### 5.2.4 Shell-out to Operating System

`Pre_Scene_Command=`s | Set command before entire scene
`Pre_Frame_Command=`s | Set command before each frame
`Post_Scene_Command=`s | Set command after entire scene
`Post_Frame_Command=`s | Set command after each frame
`User_Abort_Command=`s | Set command when user aborts POV-Ray
`Fatal_Error_Command=`s | Set command when POV-Ray has fatal error

Note: no `+` or `-` switches are available for these options. They cannot be used from the command line. They may only be used from INI files.

POV-Ray offers you the opportunity to shell-out to the operating system at several key points to execute another program or batch file. Usually this is used to manage files created by the internal animation loop however the shell commands are available for any scene. The string s is a single line of text which is passed to the operating system to execute a program. For example

     Post_Scene_Command=tga2gif -d -m myfile

would use the utility `tga2gif` with the `-D` and `-M` parameters to convert `myfile.tga` to `myfile.gif` after the scene had finished rendering.

Note: individual platforms may provide means of preventing shell-outs from occurring. For example, the Windows version provides a menu command to turn shell-outs off (which is the default setting for that platform). The reason for this (along with file I/O restrictions) is to attempt to prevent untrusted INI files from doing harm to your system.

#### 5.2.4.1 String Substitution in Shell Commands

It could get cumbersome to change the `Post_Scene_Command` every time you changed scene names. POV-Ray can substitute various values into a command string for you. For example:

     Post_Scene_Command=tga2gif -d -m %s

POV-Ray will substitute the `%s` with the scene name in the command. The scene name is the `Input_File_Name` or `+I` setting with any drive, directory and extension removed. For example:

     Input_File_Name=c:\povray3\scenes\waycool.pov

is stripped down to the scene name `waycool` which results in...

     Post_Scene_Command=tga2gif -d -m waycool

In an animation it may be necessary to have the exact output file name with the frame number included. The string `%o` will substitute the output file name. Suppose you want to save your output files in a zip archive using the utility program `pkzip`. You could do...

     Post_Frame_Command=pkzip -m %s %o

After rendering frame 12 of `myscene.pov` POV-Ray would shell to the operating system with

     pkzip -m myscene mysce012.tga

The `-M` switch in `pkzip` moves `mysce012.tga` to `myscene.zip` and removes it from the directory. Note that `%o` includes frame numbers only when in an animation loop. During the `Pre_Scene_Command` and `Post_Scene_Command` there is no frame number so the original, unnumbered `Output_File_Name` is used. Any `User_Abort_Command` or `Fatal_Error_Command` not inside the loop will similarly give an unnumbered `%o` substitution.

Here is the complete list of substitutions available for a command string.

`%o` | Output file name with extension and embedded frame number if any
`%s` | Scene name derived by stripping path and ext from input name
`%n` | Frame number of this frame
`%k` | Clock value of this frame
`%h` | Height of image in pixels
`%w` | Width of image in pixels
`%%` | A single % sign.

#### 5.2.4.2 Shell Command Sequencing

Here is the sequence of events in an animation loop. Non-animated scenes work the exact same way except there is no loop.

1. Process all INI file keywords and command line switches just once.
2. Open any text output streams and do Create_INI if any.
3. Execute Pre_Scene_Command if any.
4. Loop through frames (or just do once on non-animation).
   1. Execute Pre_Frame_Command if any.
   2. Parse entire scene file, open output file and read settings, turn on display, render the frame, destroy all objects, textures etc., close output file, close display.
   3. Execute Post_Frame_Command if any.
   4. Repeat above steps until all frames are done.
5. Execute Post_Scene_Command if any.
6. Finish

If the user interrupts processing the `User_Abort_Command`, if any, is executed. User aborts can only occur during the parsing and rendering parts of step (4b) above. If a fatal error occurs that POV-Ray notices the `Fatal_Error_Command`, if any, is executed. Sometimes an unforeseen bug or memory error could cause a total crash of the program in which case there is no chance to shell out. Fatal errors can occur just about anywhere including during the processing of switches or INI files. If a fatal error occurs before POV-Ray has read the `Fatal_Error_Command` string then obviously no shell can occur.

Note: the entire scene is re-parsed for every frame. Future versions of POV-Ray may allow you to hold over parts of a scene from one frame to the next but for now it starts from scratch every time.

Note: that the `Pre_Frame_Command` occurs before the scene is parsed. You might use this to call some custom scene generation utility before each frame. This utility could rewrite your `.pov` or `.inc` files if needed. Perhaps you will want to generate new `.gif` or `.tga` files for image maps or height fields on each frame.

#### 5.2.4.3 Shell Command Return Actions

`Pre_Scene_Return=`s | Set pre scene return actions
`Pre_Frame_Return=`s | Set pre frame return actions
`Post_Scene_Return=`s | Set post scene return actions
`Post_Frame_Return=`s | Set post frame return actions
`User_Abort_Return=`s | Set user abort return actions
`Fatal_Error_Return=`s | Set fatal return actions

Note: that no `+` or `-` switches are available for these options. They cannot be used from the command line. They may only be used from INI files.

