"for file in" numerically instead of alphabetically? [duplicate]

Question

I have a script generating an index based on each file in a folder. All file names are a number with extension.

How can I modify my loop to process them in numeric order?

for file in xml/*.xml; do
  ...
done

Answer 1

If you have GNU sort that has the option to de-limit on the \0 delimiter you can do. This way the while loop will start getting files in the sorted order for you to process. Replace the printf option with your own custom logic.

shopt -s nullglob

printf '%s\0' xml/*.xml | sort -zV | while read -rd '' file; do
    printf "%s\n" "$file"
done

shopt -u nullglob

The nullglob option is to prevent shell from expanding an empty glob if no xml files are found in the current folder. The option -u unsets it after your processing is done.

---

As Tony Speight rightly points out, if you don't want to mess with the shell options (e.g. it may be enabled for other reasons) you could just set in the sub-shell and let the glob expansion happen

( shopt -s nullglob; printf '%s\0' xml/*.xml ) | sort -zV |  while read -rd '' file; do
    printf "%s\n" "$file"
done

Answer 2

If you can use zsh (for a script, it's a trade-off between convenient development and convenient deployment):

for file in xml/*.xml(n); do
  ...
done

The glob qualifier n specifies that when ordering the matches for the glob pattern, sequences of digits must be ordered according to the numerical order of the integer they represent rather than in lexicographic order, i.e. foo10bar comes after foo9bar.

You can also use setopt numeric_glob_sort to make all glob patterns use this setting.

Answer 3

Along the lines of one of Kusalananda's suggestions:

Prep:

mkdir xml && cd xml
touch 1.xml 10.xml 23.xml 999.xml
cd ..

Loop:

for f in {1..100}; do [ -f "xml/$f.xml" ] && echo process "xml/$f.xml"; done

Results in:

process xml/1.xml
process xml/10.xml
process xml/23.xml

Make the brace expansion loop large enough to cover the largest numeric file.

Answer 4

ls -Q xml*.xml | while read line;do 
   ...
done

ls will list the files in numeric order and -Q will ensure that any spaces are dealt with.