Identify and assign most recent file to shell variable

Question

I have a directory of .jpg files that continuously grows. I want to copy the most recent one elsewhere. This is what I currently have and it works, just curious if there's a better way to identify the most recent jpg file rather than using ls and tail.

#!/bin/bash
cd /home/pi/JPGS
fn=$(ls -rt1 | tail -1)
mv -f $fn /home/pi/WWW/webpic.jpg

Solution 1

There's a number of characters in file names that would make that fail. You can improve it with:

#! /bin/sh -
cd /home/pi/JPGS || exit
fn=$(ls -t | head -n1)
mv -f -- "$fn" /home/pi/WWW/webpic.jpg

Leaving a variable unquoted in list context (in Bourne-like shells other than zsh) is the split+glob operator, you almost never want to do that. -- marks the end of the options so "$fn" will not be taken as an option if it starts with -.

That still fails if filenames contain newline characters, but not space, tab, star, question mark, right square bracket, or start with dash.

Best is to use zsh here:

#! /bin/zsh -
mv -f /home/pi/JPGS/*.jpg(.om[1]) /home/pi/WWW/webpic.jpg

(.om[1]) are glob qualifiers, they are a zsh specific feature. . restricts the glob to regular files (will not include symlinks, directories, devices...), om is to order on modification time, and [1] to only take the first file.

Note that if you want to assign that to a shell variable, that would have to be an array variable:

fn=(/home/pi/JPGS/*.jpg(.om[1]))

(not that it makes a lot of difference on how you use it later).

Solution 2

Listing file

You could reverse the logic on the ls a bit.

$ ls -t | head -n1

Details

   -t     sort by modification time, newest first

Now it shows up first so we can use head to return the first result.

NOTE: You could also sort the list by change time (ctime), though you're probably going to want to use modify time above - (mtime). The ctime is the last time the file status meta information was changed.

   -c     with -lt: sort by, and show, ctime (time of last modification of 
          file status information) with -l: show ctime and sort by name
          otherwise: sort by ctime, newest first

For example:

$ ls -tc | head -n1

Moving the file

To do the move more cleanly you'll want to wrap the filename in double quotes.

Example

$ mv -f -- "$fn" /home/pi/WWW/webpic.jpg

This will work in the majority of cases, there are a handful of legal filenames where it won't, for example, files with new lines. But these, though legal, are rarely ever intentionally used.

Comments

[Levon]

Thanks - I have a few questions: What's the significance of the -- and why is zsh preferable over bash here? I'm also not sure (.om[1]) does.

[Levon]

Thanks - I do know the filenames, so this won't be an issue. What is the advantage of using double-quotes around the variable?

[Stéphane Chazelas]

he'll probably want to sort on mtime (which reflects the modification time of the content of the file) rather than ctime (which reflects the modification time of metadata as well).

[Stéphane Chazelas]

Note that, portably (and as the documentation you quote mentions), you need the -t option as well: ls -tc to sort on the inode-change time (and ls -lc to display the inode-change time, and ls -ut to sort on access time)

[Stéphane Chazelas]

@Levon, answer updated with details.

[slm]

@Levon - double quotes protects the mv command in cases where there are spaces within the file names.

[Levon]

Thanks .. good to store this away for future possible use, my filenames are quite regular so I won't have to worry about special cases for this.

[Levon]

Thanks, very useful info re the use of "" .. my current filenames won't have that problem, but I will start using this as the default way of doing things.

[Levon]

You are right of course, my comment was with regard to the globing, not re -- which is clearly important from a security standpoint too - thanks again

[hajikelist]

instead of "ls -t" you could perhaps use "find . -maxdepth 1 -type f | head -n1" to limit to regular files...

[Stéphane Chazelas]

@hajikelist, that would not sort the list by modification time.