Is there any way or command in Tcl for writing in the middle of {data.txt} and also specific line number ... ? for example after writing data in text file, when I'm writing in line number 1000, is there any way for turning back to line number 20 and adding the data in this line for output. (something look like llappend & append for list variables, but in puts command)
You can use seek to move the current location in a channel opened on a file (it's not meaningful for pipes, sockets and serial lines); the next read (gets, read) or write (puts) will happen at that point. Except in append mode, when writes always go to the end.
seek $channel $theNewLocation
However, the location to seek to is in bytes; the only locations that it is trivially easy to go to are the start of the file and the end of the file (the latter using end-based indexing). Thus you need to either remember where “line 20” really is from the first time, or go to the start and read forward a few lines.
seek $channel 0
for {set i 0} {$i < 20} {incr i} {gets $channel}
Also be aware that the data afterwards does not shuffle up or down to accommodate what you've written the second time. If you don't write exactly the same length of data as was already there, you end up with partial lines in the file. Truncating the file with chan truncate might help, but might also be totally the wrong thing to do. If you're going to go back and patch a text file, writing an extra long line where you're going to do the patch (e.g., with lots of spaces on the end) can make it much easier. It's a cheesy hack, but simple to do.