Question

In my script I’m trying to take a string, then output the extension of a string if it has one. So essentially I take the basename of a file, then output anything that comes after a period.

What’s the syntax to do something like this?

For example

dotcutter.sh
file=testfile.jpg

the script should output .jpg

EDIT:

I’ve solved my problem now with:

file=$(basename "$1" )
stub=${file%.*}
echo ${file#"$stub"}

Which reduces my argument to a basename, thank you all.

Answer 1

$ file=testfile.jpg
$ echo ${file##*.}
jpg
$ file=testfile.ext1.ext2
$ echo ${file##*.}
ext2
$ file=noextension
$ echo ${file##*.}
noextension

Notice that it doesn’t work if the file has no extension. If that’s important then try this two-step solution:

$ file=testfile.ext1.ext2
$ stub=${file%.*}
$ echo ${file#"$stub"}
.ext2

Or this regex-based one, which will only call echo if there’s actually an extension. (&& is shorthand for “if then”.)

$ regex='(\.[^.]*)$'
$ file=testfile.ext1.ext2
$ [[ $file =~ $regex ]] && echo ${BASH_REMATCH[1]}
.ext2
$ file=noextension
$ [[ $file =~ $regex ]] && echo ${BASH_REMATCH[1]}

See also:

• basename(1)
• dirname(1)
• Bash FAQ: How can I use parameter expansion? How can I get substrings? How can I get a file without its extension, or get just a file’s extension?]