Why can't I use environment variable with chmod command? [duplicate]

Question

I can't use environment variables with chmod command. Is there any workaround to run this?

The way I'm trying to run in a sh file

export PEM_FILE="~/.ssh/xxx.pem"
chmod 400 $PEM_FILE

## Output
chmod: ~/.ssh/xxx.pem: No such file or directory

However, it work perfectly fine without the variable like this: chmod 400 ~/.ssh/xxx.pem

P.S: I am on macOS. Kindly let me know if this works fine in windows or linux.

Thanks!

Answer 1

Unfortunately, special characters like ~ do not work in strings. Instead, use a variable like $HOME to point to the home directory, or remove the quotes. (not recommended because it breaks compatibility with some shells)

So you would do something like: export PEM_FILE="$HOME/.ssh/xxx.pem" or export PEM_FILE=~/.ssh/xxx.pem.

Answer 2

The issue is that ~ is wildcard, and bash usually expands all wildcards and perform variable expansion BEFORE executing command. But in your case, you are using quotas and this prevent bash from wildcard resolution, so

export PEM_FILE="~/.ssh/xxx.pem"

PEM_FILE will contains not the path to the file but the string. And chmod command will receive this string as an argument, again without performing resolving wildcard.

So just remove quota

export PEM_FILE=~/.ssh/xxx.pem

or use full path

export PEM_FILE="/home/username/.ssh/xxx.pem"

Both will works.