Bash parameter expansion and array index operations

Question

The ${parameter,,pattern} parameter expansion converts alphabetic characters in parameter to lower-case.

On cygwin 1.7.11-1 Bash 4.1.10(4) and also on my debian squeeze Bash 4.1.5(1);
if i run the following, i get a curious result:

$ declare -a a=(Zero One Two Three); n=0; echo "${a[n],,}->${n}"; echo "${a[++n]}->${n}"; echo "${a[++n],,}->${n}"
zero->0
One->1
three->3
$

NB: similar results happen:
for ,,* or ^^ case conversion;
for some other expansions such as ${parameter##word};
for using either prefix/postfix ++ or -- operator;
for using $((++n)) instead of just ++n.

However, the length expansion ${#parameter} works as i might expect:
in the above snippet, echo "${#a[++n]}->${n}" instead of echo "${a[++n],,}->${n} would yield 3->2 instead of three->3 ~& the length of a[2]="two" is indeed 3 characters.

I imagine that the parameter expansion is happening twice. But why is this happening?

i mean exactly that i would expect to get returned:zero->0 One->1 two->2 — violet313
– violet313, Commented Apr 8, 2012 at 19:27
+1 for a well-investigated question, but I don't think you'll get a good answer, because I think it's simply a bug. On my system, Bash crashes (!) if I add another "${a[++n],,}" that should move to the last element but would (per your results) move one past the end of the array. — ruakh
– ruakh, Commented Apr 8, 2012 at 20:58
By the way, if you can confirm that it happens in the latest version of Bash, and if no one offers a theory for how it could be anything other than a bug, gnu.org/software/bash/manual/html_node/Reporting-Bugs.html has instructions for how to report it. — ruakh
– ruakh, Commented Apr 8, 2012 at 20:59
ruakh, agreed. it probably should be flagged as a bug -since as you say it can be made to seg-fault. i am grabbing the bash source now & will raise a report once i've taken a look inside — violet313
– violet313, Commented Apr 9, 2012 at 9:30

violet313 · Accepted Answer · 2012-04-09 12:10:50Z

I grabbed the latest bash source & it appears that this issue is resolved for this version:

$ ./bash --version
GNU bash, version 4.2.24(1)-release (x86_64-unknown-linux-gnu)
Copyright (C) 2011 Free Software Foundation, Inc.
License GPLv3+: GNU GPL version 3 or later <http://gnu.org/licenses/gpl.html>
$

Note however that on my Debian Squeeze stable, I have:

$ apt-cache policy bash
bash:
  Installed: 4.1-3
  Candidate: 4.1-3
  Version table:
 *** 4.1-3 0
        500 http://ftp.uk.debian.org/debian/ squeeze/main amd64 Packages
        100 /var/lib/dpkg/status
$
$ bash --version
GNU bash, version 4.1.5(1)-release (x86_64-pc-linux-gnu)
Copyright (C) 2009 Free Software Foundation, Inc.
License GPLv3+: GNU GPL version 3 or later <http://gnu.org/licenses/gpl.html>

I think this means that currently on Debian Squeeze, the latest stable version of bash is 4.1.5(1) & contains this bug. Also on Cygwin 1.7.11-1 currently the latest available bash package appears to be version 4.1.10(4) & contains this bug. Of course the issue is easy to work around, so no real need to build the 4.2 sources.

pizza · Accepted Answer · 2012-04-09 06:45:14Z

0

I got

Zero->0
One->1
Two->2

However I think "${a[++n]}->${n}" may not be well defined.

try substitute with

let n=$n+1 ; echo "${a[n],,}->${n}";

To inject the notion of sequence.

answered Apr 9, 2012 at 6:45

pizza

7,7081 gold badge27 silver badges24 bronze badges

2 Comments

violet313 Over a year ago

if you look closely at your results, you will notice that neither the Zero nor the Two have been converted to lower-case. Now try instead with echo "${a[++n]foo}->${n}" same results!. At this point you might want to check your version of bash. For example, on Bash 3.2.25(1) i get the above results also ;)

pizza Over a year ago

The lowercasing expression is not valid in bash 3.

Collectives™ on Stack Overflow

Bash parameter expansion and array index operations

2 Answers 2

Comments

2 Comments

Your Answer

Hot Network Questions

Collectives™ on Stack Overflow

2 Answers 2

Comments

2 Comments

Your Answer

Sign up or log in

Post as a guest

Related