How can I re-add a unicode byte order marker in linux?

Question

I have a rather large SQL file which starts with the byte order marker of FFFE. I have split this file using the unicode aware linux split tool into 100,000 line chunks. But whe

Answer 1

Matthew Flaschen's answer is a good one, however it has a couple of flaws.

• There's no check that the copy succeeded before the original file is truncated. It would be better to make everything contingent on a successful copy, or test for the existence of the temporary file, or to operate on the copy. If you're a belt-and-suspenders kind of person, you'd do a combo as I've illustrated below
• The ls is unnecessary.
• I'd use a better variable name than "i" - perhaps "file".

Of course, you could be very paranoid and check for the existence of the temporary file at the beginning so you don't accidentally overwrite it and/or use a UUID or a generated file name. One of mktemp, tempfile or uuidgen would do the trick.

td=TMPDIR
export TMPDIR=

usertemp=~/temp            # set this to use a temp directory on the same filesystem
                           # you could use ./temp to ensure that it's one the same one
                           # you can use mktemp -d to create the dir instead of mkdir

if [[ ! -d $usertemp ]]    # if this user temp directory doesn't exist
then                       # then create it, unless you can't 
    mkdir $usertemp || export TMPDIR=$td    # if you can't create it and TMPDIR is/was
fi                                          # empty then mktemp automatically falls
                                            # back to /tmp

for file in *.sql
do
    # TMPDIR if set overrides the argument to -p
    temp=$(mktemp -p $usertemp) || { echo "$0: Unable to create temp file."; exit 1; }

    { printf '\xFF\xFE' > "$temp" &&
    cat "$file" >> "$temp"; } || { echo "$0: Write failed on $file"; exit 1; }

    { rm "$file" && 
    mv "$temp" "$file"; } || { echo "$0: Replacement failed for $file; exit 1; }
done
export TMPDIR=$td

Traps might be better than all the separate error handlers I've added.

No doubt all this extra caution is overkill for a one-shot script, but these techniques can save you when push comes to shove, especially in a multi-file operation.