# Executing commands

This section documents the following functions:

-   `lammps_file`{.interpreted-text role="cpp:func"}
-   `lammps_command`{.interpreted-text role="cpp:func"}
-   `lammps_commands_list`{.interpreted-text role="cpp:func"}
-   `lammps_commands_string`{.interpreted-text role="cpp:func"}

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Once a LAMMPS instance is created, there are multiple ways to \"drive\"
a simulation. In most cases it is easiest to process single or multiple
LAMMPS commands like in an input file. This can be done through reading
a file or passing single commands or lists of commands or blocks of
commands with the following functions.

Via these functions, the calling code can have LAMMPS act on a series of
[input file commands](Commands_all) that are either read from a file or
passed as strings. For example, this allows setup of a problem from an
input script, and then running it in stages while performing other
operations in between or concurrently. The caller can interleave the
LAMMPS function calls with operations it performs, such as calls to
extract information from or set information within LAMMPS, or calls to
another code\'s library.

Just as with [input script parsing](Commands_parse) comments can be
included in the file or strings, and expansion of variables with
`${name}` or `$(expression)` syntax is performed. Below is a short
example using some of these functions.

``` c
/* define to make the otherwise hidden prototype for "lammps_open()" visible */
#define LAMMPS_LIB_MPI
#include "library.h"

#include <mpi.h>
#include <stdio.h>

int main(int argc, char **argv)
{
  void *handle;
  int i;

  MPI_Init(&argc, &argv);
  handle = lammps_open(0, NULL, MPI_COMM_WORLD, NULL);
  lammps_file(handle,"in.sysinit");
  lammps_command(handle,"run 1000 post no");

  for (i=0; i < 100; ++i) {
    lammps_commands_string(handle,"run 100 pre no post no\n"
                                  "print 'PE = $(pe)'\n"
                                  "print 'KE = $(ke)'\n");
  }
  lammps_close(handle);
  MPI_Finalize();
  return 0;
}
```

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::: {.doxygenfunction project="progguide"}
lammps_file
:::

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::: {.doxygenfunction project="progguide"}
lammps_command
:::

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::: {.doxygenfunction project="progguide"}
lammps_commands_list
:::

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::: {.doxygenfunction project="progguide"}
lammps_commands_string
:::