# fix print command

## Syntax

    fix ID group-ID print N string keyword value ...

-   ID, group-ID are documented in [fix](fix) command

-   print = style name of this fix command

-   N = print every N steps; N can be a variable (see below)

-   string = text string to print with optional variable names

-   zero or more keyword/value pairs may be appended

-   keyword = *file* or *append* or *screen* or *title*

        *file* value = filename
        *append* value = filename
        *screen* value = *yes* or *no*
        *title* value = string
          string =  text to print as 1st line of output file

## Examples

``` LAMMPS
fix extra all print 100 "Coords of marker atom = $x $y $z"
fix extra all print 100 "Coords of marker atom = $x $y $z" file coord.txt
```

## Description

Print a text string every N steps during a simulation run. This can be
used for diagnostic purposes or as a debugging tool to monitor some
quantity during a run. The text string must be a single argument, so it
should be enclosed in single or double quotes if it is more than one
word. If it contains variables it must be enclosed in double quotes to
ensure they are not evaluated when the input script line is read, but
will instead be evaluated each time the string is printed.

::: versionadded
15Jun2023

support for vector style variables
:::

See the [variable](variable) command for a description of *equal* and
*vector* style variables which are typically the most useful ones to use
with the print command. Equal- and vector-style variables can calculate
formulas involving mathematical operations, atom properties, group
properties, thermodynamic properties, global values calculated by a
[compute](compute) or [fix](fix), or references to other
[variables](variable). Vector-style variables are printed in a
bracketed, comma-separated format, e.g. \[1,2,3,4\] or
\[12.5,2,4.6,10.1\].

:::: note
::: title
Note
:::

As discussed on the [Commands parse](Commands_parse) doc page, the text
string can use \"immediate\" variables, specified as \$(formula) with
parenthesis, where the numeric formula has the same syntax as
equal-style variables described on the [variable](variable) doc page.
This is a convenient way to evaluate a formula immediately without using
the variable command to define a named variable and then use that
variable in the text string. The formula can include a trailing colon
and format string which determines the precision with which the numeric
value is output. This is also explained on the [Commands
parse](Commands_parse) doc page.
::::

Instead of a numeric value, N can be specified as an [equal-style
variable](variable), which should be specified as v_name, where name is
the variable name. In this case, the variable is evaluated at the
beginning of a run to determine the **next** timestep at which the
string will be written out. On that timestep, the variable will be
evaluated again to determine the next timestep, etc. Thus the variable
should return timestep values. See the stagger() and logfreq() and
stride() math functions for [equal-style variables](variable), as
examples of useful functions to use in this context. For example, the
following commands will print output at timesteps
10,20,30,100,200,300,1000,2000,etc:

``` LAMMPS
variable        s equal logfreq(10,3,10)
fix extra all print v_s "Coords of marker atom = $x $y $z"
```

The specified group-ID is ignored by this fix.

If the *file* or *append* keyword is used, a filename is specified to
which the output generated by this fix will be written. If *file* is
used, then the filename is overwritten if it already exists. If *append*
is used, then the filename is appended to if it already exists, or
created if it does not exist.

If the *screen* keyword is used, output by this fix to the screen and
logfile can be turned on or off as desired.

The *title* keyword allow specification of the string that will be
printed as the first line of the output file, assuming the *file*
keyword was used. By default, the title line is as follows:

    # Fix print output for fix ID

where ID is replaced with the fix-ID.

## Restart, fix_modify, output, run start/stop, minimize info

No information about this fix is written to [binary restart
files](restart). None of the [fix_modify](fix_modify) options are
relevant to this fix. No global or per-atom quantities are stored by
this fix for access by various [output commands](Howto_output). No
parameter of this fix can be used with the *start/stop* keywords of the
[run](run) command. This fix is not invoked during [energy
minimization](minimize).

## Restrictions

> none

## Related commands

[variable](variable), [print](print)

## Default

The option defaults are no file output, screen = yes, and title string
as described above.