Ubuntu – How to know the encoding Gedit assumes for a given file

iconv is probably what you'll want to use. iconv -l will show you the available encodings and then you can use a couple of commands to recode them all:

# all text files are in ./originals/
# new files will be written to ./newversions/

mkdir -p newversions
cd originals
for file in *.txt; do
    cat $file | iconv -f ASCII -t utf-8 > ../newversions/$file;
done

If you want to do this with files you don't the encoding of (because they're all over the place), you want to bring in a few more commands: find, file, awk and sed. The last two are just there to process the output of file.

for file in find . -type f -exec file --mime {} \; | grep "ascii" | awk '{print $1}' | sed s/.$//; do
    ...

I've no idea if this actually works so I certainly wouldn't run it from anything but the least important directory you have (make a testing folder with some known ASCII files in). The syntax of find might preclude it from being within a for loop. I'd hope that somebody else with more bash experience could jump in there and sort it out so it does the right thing.

These Windows files with Persian text are encoded in Windows-1256. So it can be deciphered by command similar to OP tried, but with different charsets. Namely:

recode Windows-1256..UTF-8 <Windows_file.txt > UTF8_file.txt
(denounced upon original poster’s complaints; see comments)

iconv -f Windows-1256 Windows_file.txt > UTF8_file.txt

This one assumes that the LANG environment variable is set to a UTF-8 locale. To convert to any encoding (UTF-8 or otherwise), regardless of the current locale, one can say:

iconv -f Windows-1256 Windows_file.txt -t ${output_encoding} > ${output_file}

Original poster is also confused with semantic of text recoding tools (recode, iconv). For source encoding (source.. or -f) one must specify encoding with which the file is saved (by the program that created it). Not some (naïve) guesses based on mojibake characters in programs that try (but fail) to read it. Trying either ISO-8859-15 or WINDOWS-1252 for a Persian text was obviously an impasse: these encodings merely do not contain any Persian letter.