man bash, search for section Special Parameters. For
_ you'll find:
expands to the last argument to the previous command, after expansion.
So in a command line context the difference is the expansion. For example if I have a variable
FOO set to 'bar' and I execute
echo $FOO then
$_ on the next line will result in
bar being submitted in the next command while
!$ will result in
This behavior might not be obvious by default but with
shopt -s histverify the distinction is a little more evident. (With
histverify enabled Bash will not submit history expansion immediately after Enter, it will first display the command, post-expansion, and wait for a second Enter). So we would see something like this after
$ echo $FOO bar $ ls !$ $_ # We hit Enter and then see $ ls $FOO $_
$_ is not expanded until the command is submitted ('parameter expanstion'). It's a subtle distinction but there you go. (Would love to hear of a truly practical use for one vs the other in the comments...beyond the side effects in history and such.)
$_ has different meanings in other contexts:
At shell startup, set to the absolute pathname used to invoke the shell or shell script being executed as passed in the environment or argument list.
Also set to the full pathname used to invoke each command executed and placed in the environment exported to that command.
When checking mail, this parameter holds the name of the mail file currently being checked.