GIMP using command line

I don't know how to use Gimp for this task, but actually I think the tools from the package imagemagick are better choice for such CLI tasks. This package is widely used as conversion tool on the web server's applications as MediaWiki and WordPress, also it is a back end for some operations performed by graphical apps such PhotoShop. First you need to install the package:

sudo apt install imagemagick

Then use the following command to accomplish the task (reference):

convert input-file.png -set colorspace Gray -separate -average output-file.jpg

If you need to convert all PNG files in the current directory you can use a loop like this:

for f in *.png; do convert "$f" -set colorspace Gray -separate -average "${f%.*}.jpg"; done

1. Open an image is really easy (image.png is the image that you want to open)

   gimp image.png
   

2. Convert RGB image to Grayscale :

   Create a GIMP Script-Fu file (named here dmmConvertPNGtoGrayscale.scm and saved in $HOME/.gimp-2.8/scripts) :

   ; dmmPNGtoGrayscale - GIMP Script-Fu to convert a PNG image to Grayscale
   ;    This Script-Fu must be put in The GIMP's script directory
   ;    (e.g., $HOME/.gimp-1.2/scripts).
   ;    For command-line invocation, use the shell script rgbtogs.sh
   ;    For interactive invocation, run The GIMP and go to
   ;    Xtns -> Script-Fu -> dmm
   ;
   (define (dmmPNGtoGrayscale infile outfile)
      (let* ((image (car (file-png-load 1 infile infile)))
                (drawable (car (gimp-image-active-drawable image)))
             )
   
            (gimp-convert-grayscale image)
   
            (file-png-save 1 image drawable outfile outfile 
                 1 0 0 0 0 0 0 )
               ; 1 Adam7 interlacing?
               ;   0 deflate compression factor (0-9)
               ;     0 Write bKGD chunk?
               ;       0 Write gAMMA chunk?
               ;         0 Write oFFs chunk?
               ;           0 Write tIME chunk?    ?? backwards in DB Browser
               ;             0 Write pHYS chunk?  ?? backwards in DB Browser
      )
   )
   
   (script-fu-register                                 ; I always forget these ...
      "dmmPNGtoGrayscale"                              ; script name to register
      "<Toolbox>/Xtns/Script-Fu/dmm/dmmPNGtoGrayscale" ; where it goes
      "dmm PNG (RGB or Indexed) to Grayscale"          ; script description
      "David M. MacMillan"                             ; author
      "Copyright 2004 by David M. MacMillan; GNU GPL"  ; copyright
      "2004-02-08"                                     ; date
      ""                                               ; type of image
      SF-FILENAME "Infile" "infile.png"                ; default parameters
      SF-FILENAME "Outfile" "outfile.png"
   )
   

   And launch it using this script (I named it rgbtogs.sh for example) :

   # rgbtogs.sh
   # Invoke The GIMP with Script-Fu dmmPNGtoGrayscale.scm
   # No error checking.
   
   if [ -e $1 ] 
   then
      echo "Usage: rgbtogs.sh degrees filebasename"
      echo "Error: Parameter (filename base) required"
      exit 1
   fi
   
   gimp -c -i -d -b "(dmmPNGtoGrayscale \"$1.png\" \"$1-gray.png\")" "(gimp-quit 0)"
   

   Give execution to the script and start it :

   chmod +x rgbtogs.sh
   ./rgbtogs.sh image
   

3. Convert a PNG image to JPEG (or JPG) :

   Create a GIMP Script-Fu file (named here dmmConvertPNGtoJPG.scm and saved in $HOME/.gimp-2.8/scripts) :

   ; dmmConvertPNGtoJPG.scm - GIMP Script-Fu to Convert PNG to JPG
   ;    This Script-Fu must be put in The GIMP's script directory
   ;    (e.g., $HOME/.gimp-1.2/scripts).
   ;    For command-line invocation, use the shell script pngtojpg.sh
   ;    For interactive invocation, run The GIMP and go to
   ;    Xtns -> Script-Fu -> dmm
   ;
   (define (dmmConvertPNGtoJPG infile outfile)
      (let* ((image (car (file-png-load 1 infile infile)))
             (drawable (car (gimp-image-active-drawable image)))
            )
   
            (file-jpeg-save 1 image drawable outfile outfile 
                 0.75 0 1 1 "GIMP" 0 1 0 0 )
               ; 0.75 quality (float 0 <= x <= 1)
               ;      0 smoothing factor (0 <= x <= 1)
               ;        1 optimization of entropy encoding parameter (0/1)
               ;          1 enable progressive jpeg image loading (0/1)
               ;            "xxxx"  image comment
               ;                   0 subsampling option number
               ;                     1 force creation of a baseline JPEG
               ;                       0 frequency of restart markers 
               ;                         in rows, 0 = no restart markers
               ;                         0 DCT algoritm to use 
      )
   )
   
   (script-fu-register                                 ; I always forget these ...
      "dmmConvertPNGtoJPG"                             ; script name to register
      "<Toolbox>/Xtns/Script-Fu/dmm/dmmConvertPNGtoJPG" ; where it goes
      "dmm Convert PNG to JPG"                         ; script description
      "David M. MacMillan"                             ; author
      "Copyright 2004 by David M. MacMillan; GNU GPL"  ; copyright
      "2004-01-27"                                     ; date
      ""                                               ; type of image
      SF-FILENAME "Infile" "infile.png"                ; default parameters
      SF-FILENAME "Outfile" "outfile.png"
   )
   

   And launch it using this script (I named it pngtojpg.sh for example) :

   # pngtojpg.sh
   # Invoke The GIMP with Script-Fu dmmConvertPNGtoJPG.scm
   # No error checking.
   
   if [ -e $1 ] 
   then
      echo "Usage: pngtojpg.sh filebasename"
      echo "Error: Parameter 1 (filename base) required"
      exit 1
   fi
   
   gimp -c -i -d -b "(dmmConvertPNGtoJPG \"$1.png\" \"$1.jpg\")" "(gimp-quit 0)"
   

   Give execution to the script and start it :

   chmod +x pngtojpg.sh
   ./pngtojpg.sh image
   

Source : http://beefchunk.com/documentation/lang/gimp/GIMP-Scripts-Fu.html

NB : inside GIMP Script-Fu file, you can remove all lines which begins with ; character (or everything on the right on this character), these are just comments

NB: These .scm script-fu were made originally for GIMP 1.2, but I tested with GIMP 2.8 and there is no issue

You can run gimp -h to get the different options availabe for gimp

1.Open an image gimp -n image_name.xxx

2.Convert an image to gray-scale I have no idea if it's possible using Gimp navigate these links: https://www.gimp.org/tutorials/Basic_Batch/

https://superuser.com/questions/1334386/how-to-load-an-image-in-gimp-from-the-terminal-and-exiting-afterwards

3.convert png image to jpeg

I suggest using convert command line which is part of the ImageMagick

convert image1.xxx image2.yyy