Ash shell (busybox), check file extension?

Question

I'm porting a script from bash to the busybox shell, ash - how can I check what file extension a file has (and strip that extension to get a generate a directory name) ?

The bash code fragment looks like this:

 if [[ "$filename" =~ '\\.tar\\.gz' ]]; then tar -xzf "$filename" dirname="${filename::${#filename}-7}" elif [[ "$filename" =~ '\\.tar\\.bz2' ]]; then tar -xjf "$filename" dirname="${filename::${#filename}-8}" elif [[ "$filename" =~ '\\.tar\\.xz' ]]; then xzcat "$filename" | tar -x dirname="${filename::${#filename}-7}" else echo "Unknown extension for file: ${filename}" >&2 exit 1

On running the script it contains I get output showing that the code to check the extension is not recognised:

Unknown extension for file: zlib-1.2.11.tar.gz

Answer 1

Use case . Use basename to remove the extension (which I find cleaner then ${var::} expansion anyway).

case "$filename" in
*.tar.gz)
    tar something
    dirname=$(basename "$filename" .tar.gz)
    ;;
*.tar.bz2)
    tar something
    dirname=$(basename "$filename" .tar.gz2)
    ;;
# and so on
esac

Answer 2

Use the expr command for regular expression matching, and the % operator to strip a known suffix. expr implicitly anchors all regular expressions to the beginning of the string, so you'll need to start each one with .* to match a suffix. expr also writes the number of characters matched to standard output, but you probably don't care to see that.

if expr "$filename" : '.*\.tar\.gz' > /dev/null; then
    tar -xzf "$filename"
    dirname="${filename%.tar.gz}"
elif expr "$filename" : '.*\.tar\.bz2' > /dev/null; then
    tar -xjf "$filename"
    dirname="${filename%.tar.gz}"
elif expr "$filename" : '.*\.tar\.xz' > /dev/null; then
    xzcat "$filename" | tar -x
    dirname="${filename%.tar.zx}"
else
    echo "Unknown extension for file: ${filename}" >&2
    exit 1
fi