Bash – Why does `bash -c ‘echo $0 ‘ ` output “bash”

-exec is a predicate that runs a command (not a shell) and evaluates to true or false based on the outcome of the command (zero or non-zero exit status).

So:

find . -iname '*.csv' -exec grep foo {} \; -print

would print the file path if grep finds foo in the file. Instead of -print you can use another -exec predicate or any other predicate

find . -iname '*.csv' -exec grep foo {} \; -exec echo {} \;

See also the ! and -o find operators for negation and or.

Alternatively, you can start a shell as:

find . -iname '*.csv' -exec sh -c '
   grep foo "$1" && echo "$1"' sh {} \;

Or to avoid having to start a shell for every file:

find . -iname '*.csv' -exec sh -c '
  for i do
    grep foo "$i" && echo "$i"
  done' sh {} +

!$ is a word designator of history expansion, it expands to the last word of previous command in history. IOW, the last word of previous entry in history. This word is usually the last argument to command, but not in case of redirection. In:

echo "hello" > /tmp/a.txt

the whole command 'echo "hello" > /tmp/a.txt' appeared in history, and /tmp/a.txt is the last word of that command.

_ is a shell parameter, it expands to last argument of previous command. Here, the redirection is not a part of arguments passed to the command, so only hello is the argument passed to echo. That's why $_ expanded to hello.

_ is not one of shell standard special parameters anymore. It works in bash, zsh, mksh and dash only when interactive, ksh93 only when two commands are on separated lines:

$ echo 1 && echo $_
1
/usr/bin/ksh

$ echo 1
1
$ echo $_
1