Most operating systems allow application programs to return an error code if something goes wrong. When POV-Ray executes a shell command it can make use of this error code returned from the shell process and take some appropriate action if the code is zero or non-zero. POV-Ray itself returns such codes. It returns 0 for success, 1 for fatal error and 2 for user abort.

The actions are designated by a single letter in the different `..._Return`=s options. The possible actions are:

`I` | ignore the code
`S` | skip one step
`A` | all steps skipped
`Q` | quit POV-Ray immediately
`U` | generate a user abort in POV-Ray
`F` | generate a fatal error in POV-Ray

For example if your `Pre_Frame_Command` calls a program which generates your height field data and that utility fails then it will return a non-zero code. We would probably want POV-Ray to abort as well. The option `Pre_Frame_Return=F` will cause POV-Ray to do a fatal abort if the `Pre_Frame_Command` returns a non-zero code.

Sometimes a non-zero code from the external process is a good thing. Suppose you want to test if a frame has already been rendered. You could use the `S` action to skip this frame if the file is already rendered. Most utilities report an error if the file is not found. For example the command...

     pkzip -V myscene mysce012.tga

tells pkzip you want to view the catalog of `myscene.zip` for the file `mysce012.tga`. If the file isn't in the archive `pkzip` returns a non-zero code.

However we want to skip if the file is found. Therefore we need to reverse the action so it skips on zero and doesn't skip on non-zero. To reverse the zero vs. non-zero triggering of an action precede it with a "`-`" sign (note a "`!`" will also work since it is used in many programming languages as a negate operator).

`Pre_Frame_Return=S` will skip if the code shows error (non-zero) and will proceed normally on no error (zero). `Pre_Frame_Return=-S` will skip if there is no error (zero) and will proceed normally if there is an error (non-zero).

The default for all shells is `I` which means that the return action is ignored no matter what. POV-Ray simply proceeds with whatever it was doing before the shell command. The other actions depend upon the context. You may want to refer back to the animation loop sequence chart in the previous section "Shell Command Sequencing". The action for each shell is as follows.

On return from any User_Abort_Command if there is an action triggered...

...and you have specified... | ...then POV-Ray will..
`F` | Then turn this user abort into a fatal error. Do the `Fatal_Error_Command`, if any. Exit POV-Ray with error code 1.
`S`, `A`, `Q`, or `U` | Then proceed with the user abort. Exit POV-Ray with error code 2.

On return from any `Fatal_Error_Command` then POV-Ray will proceed with the fatal error no matter what. It will exit POV-Ray with error code 1.

On return from any `Pre_Scene_Command`, `Pre_Frame_Command`, `Post_Frame_Command` or `Post_Scene_Commands` if there is an action triggered...

...and you have specified... | ...then POV-Ray will...
`F` | ...turn this user abort into a fatal error. Do the `Fatal_Error_Command`, if any. Exit POV-Ray with error code 1.
`U` | ...generate a user abort. Do the `User_Abort_Command`, if any. Exit POV-Ray with an error code 2.
`Q` | ..quit POV-Ray immediately. Acts as though POV-Ray never really ran. Do no further shells, (not even a `Post_Scene_Command`) and exit POV-Ray with an error code 0.

On return from a `Pre_Scene_Command` if there is an action triggered...

...and you have specified... | ...then POV-Ray will...
`S` | ...skip rendering all frames. Acts as though the scene completed all frames normally. Do not do any `Pre_Frame_Command` or `Post_Frame_Commands`. Do the `Post_Scene_Command`, if any. Exit POV-Ray with error code 0. On the earlier chart this means skip step #4.
`A` | ...skip all scene activity. Works exactly like `Q` quit. On the earlier chart this means skip to step #6. Acts as though POV-Ray never really ran. Do no further shells, (not even a `Post_Scene_Command`) and exit POV-Ray with an error code 0.

On return from a `Pre_Frame_Command` if there is an action triggered...

...and you have specified... | ...then POV-Ray will...
`S` | ...skip only this frame. Acts as though this frame never existed. Do not do the `Post_Frame_Command`. Proceed with the next frame. On the earlier chart this means skip steps (4b) and (4c) but loop back as needed in (4d).
`A` | ...skip rendering this frame and all remaining frames. Acts as though the scene completed all frames normally. Do not do any further `Post_Frame_Commands`. Do the `Post_Scene_Command`, if any. Exit POV-Ray with error code 0. On the earlier chart this means skip the rest of step (4) and proceed at step (5).

On return from a `Post_Frame_Command` if there is an action triggered...

...and you have specified... | ...then POV-Ray will...
`S` or `A` | ...skip all remaining frames. Acts as though the scene completed all frames normally. Do not do any further `Post_Frame_Commands`. Do the `Post_Scene_Command`, if any. Exit POV-Ray with error code 0. On the earlier chart this means skip the rest of step (4) and proceed at step (5).

On return from any `Post_Scene_Command` if there is an action triggered and you have specified `S` or `A` then no special action occurs. This is the same as `I` for this shell command.