difference between "bash -eu" . and "bash -e"

Question

Apologies if this is a repost. I did search (no luck) before posting this.

"bash -e" will error/fail if there is any error. Doesn't it include the "bash -u" condition? If a parameter is not set won't a command using that parameter fail and caught by "bash -e" ?

Isn't "bash -eu" equal to "bash -e" in that case?

Answer 1

No, bash -e (bash started with the errexit shell option set) is not the same as bash -e -u (bash started with both errexit and nounset set).

Example:

$ bash -e -c 'echo "hello $string"'
hello
$ echo "$?"
0

$ bash -e -u -c 'echo "hello $string"'
bash: string: unbound variable
$ echo "$?"
1

Using an unset variable under only errexit is not an error, it just expands to an empty string.

Also:

$ bash -u -c 'echo "hello $string"'
bash: string: unbound variable
$ echo "$?"
127

This shows a subtle difference between -e and -u. With only -u, bash exits with code 127, which translates into a "command not found" error. With both -e and -u, bash exits with a more generic error code of 1.

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These things holds true for the POSIX sh shell as well, although I don't believe that the 127 exit status is explicitly required for the last example.