Command substitution in alias resolved in bash profile?

Question

I want to make an alias for randomly changing my mac address

alias chrandmac="sudo ifconfig en0 ether $(openssl rand -hex 6 | sed 's/\(..\)/\1:/g; s/.$//')"

but the command substitution part is already resolved when executing the profile.

alias chrandmac='sudo ifconfig en0 ether 83:3a:bf:fc:4e:29'

Any thoughts as to why this occurs?

The `$(....)` part is done when the alias is _defined_, not when it runs. Use a shell function. — vonbrand, Mar 30 '13 at 22:54
@vonbrand it's done when it's defined because `$()` happens inside of double quotes. That can be avoided. — jordanm, Mar 31 '13 at 00:58
@jordanm, I've been badly bitten by aliases with arguments that seemed to work (along the lines you state). Better avoid that, use aliases _only_ with fixed text replacement. — vonbrand, Mar 31 '13 at 01:03

jordanm · Accepted Answer · 2013-03-30T21:24:27.117

25

You want to use a function instead of an alias. It can be put in your startup file just like an alias:

chrandmac() {
    sudo ifconfig en0 ether $(openssl rand -hex 6 | sed 's/\(..\)/\1:/g; s/.$//')
}

In order to get it to work with an alias, you need to use single quotes to prevent the expansion of the command substitution.

alias chrandmac='sudo ifconfig en0 ether $(openssl rand -hex 6 | sed '\''s/\(..\)/\1:/g; s/.$//'\'')'

edited Mar 30 '13 at 21:24

answered Mar 30 '13 at 20:31

jordanm

Thanks, but I'm curious how to make this with an alias. – 1.61803 Mar 30 '13 at 20:58
1

@1.61803 answer updated – jordanm Mar 30 '13 at 21:24
1

Great, the alias with your modifications works fine, thanks. – 1.61803 Mar 31 '13 at 01:22

1 Answers1