How to extract specific elements from a filename?

Question

I have a bunch of files in the following format:

2014-11-19.8.ext
2014-11-26.1.ext
2014-11-26.2.blah.ext
2014-11-26_3.ext
2014-11-26.4.stuff_here.ext
2014-12-03.1. could be anything.ext
2014-12-032b.ext
2014-11-26 613 adva.ext

My goal is to iterate over the entire list of files and to take the date formatting from YYYY-MM-DD and store that in a variable in the format of YYYYMMDD for further processing (in my case It's going to be pushed into a touch command).

So normally I would match against this regular expression: (\d{4})-(\d{2})-(\d{2}).*

And then use $1$2$3 to get my desired pattern, however I'm not sure how to do this in bash / zsh.

How can this be done within a shell script as such?

Answer 1

Using parameter expansion

$ touch 2014-11-19.8.ext 2014-11-26.1.ext
$ for f in *.ext; do d="${f:0:4}${f:5:2}${f:8:2}"; echo "$d"; done
20141119
20141126

• ${f:0:4} means 4 characters starting from index 0 and f is variable name
• replace echo "$d" with your code

Answer 2

To loop over every file in the current directory and compare their filenames to the desired pattern, then set a variable containing the date pieces

for f in *
do 
  [[ $f =~ ^([0-9][0-9][0-9][0-9])-([0-9][0-9])-([0-9][0-9])(.*) ]] && 
  yourvar="${BASH_REMATCH[1]}${BASH_REMATCH[2]}${BASH_REMATCH[3]}"
done

This uses bash's [[ ability to use regular expression matching to place the date pieces into the BASH_REMATCH array.

Answer 3

You can do it interactively by using GNU sed:

$ sed 's/^\([0-9]\{4\}\)-\([0-9]\{2\}\)-\([0-9]\{2\}.*\)/\1\2\3/g' stuff.txt

For multiple files (if in same directory and no other considered files in directory):

for file in *
do
    if [ -f "$file" ]
    then
          sed 's/^\([0-9]\{4\}\)-\([0-9]\{2\}\)-\([0-9]\{2\}\).*/\1\2\3/g' "$file"
    fi
done

Answer 4

Here is a zsh way of doing this, without loops:

autoload -U zmv
zmv -n '([0-9](#c4))-([0-9](#c2))-([0-9](#c2))(*)' '$1$2$3$4'

• [0-9](#c4) means any digit repeated 4 times
• $1-$2 refer to previously used parenthesis
• -n prevents execution (only prints), remove this flag if you are happy with the result

As zsh takes care of globbing all corner cases (whitespaces, special characters, etc) should be taken automatically into account.

Answer 5

If you're on GNU Coreutils, you have this:

$ date --date=2014-11-13 +"%Y%m%d"
20141113

However:

$ date --date=2014-11-130ABCJUNK +"%Y%m%d"
date: invalid date ‘2014-11-130ABCJUNK’

So the task is much simpler: extact the first ten characters of each YYYY-MM-DDetc filename to get the date by itself, then pass to date for reformatting.

But, if we are on GNU Coreutils, we can skip the date command because touch has the exact same --date=STRING option.

for file in * ; do
  date=${file%${file##??????????}} # chop all but first ten
  touch --date=$date -- "$file"
done

But why do this ten character chopping in the POSIX portable way when we are relying on touch to be from GNU Coreutils?

for file in * ; do
  date=${file:0:10}
  touch --date=$date -- "$file"
done

Answer 6

Try pattern substitution:

${parameter/pattern/string}

parameter is the basename of the file. pattern is a dash. In this /- to replace pattern globally. string is empty, since you want delete the dashes.

mv "${f}" "${f//-/}"

Caveat: I didn't get this working with the case of spaces in the extention.