变量替换无法将 --exclude 选项传递给 tar 命令

Question

I am trying to pass some --exclude statements to my tar command via an env var (in reality I would be passing several --exclude statements, and trying to improve readability by putting the definition on a separate line):

# EXCLUDE_STUFF="--exclude='/captain/generated'"
# tar ${EXCLUDE_STUFF} -cvzf /dev/null /captain
/captain/
/captain/generated/
/captain/generated/registry-auth
... etc ...

ie it's not excluding the folder I've specified. It doesn't make a difference if I surround ${EXCLUDE_STUFF} in quotes.

This is what I get when I echo the above command:

# echo tar ${EXCLUDE_STUFF} -cvzf /dev/null /captain
tar --exclude='/captain/generated' -cvzf /dev/null /captain

So it looks like the command is correct. And when I run the expanded command directly, I get:

# tar --exclude='/captain/generated' -cvzf /dev/null /captain
/captain/
/captain/temp/
/captain/data/
... etc ...

ie the specified folder has been excluded.

So why is the variable substitution not working in this context?

Answer 1

why is the variable substitution not working in this context?

After assignment the variable EXCLUDE_STUFF contains literally the string --exclude='/captain/generated' .

You are passing --exclude='/captain/generated' from the variable substitution. Because there is no directory named literally ' relative to your current working directory, '/captain/generated' does not match anything, so it does not exclude anything.

In short tar --exclude='/captain/generated' is not the same as tar "--exclude='/captain/generated'" .

In this case, just

exclude_stuff="--exclude=/captain/generated"
tar "$exclude_stuff" ...

Or use bash arrays:

exclude_stuff=( --exclude='/captain/generated' )  # note: quotes are interpreted
tar "${exclude_stuff[@]}" ...

Read https://mywiki.wooledge.org/BashFAQ/050 . Research shell quoting, how it works, and when by it, research word splitting expansion and filename expansion. Check your scripts with https://shellcheck.net . Prefer to use lower case variables for script local variables.