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Document the rather suprising behavior with ' inside action rules.

Browse Source 
To prevent issues with odd shell characters appearing in, a surprising
shell feature is used. Document it and a workaround for it.

Differential Revision: https://reviews.freebsd.org/D26723
This commit is contained in:
Warner Losh 2020-10-13 05:19:00 +00:00
parent e694696956
commit eb93b08fe0
Notes: svn2git 2020-12-20 02:59:44 +00:00 
svn path=/head/; revision=366661
1 changed files with 57 additions and 1 deletions
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58
sbin/devd/devd.conf.5
View File

@ -40,7 +40,7 @@
.\" ACTION, ARISING OUT OF OR IN CONNECTION WITH THE USE OR PERFORMANCE OF THIS
.\" SOFTWARE.
.\"
.Dd August 18, 2020
.Dd October 12, 2020
.Dt DEVD.CONF 5
.Os
.Sh NAME
@ -662,6 +662,59 @@ For example:
// is a new comment, even though it is logically
// part of the previous comment.
.Ed
.Ss Notes on Variable Expansion
To prevent issues with special shell characters, the following happens for each
variable
.Ic $foo .
.Bl -enum
.It
The characters
.Dq $'
are inserted.
.It
The string
.Dq $foo
is removed.
.It
The value of the
.Ic foo
variable is inserted into the buffer with all single quote characters
prefixed by a backslash.
.El
.Pp
See
.Xr sh 1
for what this construct means.
It is safe in all context, except one: inside single quotes.
If foo=meta and bar=var, then a rule like the following:
.Bd -literal -offset indent -compact
action "echo '$foo $bar'";
.Ed
will be presented to the shell via
.Xr system 3
as
.Bd -literal -offset indent -compact
echo '$'meta' $'var''
.Ed
which produces the following output:
.Bd -literal -offset indent -compact
$meta $var
.Ed
as its output.
This is an unanticipated result.
A future version of this software will change this behavior.
Users are discouraged from using single quotes inside
.Ic action
value without due care.
.Pp
The above should be written as
.Bd -literal -offset indent -compact
action "echo $foo' '$bar"
.Ed
to produce a single argument to echo.
Given the above expansion, juxtaposing bare variables with
single quote expressions will produce the right output,
regardless of the value of the variable.
.Sh FILES
.Bl -tag -width ".Pa /etc/devd.conf" -compact
.It Pa /etc/devd.conf
@ -733,6 +786,9 @@ detach 0 {
The installed
.Pa /etc/devd.conf
has many additional examples.
.Sh BUGS
The variable expansion's interaction with single quotes is
suboptimal and surprising.
.Sh SEE ALSO
.Xr cam 4 ,
.Xr coretemp 4 ,