttrf

I have defined the following command:

newcommandformatcommand[1]
 deftemp#1
 defsifxtempempty emptyelse not emptyfi
 echo "s"

This works correctly:

formatcommand
formatcommand 

However, when using the above command inside write18,

immediatewrite18formatcommand

the defs seem to not be expanding, since I'm getting def is not an executable..., meaning that write18 sent the body of formatcommand without expanding it to the command line. How can this be fixed?

MWE

documentclass[border=1cm]standalone


begindocument

newcommandformatcommand[1]
 deftemp#1
 defsifxtempempty emptyelse not emptyfi
 echo "s"


formatcommand
formatcommand 

immediatewrite18formatcommand

enddocument

When TeX writes the contents of a write command, it expands all material similarly to what happens on an edef defintion, i.e. all expandable commands are expanded but no execution happens. Macro definitions (or other assignments) are not expandable, so the defs remain in the input stream when they are written as shell commands.

The solution is to use a fully-expandable test for an empty token list, like the following ifempty macro:

makeatletter
newcommandifempty[1]%
 ifrelaxdetokenize#1relax
 expandafter@firstoftwo
 else
 expandafter@secondoftwo
 fi

makeatother

newcommandformatcommand[1]
 echo "ifempty#1emptynot empty"

Generally I recommend siracusa'a solution, but if you LuaTeX there is another trick which you can use if your problem can't be written in an expandable manner:

LuaTeX has primitives immediateassignment and immediateassigned which allow expandable defs:

documentclass[border=1cm]standalone


begindocument

newcommandformatcommand[1]
 immediateassignmentdeftemp#1
 immediateassignmentdefsifxtempempty emptyelse not emptyfi
 echo "s"


formatcommand
formatcommand 

immediatewrite255formatcommand

enddocument

(I replaced write18 with write255 to show the command on the terminal because shell-escape works differently in LuaTeX, so executing the command would have needed bigger changes.)

Using immediateassigned, you can make all assignments in a block expandable:

documentclass[border=1cm]standalone


begindocument

newcommandformatcommand[1]
 immediateassigned%
 deftemp#1
 defsifxtempempty emptyelse not emptyfi
 %
 echo "s"


formatcommand
formatcommand 

immediatewrite255formatcommand

enddocument