Variable substitution not working to pass --exclude options to tar command-CodePudding

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 ...

i.e. 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 ...

i.e. the specified folder has been excluded.

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

CodePudding user response：

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.