Updated Sc2044 (markdown)

Sc2044.md

@@ -1,26 +1,98 @@
 ## For loops over find output are fragile. Use find -exec or a while read loop.
 
-It is difficult to correctly use "find" in a for loop, because filenames can contain bytes such as newlines and "*".  Where practical, consider using find's "-exec" like this:
+### Problematic code:
 
-    find . -exec COMMAND... {} \;
-    find . -exec COMMAND... {} \+
-
-A portable though complicated approach is to use a subshell (use '\'' for each single quote), though since a subshell is used, all variable assignments in the loop are lost after the loop ends:
-
-    find . -exec sh -c '
-     for file do
-        ...  # Use "$file" not $file
-     done' sh {} +
-
-
-A nonstandard but widely-working simple alternative is:
-
-    find . -print0 | xargs -0 COMMAND
-
-Using a find... while loop is another alternative.  This requires nonstandard extensions to find (-print0) and shell (read -d), and fails in some cases on Cygwin (in while loops,  filenames ending in \r \n and \n look identical to Cygwin).  Variable values may be lost after the loop because the loop may run in a subshell:
-
-    find . -print0 | while IFS="" read -r -d "" file ; do ...
-      COMMAND "$file" # Use quoted "$file", not $file, everywhere.
+    for file in $(find mydir -mtime -7 -name '*.mp3')
+    do
+      let count++
+      echo "Playing file no. $count"
+      play "$file"
     done
+    echo "Played $count files"
 
-For more information, see "[Filenames and Pathnames in Shell: How to do it Correctly](http://www.dwheeler.com/essays/filenames-in-shell.html)".
+This will fail for filenames containing spaces and similar, such as `My File.mp3`, and has a series of potential globbing issues depending on other filenames in the directory like (if you have `MyFile2.mp3` and `MyFile[2014].mp3`, the former file will play twice and the latter will not play at all).
+
+### Correct code:
+
+There are many possible fixes, each with its pros and cons. 
+
+The most general fix (that requires the least amount of thinking to apply) is having `find` output a `\0` separated list of files and consuming them in a `while read` loop:
+
+    while IFS= read -r -d '' file
+    do
+      let count++
+      echo "Playing file no. $count"
+      play "$file"
+    done <   <(find mydir -mtime -7 -name '*.mp3' -print0)
+    echo "Played $count files"
+
+In usage it's very similar to the `for` loop: it gets its output from a `find` statement, it executes a shell script body, it allows updating/aggregating variables, and the variables are available when the loop ends. 
+
+It requires Bash, and works with GNU, Busybox, OS X, FreeBSD and OpenBSD find, but not POSIX find.
+
+##### If `find` is just matching globs recursively
+
+If you don't need `find` logic like `-mtime -7` and just use it to match globs recursively (all `*.mp3` files under a directory), you can instead use `globstar` and `nullglob` instead of `find`, and still use a `for` loop:
+
+    shopt -s globstar nullglob
+    for file in mydir/**/*.mp3
+    do
+      let count++
+      echo "Playing file no. $count"
+      play "$file"
+    done
+    echo "Played $count files"
+
+This is bash specific.
+
+
+##### For POSIX
+
+If you need POSIX compliance, this is a fair approach:
+
+    find mydir ! -name "$(printf "*\n*")" -name '*.mp3' > tmp
+    while IFS= read -r file
+    do
+      let count++
+      echo "Playing file #$count"
+      play "$file"
+    done < tmp
+    rm tmp
+    echo "Played $count files"
+
+The only problem is for filenames containing line feeds. A `! -name "$(printf "*\n*")"` has been added to simply skip these files, just in case there are any. 
+
+If you don't need variables to be available after the loop (here, if you don't need to print the final play count at the end), you can skip the `tmp` file and just pipe from `find` to `while`. 
+
+##### For simple commands with no aggregation
+
+If you don't need a shell script loop body or any form of variable  like if we only wanted to play the file, we can dramatically simplify while maintaining POSIX compatibility:
+
+    # Simple and POSIX
+    find mydir -name '*.mp3' -exec play {} \;
+
+This does not allow things like `let counter++` because `let` is a shell builtin, not an external command.
+
+##### For shell commands with no aggregation
+
+If we do need a shell script body but no aggregation, you can do the above but invoking `sh` (this is still POSIX):
+
+    find mydir -name '*.mp3' -exec sh -c '
+        echo "Playing ${1%.mp3}"
+        play "$1"
+      ' sh {} \;
+
+This would not be possible without `sh`, because `${1%.mp3}` is a shell construct that `find` can't evaluate by itself. If we had tried to `let counter++` in this loop, we would have found that the value never changes.
+
+Note that using `+` instead of `\;`, and using an embedded `for file in "$@"` loop rather than `"$1"`, will not allow aggregating variables. This is because for large lists, `find` will invoke the command multiple times, each time with some chunk of the input.
+
+
+### Rationale:
+
+`for var in $(find ...)`  loops rely on word splitting and will evaluate globs, which will wreck havoc with filenames containing whitespace or glob characters. 
+
+`find -exec` `for i in glob` and `find`+`while` do not rely on word splitting, so they avoid this problem. 
+
+### Contraindications
+
+If you know about and carefully apply `IFS=$'\n'` and `set -f`, you could choose to ignore this message.