Ramsay Jones <ramsay@xxxxxxxxxxxxxxxxxxx> writes:
> On systems which have dash as /bin/sh, such as Ubuntu, the final
> test (master@{n} for various n) fails with a syntax error while
> processing an arithmetic expansion. The syntax error is caused by
> using a bare name ('N') as a variable reference in the expression.
>
> In order to avoid the syntax error, we spell the variable reference
> as '$N' rather than simply 'N'.
>
> Signed-off-by: Ramsay Jones <ramsay@xxxxxxxxxxxxxxxxxxx>
Thanks.
POSIX wants shells to support both "N" and "$N" and requires them to yield
the same answer to $((N)) and $(($N)), but we should aim for portability
in a case like this, especially when the price we pay to do so is so
small, i.e. a few extra dollars.
By the way, on my box, I get this:
$ ls l /bin/dash
-rwxr-xr-x 1 root root 104024 2008-08-26 02:36 /bin/dash*
$ dpkg -l dash | grep '^ii'
ii dash 0.5.4-12 POSIX-compliant shell
$ /bin/dash -c 'N=20 ; echo $(( N + 3 ))'
23
I just left it vague by saying "e.g. older dash" in below, but we may want
to be more precise in the documentation.
-- >8 --
CodingGuidelines: spell Arithmetic Expansion with $(($var))
Signed-off-by: Junio C Hamano <gitster@xxxxxxxxx>
---
Documentation/CodingGuidelines | 4 ++++
1 files changed, 4 insertions(+), 0 deletions(-)
diff --git a/Documentation/CodingGuidelines b/Documentation/CodingGuidelines
index b8bf618..2cdd76f 100644
--- a/Documentation/CodingGuidelines
+++ b/Documentation/CodingGuidelines
@@ -43,6 +43,10 @@ For shell scripts specifically (not exhaustive):
- We use Arithmetic Expansion $(( ... )).
+ - Inside Arithmetic Expansion, spell shell variables with $ in front
+ of them, as some shells do not grok $((x)) while accepting $(($x))
+ just fine (e.g. older dash).
+
- No "Substring Expansion" ${parameter:offset:length}.
- No shell arrays.
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