If you want to learn how to use your computer like a boss, it's a good idea to get comfortable working on the command line with a shell. Oftentimes it's a more effective way to get certain things done.

If you are on MacOS or Linux, you probably already have the bash shell installed. Limited bash support was added to Windows in Windows 10. Once you are comfortable working in the shell you'll want to learn to use command line utilities like find, xargs, grep (and much more) to get stuff done and make your day to day computing life easy.

There are so many practical applications of these utilities. In this post, I'll introduce find which is a utility for finding files based on criteria like name, file size, or the timestamp when the file was created or last accessed or modified.

Let's say I've got a bunch of files in a folder, and I want to copy some (but not all) of the files to Dropbox. I only want to copy image files that have been created in the last week. We will explore the use of command line tools to get this job done.

Using ls to list files by extension

The ls utility can be used to list the files in the current directory:

ls -1 *.jpg

This works well in simple situations, but it's not the most robust solution. What if some files have the extension '.JPG' or '.jpeg' or '.png'? And how do we only list some files based on creation time? ls cannot handle these situations.

Using find to list files by extensions

The find utility can be used to find files based on various search criteria. To filter files by name pattern, ignoring case:

find . -iname '*.jpg'

This means "find in this directory or the subdirectories contained therein any file that has an extension .jpg, case insensitive." You can use -name for case sensitive searches.

Note the quotes around the pattern: '*.jpg'. We must use quotes to prevent the shell from performing shell expansion.

Multiple Criteria

To accommodate multiple possible extensions, we must add arguments to find,

By default, each criteria provided to find is added using AND logic (i.e. all the listed criteria must be true for a file to match). Here we must use -o to add criteria using OR logic, where a file must match only some of the search criteria.

find .  \
 -iname '*.jpg' \
 -o -iname '*.jpeg' \
 -o -iname '*.png'

This means "find in this directory or the subdirectories contained therein any file that has an extension '.jpg', or '.jpeg' or '.png', case insensitive".

If you forgot the -o flags then find would come up empty because file names cannot end in '.jpg' AND '.jpeg'.

Using maxdepth

We can make this search even more robust by requiring the file be a regular file (i.e. not a directory) and the results be in the current working directory, but not a subdirectory.

The argument -maxdepth 1 says look in this folder, but not any deeper. The argument -type f means look for regular files, but not directories.

To get the OR logic to work right, we need to group our file name criteria in the matching parentheses \( and \). The parentheses must be escaped with the backlash (common practice when working on the command line. (Sometimes you gotta escape stuff.)

find .  \
 -maxdepth 1 \
 -type f \
 \( \
     -iname '*.jpg' \
     -o -iname '*.jpeg' \
     -o -iname '*.png' \
 \)

Using -ctime for creation time

We use the -ctime argument to find files based on file creation time. To find created in the last 7 days, we use -ctime -7

find .  \
 -maxdepth 1 \
 -type f \
 -ctime -7 \
 \( \
     -iname '*.jpg' \
     -o -iname '*.jpeg' \
     -o -iname '*.png' \
 \) 

Using find + xargs

Alright, we are novice experts at finding files now. I want to find files and do something with those files I find - like move them to Dropbox.

Since I've installed Dropbox on my MacBook, if I copy files into ~/Dropbox the Dropbox app will sync them to the Dropbox service. We can use the xargs utility to do the heavy lifting.

xargs is processor of sorts: for each argument in its input, it runs a program and constructs the command line argumants passed to that program.

For example, I effectively want to do something like this:

cp photo1.jpg ~/Dropbox
cp photo2.jpg ~/Dropbox
cp photo3.jpg ~/Dropbox

etc., but 100 times. or 1000 times, once for each photo. This is a job for xargs. You can pass the results of find into xargs using the pipe operator |, as so:

find .  \
 -maxdepth 1 \
 -type f \
 -ctime -7 \
 \( \
     -iname '*.jpg' \
     -o -iname '*.jpeg' \
     -o -iname '*.png' \
 \) \
 | xargs -I {} cp {} ~/Dropbox

For each file returned by find (photo1.jpg, photo2.jpg, etc.), xargs will run cp {} ~/Dropbox where the placeholder {} is replaced by the arguments.

There is one gotcha - sometimes filenames have spaces (boooo!). This can create a problem for this find into xargs pipeline. The solution is to instruct find to separate results with a null character, and then have xargs parse it's input list of arguments using this separator. We use -print0 argument with find, and -0 with xargs:

find . \
 -maxdepth 1 \
 -type f \
 -ctime -7 \
 \( \
     -iname '*.jpg' \
     -o -iname '*.jpeg' \
     -o -iname '*.png' \
 \) \
 -print0 \
 | xargs -0 -I {} cp {} ~/Dropbox

-print0 must come at the end, after the search criteria.

Alright good stuff. This is nice, powerful, and flexible!

Other arguments to find

File size

Use '-size' to search based on file size. Here's a way to search for large files over 100 MB in your home directory:

find ~ -type f -size +100M

use k for kilobytes, 'M' for Megbytes, G for gigabytes.

Timestamps in depth

Use -ctime, -mtime, -atime to find files that have been created, modified, accessed n days ago. There are some rounding affects, where fractional parts of days are ignored. Check out the manpage (man find) for more information. Here are some quick examples:

To find files modified between 1 and 2 days ago, supply argument -mtime 1.

find . -mtime 1

Since the fractional part of days are ignored, files modified 1.001 days and 1.999 days ago both match -mtime 1 exactly.

To find files modified less than 1 days ago:

find . -mtime -1

Find files modified more than 1 days ago:

find . -mtime +1

Notice how the + and - affect the search. Like all of the find criteria, you can combine them. For example:

find . -mtime +0 -mtime -2

means find files that have modified 1 or more days ago (-mtime +0) and less than 2 days ago (-mtime -2). This is the same as find . -mtime 1.

Use -cmin, -mmin, -amin to find files that have been created, modified, accessed n minutes ago. This works just like the examples above, accept where n represents minutes instead of days.

Conclusion

find is a very useful utility for finding files. It works great with xargs, for running additional commands for every file that matches your search criteria.