Path parsing

Parsing is the ability to parse, i.e. to write strings based on data contained in the FITS header. Path parsing, introduced in Siril 1.2.0, aims at giving more flexibility to scripting by using header data to write/read filenames or paths. For now, this is used with the following commands:

and of course, their GUI counterparts.

Syntax example

We will take an easy example to begin with. Say you have a file named and you would like to locate a masterdark from your masters library that matches the characteristics of said light. As you have chosen a convention to name your masterdarks, you know that the correct dark should be named something like:


with the terms between quotes replaced with the values read from your light header. For an exposure of 120s, temperature of -10C, gain/offset of 120/30 and binning 1, the masterdark would be named:

Well, that's exactly what this feature allows to do. If you specify the dark name with the conventions explained just after, you can tell Siril to open the light image, read its header and use its values to write such string (and then use it to calibrate your light).

You can read the info contained in the header either through dumpheader command or by right-clicking on an opened image and selecting FITS Header.... You would normally get a print like the one below (some keys removed for brevity):

SIMPLE  =                    T / C# FITS
BITPIX  =                   16 /
NAXIS   =                    2 / Dimensionality
NAXIS1  =                 4144 /
NAXIS2  =                 2822 /
BZERO   =                32768 /
EXTEND  =                    T / Extensions are permitted
IMAGETYP= 'LIGHT'              / Type of exposure
EXPOSURE=                120.0 / [s] Exposure duration
DATE-OBS= '2022-01-24T01:03:34.729' / Time of observation (UTC)
XBINNING=                    1 / X axis binning factor
YBINNING=                    1 / Y axis binning factor
GAIN    =                  120 / Sensor gain
OFFSET  =                   30 / Sensor gain offset
INSTRUME= 'ZWO ASI294MC Pro'   / Imaging instrument name
SET-TEMP=                -10.0 / [degC] CCD temperature setpoint
CCD-TEMP=                -10.0 / [degC] CCD temperature
BAYERPAT= 'RGGB'               / Sensor Bayer pattern
TELESCOP= '61EDPH'             / Name of telescope
FILTER  = 'DualBand'           / Active filter name
OBJECT  = 'Rosette Nebula     '/ Name of the object of interest
OBJCTRA = '06 30 36'           / [H M S] RA of imaged object
OBJCTDEC= '+04 58 51'          / [D M S] Declination of imaged object
RA      =     97.6960081674312 / [deg] RA of telescope
DEC     =     4.99212765957446 / [deg] Declination of telescope

The format used to specify the dark name would then be:


All the terms to be parsed are formed as follows: $KEY:fmt$

For instance, $EXPTIME:%d$ will be parsed to 120 if the light have been exposed for 120s. But would be parsed to 120.0 if you specify $EXPTIME:%0.1f$, thanks to the formatter %x.yf.

The full expression above would therefore evaluate to:

In this first example, we have only used conversion to integers with %d. But there are other conventional formatters that you can use:

  • %x.yf for floats

  • %s for strings


For strings, leading and trailing spaces are always removed, while spaces within the strings are replaced with _ signs. Example: $OBJECT:%s$ would be converted to Rosette_Nebula.

You can also use some less conventional formatters:

  • In order to parse a date from a date-time header key, you can use the special non-standard formatter dm12, which means date minus 12h. In the header above, the key DATE-OBS has a value of '2022-01-24T01:03:34.729'. $DATE-OBS:dm12$ would convert to '2022-01-23', which was the date at the start of the night. You can also use special formatter dm0 which will just parse the date, without substracting 12h.

  • In order to parse a date-time from a date-time header key, you can use the special non-standard formatter dt, which just means date-time. In the header above, the key DATE-OBS has a value of '2022-01-24T01:03:34.729'. $DATE-OBS:dt$ would then convert to '2022-01-24_01-03-34'.

  • In order to parse RA and DEC info from OBJCTRA and OBJCTDEC header keys, you can use the special non-standard formatters ra and dec. In the header above, the keys OBJCTRA and OBJCTDEC have a value of '06 30 36' and '+04 58 51' respectively. $OBJCTRA:ra$_$OBJCTDEC:dec$ would convert to '06h30m36s_+04d58m51s'.

  • In order to parse RA and DEC info from RA and DEC header keys, when in decimal format, you can use the special non-standard formatters ran and decn. In the header above, the keys RA and DEC have a value of '97.6960081674312' and '4.99212765957446' respectively. $RA:ran$_$DEC:decn$ would convert to '06h30m47s_+04d59m32s'.

