find: current filename evaluates to empty in nested command

Question

I am trying to rename a bunch of images in a directory according to their dimensions. To that end, I am building up my command step by step, using of Imagemagik's identify command. I currently am using echo and plan to switch that to mv once echo outputs correctly.

find * -type f -exec echo "$(identify -format '%w-%h' {})" \;

This outputs a bunch of empty lines. However, when I just run it without the echo directly,

find * -type f -exec identify -format '%w-%h' {} \;

It outputs the dimensions as expected. Why isnt the inner command executing properly?

Answer 1

That's because echo doesn't know what is {} inside itself to expand it, only exec understand {}, in this case you should use in sh -c mode.

find . -type f -exec sh -c 'echo "$(identify -format '%w-%h' "$1")"' sh '{}' \;

Answer 2

Use:

find . -type f -exec sh -c 'echo "$(identify -format %w-%h "$1")"' sh {} \;

Explanation

The problem is with the way bash expands variables

find * -type f -exec echo "$(identify -format '%w-%h' {})" \;

First bash expands what is in side $(...):

find * -type f -exec echo 'identify: unable to open image `{}': No such file or directory...' \;

At which point find will use the same exec statement for all commands with a rather useless argument. What you want to do is suppress the bash expansion for inside the finds exec. You can stop bash expanding things by placing them inside single quotes

find * -type f -exec echo '$(identify -format %w-%h {})' \;

But here we lose expansion completely, we can inject it back by running the expression with sh -c

find * -type f -exec sh -c 'echo "$(identify -format %w-%h {})"' \;

Having {} within the shell script can also lead to some unexpected errors when dealing with whitespace or quotes or any character special to the shell in file names. To get around this you should move the {} as an argument to sh with:

find * -type f -exec sh -c 'echo "$(identify -format %w-%h "$1")"' sh {} \;

See this question/answer for a more detailed explanation of this.

Finally you can replace the * with . to let find find the files in the current directory by itself (and include hidden ones and not fail for filenames starting with -).

Additionally, you can add the -x flag to sh to get it to print out what was it executed to help with debugging the command - this can be very noisy for a lot of files.