sed error: bad option in substitution expression

Question

I trying to replace password in a simple password file. The new password is garbage and is randomly generated.

For example here is a shadow file with the root account.

root:igXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXX+udyyyyyyyyyyyySgY=:10933:0:99999:7:::

I have the following expression which works 3 out of 4 try.

On the fourth try, I get this error "sed: bad option in substitution expression".

The other time, it works just fine. The password is replaced with a randomly generate string.

sed -i "/root/ s/ *:[^:]*:/:"$(openssl rand -base64 32)":/ " shadow

Can some one please help explain the problem?

Thanks, Tai

Adrian Frühwirth · Accepted Answer · 2014-04-03 14:35:33Z

2

Your generated password might include characters that are magic to the sed expression. A slightly better approach would be:

$ password=$(openssl rand -base64 32) $ awk -F: -v OFS=: -v p="$password" '$1=="root"{$2=p}1' /etc/shadow

Since most awk implementations don't have an in-place feature you need to write to a temporary file and replace /etc/shadow (but that's probably safer anyway) or use the handy sponge utility which is part of moreutils.

answered Apr 3, 2014 at 14:35

Adrian Frühwirth

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3 Comments

jaypal singh Over a year ago

@user2019842 Please consider accepting the answer if it helped you resolved your question. You can do so by clicking on the tick mark next to the answer so that it's green. For more information please read this

twalberg Over a year ago

To be more specific, the base64 encoding can include the '/' and '+' characters, as well as newlines. Since you are using '/' as a separator for the sed commands, the existence of a '/' in the base64 encoded string confuses it...

Adrian Frühwirth Over a year ago

@twalberg Sssshh, we can't do all the homework! ;-) No seriously, thanks for filling in on my laziness. I was hoping this would be apparent after my vague explanation but I guess it might not be.

anubhava · Accepted Answer · 2014-04-03 14:32:49Z

Use a different control character since / might appear in the password field also.

sed -i "/root/s^A *:[^:]*:^A:"$(openssl rand -base64 32)":^A" shadow

Here delimiter is ^A (ctrl-A - typed using ctrlVA keys together)

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