Bash function to find newest file matching pattern

The ls command has a parameter -t to sort by time. You can then grab the first (newest) with head -1.

ls -t b2* | head -1

But beware: Why you shouldn't parse the output of ls

My personal opinion: parsing ls is only dangerous when the filenames can contain funny characters like spaces or newlines. If you can guarantee that the filenames will not contain funny characters then parsing ls is quite safe.

If you are developing a script which is meant to be run by many people on many systems in many different situations then I very much do recommend to not parse ls.

Here is how to do it "right": How can I find the latest (newest, earliest, oldest) file in a directory?

unset -v latest
for file in "$dir"/*; do
  [[ $file -nt $latest ]] && latest=$file
done

The combination of find and ls works well for

filenames without newlines
not very large amount of files
not very long filenames

The solution:

find . -name "my-pattern" -print0 |
    xargs -r -0 ls -1 -t |
    head -1

Let's break it down:

With find we can match all interesting files like this:

find . -name "my-pattern" ...

then using -print0 we can pass all filenames safely to the ls like this:

find . -name "my-pattern" -print0 | xargs -r -0 ls -1 -t

additional find search parameters and patterns can be added here

find . -name "my-pattern" ... -print0 | xargs -r -0 ls -1 -t

ls -t will sort files by modification time (newest first) and print it one at a line. You can use -c to sort by creation time. Note: this will break with filenames containing newlines.

Finally head -1 gets us the first file in the sorted list.

Note: xargs use system limits to the size of the argument list. If this size exceeds, xargs will call ls multiple times. This will break the sorting and probably also the final output. Run

xargs  --show-limits

to check the limits on you system.

Note 2: use find . -maxdepth 1 -name "my-pattern" -print0 if you don't want to search files through subfolders.

Note 3: As pointed out by @starfry - -r argument for xargs is preventing the call of ls -1 -t, if no files were matched by the find. Thank you for the suggesion.

This is a possible implementation of the required Bash function:

# Print the newest file, if any, matching the given pattern
# Example usage:
#   newest_matching_file 'b2*'
# WARNING: Files whose names begin with a dot will not be checked
function newest_matching_file
{
    # Use ${1-} instead of $1 in case 'nounset' is set
    local -r glob_pattern=${1-}

    if (( $# != 1 )) ; then
        echo 'usage: newest_matching_file GLOB_PATTERN' >&2
        return 1
    fi

    # To avoid printing garbage if no files match the pattern, set
    # 'nullglob' if necessary
    local -i need_to_unset_nullglob=0
    if [[ ":$BASHOPTS:" != *:nullglob:* ]] ; then
        shopt -s nullglob
        need_to_unset_nullglob=1
    fi

    newest_file=
    for file in $glob_pattern ; do
        [[ -z $newest_file || $file -nt $newest_file ]] \
            && newest_file=$file
    done

    # To avoid unexpected behaviour elsewhere, unset nullglob if it was
    # set by this function
    (( need_to_unset_nullglob )) && shopt -u nullglob

    # Use printf instead of echo in case the file name begins with '-'
    [[ -n $newest_file ]] && printf '%s\n' "$newest_file"

    return 0
}

It uses only Bash builtins, and should handle files whose names contain newlines or other unusual characters.

Bash function to find newest file matching pattern

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