Why does -a in "#!/bin/sh -a" affect sed and "set -a" doesn't?

Question

If I run the following .sh file:

#!/bin/sh -a
echo "a" | sed -e 's/[\d001-\d008]//g'

The result is an error:

sed: -e expression #1, char 18: Invalid range end

But if I run the following .sh file:

#!/bin/sh
set -a
echo "a" | sed -e 's/[\d001-\d008]//g'

It runs without error. Isn't the second code supposed to be equivalent of the first one? Why the error in the first one?

Answer 1

When bash is called with the name sh, it does this:

if (shell_name[0] == 's' && shell_name[1] == 'h' && shell_name[2] == '\0')
    act_like_sh++;

and then later sets the POSIXLY_CORRECT shell variable to y:

if (act_like_sh)
  {
    bind_variable ("POSIXLY_CORRECT", "y", 0);
    sv_strict_posix ("POSIXLY_CORRECT");
  }

bind_variable calls bind_variable_internal, which, if the shell attribute a is on at the time (which it would be if you invoked the shell with -a), marks the shell variable as exported.

So in your first script:

#!/bin/sh -a
echo "a" | sed -e 's/[\d001-\d008]//g'

sed is invoked with POSIXLY_CORRECT=y in its environment, which will make it complain about [\d001-\d008]. (Same thing happens if sed is given the --posix option.)

In GNU sed, \dNNN is an escape code for the character whose numerical value in base-10 is NNN, but in POSIX mode, this is disabled inside a bracket expression, so [\d001-\d008], means literally the characters \, d, etc., with the range being from 1 to \. In order of character codes, 1 comes before \ (and the range includes all digits except zero, plus all uppercase letters, plus some special characters). In the en_US.UTF-8 locale which you were using, \ sorts before 1, however, so the range is invalid.

In your second script:

#!/bin/sh
set -a
echo "a" | sed -e 's/[\d001-\d008]//g'

even though POSIXLY_CORRECT is set in the shell, it isn't exported, so sed is invoked without POSIXLY_CORRECT in the environment, and sed runs with GNU extensions.

If you add export POSIXLY_CORRECT near the top of your second script, you'll also see sed complain.