Using semicolon (;) vs plus (+) with exec in find

This might be best illustrated with an example. Let's say that find turns up these files:

file1
file2
file3

---

Using -exec with a semicolon (find . -exec ls '{}' \;), will execute

ls file1
ls file2
ls file3

But if you use a plus sign instead (find . -exec ls '{}' \+), as many filenames as possible are passed as arguments to a single command:

ls file1 file2 file3

The number of filenames is only limited by the system's maximum command line length. If the command exceeds this length, the command will be called multiple times.

All of the answers so far are correct. I offer this as a clearer (to me) demonstration of the behaviour that is described using echo rather than ls:

With a semicolon, the command echo is called once per file (or other filesystem object) found:

$ find . -name 'test*' -exec echo {} \;
./test.c
./test.cpp
./test.new
./test.php
./test.py
./test.sh

With a plus, the command echo is called once only. Every file found is passed in as an argument.

$ find . -name 'test*' -exec echo {} \+
./test.c ./test.cpp ./test.new ./test.php ./test.py ./test.sh

If find turns up large numbers of results, you may find that the command being called chokes on the number of arguments.

From man find:

-exec command ;

Execute command; true if 0 status is returned. All following arguments to find are taken to be arguments to the command until an argument consisting of ';' is encountered. The string '{}' is replaced by the current file name being processed everywhere it occurs in the arguments to the command, not just in arguments where it is alone, as in some versions of find. Both of these constructions might need to be escaped (with a '\') or quoted to protect them from expansion by the shell. See the EXAMPLES sec section for examples of the use of the '-exec' option. The specified command is run once for each matched file. The command is executed in the starting directory. There are unavoidable security problems surrounding use of the -exec option; you should use the -execdir option instead.

-exec command {} +

This variant of the -exec option runs the specified command on the selected files, but the command line is built by appending each selected file name at the end; the total number of invocations of the command will be much less than the number of matched files. The command line is built in much the same way that xargs builds its command lines. Only one instance of '{}' is allowed within the command. The command is executed in the starting directory.

So, the way I understand it, \; executes a separate command for each file found by find, whereas \+ appends the files and executes a single command on all of them. The \ is an escape character, so it's:

ls testdir1; ls testdir2 

vs

ls testdir1 testdir2

Doing the above in my shell mirrored the output in your question.

example of when you would want to use \+

Suppose two files, 1.tmp and 2.tmp:

1.tmp:

1
2
3

2.tmp:

0
2
3

With \;:

 find *.tmp -exec diff {} \;
> diff: missing operand after `1.tmp'
> diff: Try `diff --help' for more information.
> diff: missing operand after `2.tmp'
> diff: Try `diff --help' for more information.

Whereas if you use \+ (to concatenate the results of find):

find *.tmp -exec diff {} \+
1c1,3
< 1
---
> 0
> 2
> 30

So in this case it's the difference between diff 1.tmp; diff 2.tmp and diff 1.tmp 2.tmp

There are cases where \; is appropriate and \+ will be necessary. Using \+ with rm is one such instance, where if you are removing a large number of files performance (speed) will be superior to \;.