Thank you for this. I think I understand. I just have a little problem understanding the letter g in %03g (I am not a computer scientist even if already wrote programms in Java and C++). . I think I have to read the c printf function quietly.

I read a bit about it but I cannot pretend I master it.
I am very interrested in learing more as I am a beginner with bash scipting

A few examples would be welcome.

Ok thank you again.
For my problem instead of using an index, I'll use a date based extension to the name like this :

ffmpeg -f alsa -ac 1 -i hw:1,0 -acodec flac -f x11grab -s $LARGEUR"x"$HAUTEUR -i :0.0+$DECALAGE_FENETRE -r 15 -vcodec libx264 -vpre lossless_ultrafast  -y video`date +%Y`.`date +%m`.`date +%H`.`date +%M`.`date +%S`.mkv &

I went a little further

This works (removing double quotes) :

filename=~/Bureau/video

index=1

mystring=${filename}${index}.mkv



ffmpeg -f alsa -ac 1 -i hw:1,0 -acodec flac -f x11grab -s $LARGEUR"x"$HAUTEUR -i :0.0+$DECALAGE_FENETRE -r 15 -vcodec libx264 -vpre lossless_ultrafast  -y  ${mystring}

filename=/home/jaaf/Bureau/video #I am jaaf

index=1

mystring=${filename}${index}.mkv



ffmpeg -f alsa -ac 1 -i hw:1,0 -acodec flac -f x11grab -s $LARGEUR"x"$HAUTEUR -i :0.0+$DECALAGE_FENETRE -r 15 -vcodec libx264 -vpre lossless_ultrafast  -y  ${mystring}

How comes ?

here is the full script (my first one -adaptation - so be indulgent)

#!/bin/sh

#la ligne précédente indique que c'est un script bash. IL ne doit y avoir aucun caractère ou espace avant.

#Une ligne qui commence par un dièse est un commentaire. Comme cette ligne.

#Nous informons l'utilisateur

echo "\n Vous démarrez un enregistrement vidéo. Veuillez cliquer sur la  fenêtre à enregistrer\n"

#Nous demandons au système les informations sur la fenêtre cliquée

INFO_FENETRE=$(xwininfo -frame)

#la commande attend le clic

#Nous affichons les informations

echo "\n Information sur la fenêtre \n" $INFO_FENETRE+"\n\n"

LARGEUR=$(echo $INFO_FENETRE | grep -oEe 'Width: [0-9]+' | grep -oEe '[0-9]+' )

echo "Largeur trouvée : "$LARGEUR

LARGEUR=$(($LARGEUR/2))

LARGEUR=$(($LARGEUR*2))

echo "Largeur corrigée : "$LARGEUR

HAUTEUR=$(echo $INFO_FENETRE | grep -oEe 'Height: [0-9]+' | grep -oEe '[0-9]+')

echo "Hauteur trouvée : "$HAUTEUR

HAUTEUR=$(($HAUTEUR/2))

HAUTEUR=$(($HAUTEUR*2))

echo "Hauteur corrigée : "$HAUTEUR

DECALAGE_FENETRE=$( echo $INFO_FENETRE | grep -oEe 'Corners:\s+\+[0-9]+\+[0-9]+' | grep -oEe '[0-9]+\+[0-9]+' | sed -e 's/\+/,/' )

echo "Décalage fenêtre: "$DECALAGE_FENETRE"\n\n"

#nous affichons la commande pour détection d'erreur s'il y a lieu.

#echo "\n ffmpeg -f alsa -ac 1 -i hw:1,0 -acodec flac -f x11grab -s "$LARGEUR"x$HAUTEUR -i :0.0+$DECALAGE_FENETRE -r 10 -vcodec libx264 -vpre lossless_ultrafast ~/Bureau/essai.mkv "+"\n\n"

#la commande proprement dite



filename="~/Bureau/video"

index=1 #will deal with index later

mystring="${filename}${index}.mkv"

echo "ffmpeg -f alsa -ac 1 -i hw:1,0 -acodec flac -f x11grab -s $LARGEUR"x"$HAUTEUR -i :0.0+$DECALAGE_FENETRE -r 15 -vcodec libx264 -vpre lossless_ultrafast  -y ${mystring}"

the output is

ffmpeg -f alsa -ac 1 -i hw:1,0 -acodec flac -f x11grab -s 1042x360 -i :0.0+ -r 15 -vcodec libx264 -vpre lossless_ultrafast  -y ~/Bureau/video1.mkv

and the output command works!

hi, thank you for your help.
I did what you suggested and it works fine.

But what to do after that ? I tried to put the last echo line between back tits but the error message is still the same.

~/Bureau/video1.mkv: No such file or directory

Hi,
I am trying to write a script that records a window on my screen. No pb with that.
To go further I would like to choose an incremental fileName for output incrementing if the fileName already exist.
I have trouble doing it so before dealing with the incrementation itself, I want to check I can substitute the calculated name in the ffmpeg command.

filename="~/Bureau/essai" 



index=1 #I will deal with the incrementation of index after this part works 



mystring="${fileName}${index}.mkv" #omce I have a proper index I concatenate the name with index and name extension 



echo ${mystring} # gives ~/Bureau/essai1.mkv seems to be what I expect 



ffmpeg -f alsa .....  -y ${mystring}  

I get this message

~/Bureau/essai1.mkv No such file or directory

despite -y to force overwriting

However if I put the file name directly in the command like this

ffmpeg -f alsa .....  -y ~/Bureau/essai1.mkv

it works perfectly. My question is : "What is wrong ?"