To test the syntax, you can load an image and use the parse command, as reminded below.

Siril command line

parse str [-r]
Parses the string str using the information contained in the header of the image currently loaded. Main purpose of this command is to debug path parsing of header keys which can be used in other commands.
Option -r specifies the string is to be interpreted in read mode. In read mode, all wilcards defined in string str are used to find a file name matching the pattern. Otherwise, default mode is write mode and wildcards, if any, are removed from the string to be parsed.

If str starts with $def prefix, it will be recognized as a reserved keyword and looked for in the strings stored in gui_prepro.dark_lib, gui_prepro.flat_lib, gui_prepro.bias_lib or gui_prepro.stack_default for $defdark, $defflat, $defbias or $defstack respectively.
The keyword $seqname$ can also be used when a sequence is loaded

Finding a file with path parsing

In the example above, we have seen that we could find the name of a masterdark based on the information contained in the header of the light to be calibrated. This is what is called in the parse command, the read mode.

This behavior is mainly used in conjuction with the calibrate command/tab. In the -dark= option of the command or in the Dark field of the GUI, you can use the syntax described above. So that you are sure that the matching dark will be fetched to calibrate the light. The same is equally applicable to Bias and Flat. You can, of course, also give a full (or relative) path to the file. And the path can also contain expressions of the same kind.

For instance, for flats, you may want to specify the path to a library, which could contain filter or telescope information, as you may have multiple setups. A path like:


Would then evaluate to:


and is a valid value for Flat input.

Of course, if you were to write this as a command in your scripts or in the GUI Flat field every time you calibrate, that could become a bit tedious. That's when reserved keywords come to the rescue. There are 3 reserved keywords for masters:

  • $defdark

  • $defflat

  • $defbias

Their values are set in Preferences ‣ Pre-processing section. You can also specify them through scripting thanks to set command. They correspond to the values of gui_prepro.dark_lib, gui_prepro.flat_lib and gui_prepro.bias_lib.

When their values are set and you chose to use them as defaults, they will be displayed in the fields of the Calibration tab. You can also start to write your scripts using these keywords. The calibration step of a new generic script for a color camera could look like:

calibrate light -dark=$defdark -cc=dark -flat=$defflat -cfa -equalizecfa -debayer

This would pick your masters directly from your libraries and make sure you never mix them up during the calibration step.

Writing a file with path parsing

Now, while it is handy to be able to find the files, it would be equally useful to use this syntax to store your files while stacking. That is exactly what the write mode is about. The syntax can be used in the -out= field of stack and stackall commands, or in the corresponding field in the GUI.

Let's say you want to write a generic script that prepares your master darks each time you renew your library. In the stack line of the script, you could write:

stack dark rej 3 3 -nonorm -out=$defdark

This line ensures that the resulting masterdark will be stored at the right location with the correct filename that can then be fetched to calibrate your lights.

In order to introduce even more flexibility with the stack commands, there are two more reserved keywords:

  • $defstack

  • $sequencename$

As for default masters, $defstack is configured in the same section of the Preferences, or with a set command on gui_prepro.stack_default. For instance, let's assume you have defined $defstack as:


The script line:

stack r_pp_light rej 3 3 -norm=addscale -output_norm -out=$defstack

would save the stack result as:

As of Siril 1.2.0, the default name for stacking is defined as $sequencename$stacked (the '_' sign is added if not present). This is similar to the behavior in previous versions, except it is now explicit that pathparsing is used.

Using wildcards

It could be that you want to use some key value in your masters name that do not match the key value in the frames to be calibrated. With an example, it may be a bit clearer. Say, you want, in your masterflats names, to keep record of their exposure time. Something like:

If you put a field $EXPTIME:%0.2f$ in $defflat, it will end up with an error at calibration step. Simply because the EXPTIME key will be read from the light frame to be calibrated, not a flat.

To deal with this situation, you can use wildcards in the expressions to be parsed:


Note the * symbol placed right before EXPTIME.

What this symbol means is the following:

  • In write mode, so basically when stacking your masterflat, the field EXPOSURE will be used to form the filename to be saved. In the example above, you would then effectively save to

  • In read mode, so when calibrating your lights, the field EXPOSURE will be replaced by *. When searching for such file, Siril will fetch all the files that marches the pattern FLAT_* Hopefully, your naming convention is robust enough so that it finds only one matching file and uses it to calibrate.


In the event Siril finds more than one file in read mode, it will emit a warning in the console and select the most recent file. As it may not produce the desired outcome, you should reconsider your naming convention if this happens.