How do you check if a mercurial repo is in a clean state?

If you issue the hg identify --id command, it will suffix the ID with a + character when the repository has modified files. (Note: this flag does not report untracked files.)

If you grep the output of this command for the + character, you can use the exit status to determine whether there are modifications or no:

$ hg init
$ hg identify --id | grep --quiet + ; echo $?
1
$ touch a
$ hg identify --id | grep --quiet + ; echo $?
1
$ hg add a
$ hg identify --id | grep --quiet + ; echo $?
0

You should use hg summary:

$ hg init
$ echo blablabla > test.txt
$ hg summary
parent: -1:000000000000 tip (empty repository)
branch: default
commit: 1 unknown (clean)
update: (current)