While I was reading the document of some Linux utilities, I found that they always quote strings using ` and '. For example, below is an excerpt from the man page of the find utility.
-exec command {} +
This variant of the -exec action 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, and it must appear at the end,
immediately before the `+'; it needs to be escaped (with a
`\') or quoted to protect it from interpretation by the
shell. The command is executed in the starting directory.
If any invocation with the `+' form returns a non-zero
value as exit status, then find returns a non-zero exit
status. If find encounters an error, this can sometimes
cause an immediate exit, so some pending commands may not
be run at all. For this reason -exec my-
command ... {} + -quit may not result in my-command
actually being run. This variant of -exec always returns
true.
As you can see, there are several such cases like `{}', `\' and `+'. Why they don't use a pair of single quotes or backtick?
