Shell-escape with LaTeX 3

Question

What is the LaTeX 3 equivalent to \write18 if there is one? I want to call lilypond from within my TeX document.

Answer 1

UPDATE

Things have changed since 2013 and shell escape is now fully supported by expl3. The relevant section in the manual (as of 2021) is currently 10.7.

Getting the output from a command

\sys_get_shell:nnN
\sys_get_shell:nnNTF

For both functions the first n argument is an unquoted shell command (" quotes around it will be automatically supplied; the standard restrictions apply, namely that " cannot appear in this argument). The second argument is for “setup” (category code changes, for instance) that is applied to the tokens in the shell command. The third argument should be a token list variable. In the second function the T argument is returned if the command is well-formed and shell escape is enabled; otherwise the F argument is returned. The usual conventions for TF apply. The token list variable will contain the output of the shell command. If shell escape is disabled, the variable will contain \q_no_value.

This is the interface to `\input{|"..."}

Checking the shell escape status

\c_sys_shell_escape_int

is set to 0, 1 or 2 at the beginning of the job, with the usual meaning: 0 means shell escape is disabled; 1 that it is unrestricted; 2` means it is restricted.

\sys_if_shell_p:
\sys_if_shell:TF
\sys_if_shell_unrestricted_p:
\sys_if_shell_unrestricted:TF
\sys_if_shell_restricted_p:
\sys_if_shell_restricted:TF

are the predicates and the conditionals for testing the shell escape status.

Executing shell commands

\sys_shell_now:n
\sys_shell_now:x

will execute the command passed as argument, the x version will do full expansion of the argument. The execution is immediate.

This is the interface to \immediate\write18{...}

\sys_shell_shipout:n
\sys_shell_shipout:x

will execute the shell command at the next shipout (useful if the command to be executed needs as input the page number, for example).

This is the interface to \write18.

ORIGINAL ANSWER

There is no function in LaTeX3, at the moment, dealing with \write18; it can be obtained by

\RequirePackage{expl3,pdftexcmds}
\ExplSyntaxOn
\int_const:Nn \c_tobi_eighteen_int { 18 }
\int_const:Nn \c_tobi_shell_escape_int { \int_eval:n { \use:c { pdf@shellescape } } }
\cs_new_protected:Npn \tobi_system_exec_now:n #1
 {
  \int_compare:nT { \c_tobi_shell_escape_int > 0 }
   { \iow_now:Nn \c_tobi_eighteen_int { #1 } }
 }
\cs_new_protected:Npn \tobi_system_exec_shipout:n #1
 {
  \int_compare:nT { \c_tobi_shell_escape_int > 0 }
   {  \iow_shipout:Nn \c_tobi_eighteen_int { #1 } }
 }
\cs_generate_variant:Nn \tobi_system_exec_now:n { x }
\cs_generate_variant:Nn \tobi_system_exec_shipout:n { x }

%% Examples
\tobi_system_exec_now:n { ls }
\tl_set:Nn \l_tmpa_tl { ls }
\tobi_system_exec_now:x { \l_tmpa_tl }

You probably would want to add messages telling that things went wrong when shell escape is disabled; or just remove the \int_compare:nT thing.

I used the tobi prefix because using \system is probably a bad idea (I don't want to get slapped by Joseph on chat, you know).

Answer 2

Since this question was answered, LaTeX3 has evolved. See section 10.7 Access to the shell, on interface3.pdf. In particular,

\immediate\write18

is replaced by

\sys_shell_now:n

and its :x variant.