tar exclude在bash脚本中不起作用

Question

I am trying to create a tar file of a folder, which has a lot of files to be excluded. So I wrote a script ( mytar ):

#!/usr/bin/env bash

# more files to be included 
IGN=""
IGN="$IGN --exclude='notme.txt'"

tar --ignore-failed-read $IGN -cvf "$1" "$2"

# following command is working perfectly
# bash -c "tar --ignore-failed-read $IGN -cvf '$1' '$2'"

Test folder:

test/
    notme.txt
    test.txt
    test2.txt 

If I execute the script, it creates a tar file but doesn't exclude the files I have listed in IGN
Apparently, the command is:

tar --ignore-failed-read --exclude='notme.txt' -cvf test1.tar test  

The command is working perfectly fine if it's directly executing in the shell. Also I have found a workaround for the script: using bash -c in script file

bash -c "tar --ignore-failed-read $IGN -cvf '$1' '$2'"

I am wondering and trying to figure out it,

Why this simple command is not working without bash -c ?
Why it's working with bash -c ?

Output:
First output shouldn't container notme.txt file like later

UPDATE 1 script updated

Answer 1

This has to do with the way bash expands variables in its shell.

When you set:

IGN="--exclude='notme.txt'"

it will be expanded as :

tar --ignore-failed-read '--exclude='\''notme.txt'\''' -cvf test1.tar test  

And as such tar will look to exlcude a file named \\''notme.txt'\\'' , which it won't find.

You may use:

IGN=--exclude='notme.txt'

which will be be interpreted correctly after shell expansion and tar will know it, but I would rather suggest you use your variable to only store the file name to be excluded:

IGN="notme.txt"
tar --exclude="$IGN" -cvf ./test1.tar ./*

Answer 2

in following command single quotes are syntactical (not literal, filename argument is not literaly surounded by quotes) to prevent shell for splitting argument in the case it contains a space or a tab

tar --ignore-failed-read --exclude='notme.txt' -cvf test1.tar test  

the closest is to use array instead of string variable :

ign=( --exclude='notme.txt' )
tar --ignore-failed-read "${ign[@]}" -cvf test1.tar test