How to Grab File Extension in Bash

bashfilenamesshell

How do I get the file extension from bash? Here's what I tried:

filename=`basename $filepath`
fileext=${filename##*.}

By doing that I can get extension of bz2 from the path /dir/subdir/file.bz2, but I have a problem with the path /dir/subdir/file-1.0.tar.bz2.

I would prefer a solution using only bash without external programs if it is possible.

To make my question clear, I was creating a bash script to extract any given archive just by a single command of extract path_to_file. How to extract the file is determined by the script by seeing its compression or archiving type, that could be .tar.gz, .gz, .bz2 etc. I think this should involve string manipulation, for example if I get the extension .gz then I should check whether it has the string .tar before .gz — if so, the extension should be .tar.gz.

Best Answer

If the file name is file-1.0.tar.bz2, the extension is bz2. The method you're using to extract the extension (fileext=${filename##*.}) is perfectly valid¹.

How do you decide that you want the extension to be tar.bz2 and not bz2 or 0.tar.bz2? You need to answer this question first. Then you can figure out what shell command matches your specification.

One possible specification is that extensions must begin with a letter. This heuristic fails for a few common extensions like 7z, which might be best treated as a special case. Here's a bash/ksh/zsh implementation:

basename=$filename; fileext=
while [[ $basename = ?*.* &&
         ( ${basename##*.} = [A-Za-z]* || ${basename##*.} = 7z ) ]]
do
  fileext=${basename##*.}.$fileext
  basename=${basename%.*}
done
fileext=${fileext%.}

For POSIX portability, you need to use a case statement for pattern matching.

while case $basename in
        ?*.*) case ${basename##*.} in [A-Za-z]*|7z) true;; *) false;; esac;;
        *) false;;
      esac
do …

Another possible specification is that some extensions denote encodings and indicate that further stripping is needed. Here's a bash/ksh/zsh implementation (requiring shopt -s extglob under bash and setopt ksh_glob under zsh):
```
basename=$filename
fileext=
while [[ $basename = ?*.@(bz2|gz|lzma) ]]; do
  fileext=${basename##*.}.$fileext
  basename=${basename%.*}
done
if [[ $basename = ?*.* ]]; then
  fileext=${basename##*.}.$fileext
  basename=${basename%.*}
fi
fileext=${fileext%.}
```
Note that this considers 0 to be an extension in file-1.0.gz.

¹ _{${VARIABLE##SUFFIX} and related constructs are in POSIX, so they work in any non-antique Bourne-style shell such as ash, bash, ksh or zsh.}

Related Solutions

Rename – Replace Dots with Underscores in Filenames

I believe that this program will do what you want. I have tested it and it works on several interesting cases (such as no extension at all):

#!/bin/bash

for fname in *; do
  name="${fname%\.*}"
  extension="${fname#$name}"
  newname="${name//./_}"
  newfname="$newname""$extension"
  if [ "$fname" != "$newfname" ]; then
    echo mv "$fname" "$newfname"
    #mv "$fname" "$newfname"
  fi
done

The main issue you had was that the ## expansion wasn't doing what you wanted. I've always considered shell parameter expansion in bash to be something of a black art. The explanations in the manual are not completely clear, and they lack any supporting examples of how the expansion is supposed to work. They're also rather cryptic.

Personally, I would've written a small script in sed that fiddled the name the way I wanted, or written a small script in perl that just did the whole thing. One of the other people who answered took that approach.

One other thing I would like to point out is my use of quoting. Every time I do anything with shell scripts I remind people to be very careful with their quoting. A huge source of problems in shell scripts is the shell interpreting things it's not supposed to. And the quoting rules are far from obvious. I believe this shell script is free of quoting issues.

Bash – Manipulate file name piped from find command

As other answers have already identified, ${f%.*} is expanded by the shell before it runs the xargs command. You need this expansion to happen once for each file name, with the shell variable f set to the file name (passing -I f doesn't do that: xargs has no notion of shell variable, it looks for the string f in the command, so if you'd used e.g. xargs -I e echo … it would have executed commands like ./somedir/somefile.wmacho .mp3).

Keeping on this approach, tell xargs to invoke a shell that can perform the expansion. Better, tell find — xargs is a largely obsolete tool and is hard to use correctly, as modern versions of find have a construct that does the same job (and more) with fewer plumbing difficulties. Instead of find … -print0 | xargs -0 command …, run find … -exec command … {} +.

find . -name '*.wma' -type f -exec sh -c 'for f; do echo "${f%.*}.mp3"; done' _ {} +

The argument _ is $0 in the shell; the file names are passed as the positional arguments which for f; do … loops over. A simpler version of this command executes a separate shell for each file, which is equivalent but slightly slower:

find . -name '*.wma' -type f -exec sh -c 'echo "${0%.*}.mp3"' {} \;

You don't actually need to use find here, assuming you're running a reasonably recent shell (ksh93, bash ≥4.0, or zsh). In bash, put shopt -s globstar in your .bashrc to activate the **/ glob pattern to recurse in subdirectories (in ksh, that's set -o globstar). Then you can run

for f in **/*.wma; do
  echo "${f%.*}.mp3"
done

(If you have directories called *.wma, add [ -f "$f" ] || continue at the beginning of the loop.)

Best Answer

Related Solutions

Rename – Replace Dots with Underscores in Filenames

Bash – Manipulate file name piped from find command

Related Question