I have a file, I want to determine if it contains only English ASCII characters.

I thought this is a very common question, but when I googled it, there is no direct answer or related.

According to some other questions, this is what I got so far, but this doesn't work. No matter ASCII or Non-ASCII chars, it will always determine as non-ascii.

if (LC_ALL=C; [[ $TEXT = *[[:ascii:]]* ]]) then
  echo "Contain Non-ASCII"

BTW, This is a shell script.


Your code tests whether the value of the variable TEXT contains at least one ASCII character. If you want to test whether $TEXT contains non-ASCII characters, you need to invert the set. By the way, [:ascii:] is locale-independent, so you don't need to set LC_CTYPE.

if [[ $TEXT = *[![:ascii:]]* ]]; then
  echo "Contain Non-ASCII"

Note that [:ascii:] is a bash (and zsh) feature. If you want your script to work in ksh or plain sh, you will need to make use of the locale. Here's a portable method:

case $TEXT in
  *[![:cntrl:][:print:]]*) echo "Contain Non-ASCII";;

If you want to check the file contents, use grep.

if LC_ALL=C grep -q '[^[:cntrl:][:print:]]' somefile; then
  echo "Contain Non-ASCII"
  • 1
    Fine! How to tell where it is?
    – Notinlist
    Apr 7 '18 at 14:58
  • yeah, how can i print the non-ascii char that was found ? Apr 26 '18 at 10:13
  • @DavidGidony Remove the -q option from the grep call so that it prints the offending lines, maybe add -n to print the line number. Apr 26 '18 at 10:32
  • The example with grep doesn't work for me. It always says Contains Non-ASCII. Where as the following command shows me that it has Non-ASCII characters: grep --color='auto' -P -n "[\x80-\xFF]" somefile or LANG=c sed 's,[\x00-\x7F],,g' somefile > somefile_without_ascii creates a new non-zero byte file. Also this (from yourself about 5 years prior to this answer) stackoverflow.com/a/3208902/319542 works! :) Jun 24 at 16:17
  • 1
    @PraveenLobo I fixed a mistake in the grep command. Jun 24 at 16:35

The file command can tell you the type of a file (ASCII, unicode, binary, etc.).

$ file my_file.txt
my_file.txt: ASCII text

So you can just check to see that the output contains the word "ASCII" and this should work:

if [[ file my_file.txt | grep -i ascii ]] ...
  • 2
    FYI - Caution grepping for "ASCII" alone. Other things than plain ascii may come from "file". For instance: Non-ISO extended-ASCII text
    – BRebey
    Nov 1 '19 at 17:44

RESULT=$(echo $TEXT | LC_COLLATE=C grep -r '[^ -~]')
if [ -z "$RESULT" ]; then
    echo "ALL ASCII"
    echo "Contain Non-ASCII"

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