Bash conditional assignment that tests a variable whilst building a string

Question

I found this question for how to do conditional assignment in bash, but what I'm trying to do is a little more complex, and I can't seem to get the syntax right. The condition in my case is to test a variable to see if it exists, and the output is concatenated to a string.

Here's what I have so far:

fwversion="${BASH_REMATCH[1]}.$(( [[ BASH_REMATCH[2] ]] ? BASH_REMATCH[2] : 0 ))"

Which produces this error message:

bash: line 41: [[ BASH_REMATCH[2] ]] ? BASH_REMATCH[2] : 0 : syntax error: 
operand expected (error token is "[[ BASH_REMATCH[2] ]] ? BASH_REMATCH[2] : 0 ")

Here's what I'm trying to achieve as C++ code:

std::string fwversion = BASH_REMATCH[1] + "." + ((BASH_REMATCH[2]) ? : BASH_REMATCH[2] : 0);

What's the correct syntax to do this in bash? Thanks.

That's actually C++ code

user4832408
– user4832408

2015-10-09 14:35:43 +00:00
Commented Oct 9, 2015 at 14:35 — user4832408
– user4832408, Commented Oct 9, 2015 at 14:35

glenn jackman · Accepted Answer · 2015-10-09 14:41:41Z

1

Looks like [[ ... ]] are not understood in an arithmetic expression.

I'd do this:

fwversion=${BASH_REMATCH[1]}
[[ ${BASH_REMATCH[2]} ]] && fwversion+=${BASH_REMATCH[2]} || fwversion+=0

or

[[ ${BASH_REMATCH[2]} ]] && ext=${BASH_REMATCH[2]} || ext=0
fwversion="${BASH_REMATCH[1]}.$ext"

On second thought, I wouldn't do that at all, I'd use the power of the shell's parameter expansion

str="foo:bar"
if [[ $str =~ ^([a-z]+):([a-z]*)$ ]]; then
    echo "${BASH_REMATCH[1]}.${BASH_REMATCH[2]:-0}"
fi

foo.bar

str="foo:"
if [[ $str =~ ^([a-z]+):([a-z]*)$ ]]; then
    echo "${BASH_REMATCH[1]}.${BASH_REMATCH[2]:-0}"
fi

foo.0

edited Oct 9, 2015 at 14:41

answered Oct 9, 2015 at 13:32

glenn jackman

249k42 gold badges233 silver badges362 bronze badges

Sign up to request clarification or add additional context in comments.

3 Comments

Sonic Atom Over a year ago

Hmmm, makes sense, although not very readable. honestly I'm tempted to do it with if then else statements.

Sonic Atom Over a year ago

Well that looks a lot nicer. Is there a reference for that :- construct?

glenn jackman Over a year ago

In the link I provided in the answer

Collectives™ on Stack Overflow

Bash conditional assignment that tests a variable whilst building a string

1 Answer 1

3 Comments

Your Answer

Linked

Hot Network Questions

Collectives™ on Stack Overflow

1 Answer 1

3 Comments

Your Answer

Sign up or log in

Post as a guest

Linked

Related