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b732e08671
The issues which this commit fixes are unlikely to be broken
in real life, but the fixes improve correctness, and would prevent
bugs in some uncommon cases, such as weird IFS values.
Listing some portability guidelines here for future reference.
I'm leaving it to someone else to decide whether to include
it in the file itself, place it as a new file, or not.
---------
The command "local" is non standard, but is allowed in this file:
- Quote initialization if it can expand (local x="$y"). See below.
- Don't assume initial value after "local x". Either initialize it
(local x=..), or set before first use (local x;.. x=..; <use $x>).
(between shells, "local x" can unset x, or inherit it, or do x= )
Other non-standard features beyond "local" are to be avoided.
Use the standard "test" - [...] instead of non-standard [[...]] .
--------
Quotes (some portability things, but mainly general correctness):
Quotes prevent tilde-expansion of some unquoted literal tildes (~).
If the expansion is undesirable, quotes would ensure that.
Tilds expanded: a=~user:~/ ; echo ~user ~/dir
not expanded: t="~"; a=${t}user b=\~foo~; echo "~user" $t/dir
But the main reason for quoting is to prevent IFS field splitting
(which also coalesces IFS chars) and glob expansion in parts which
contain parameter/arithmetic expansion or command substitution.
"Simple command" (POSIX term) is assignment[s] and/or command [args].
Examples:
foo=bar # one assignment
foo=$bar x=y # two assignments
foo bar # command, no assignments
x=123 foo bar # one assignment and a command
The assignments part is not IFS-split or glob-expanded.
The command+args part does get IFS field split and glob expanded,
but only at unquoted expanded/substituted parts.
In the command+args part, expanded/substituted values must be quoted.
(the commands here are "[" and "local"):
Good: [ "$mode" = yes ]; local s="*" x="$y" e="$?" z="$(cmd ...)"
Bad: [ $mode = yes ]; local s=* x=$y e=$? z=$(cmd...)
The arguments to "local" do look like assignments, but they're not
the assignment part of a simple command; they're at the command part.
Still at the command part, no need to quote non-expandable values:
Good: local x= y=yes; echo OK
OK, but not required: local x="" y="yes"; echo "OK"
But completely empty (NULL) arguments must be quoted:
foo "" is not the same as: foo
Assignments in simple commands - with or without an actual command,
don't need quoting becase there's no IFS split or glob expansion:
Good: s=* a=$b c=$(cmd...)${x# foo }${y- } [cmd ...]
It's also OK to use double quotes, but not required.
This behavior (no IFS/glob) is called "assignment context", and
"local" does not behave with assignment context in some shells,
hence we require quotes when using "local" - for compatibility.
The value between 'case' and 'in' doesn't IFS-split/glob-expand:
Good: case * $foo $(cmd...) in ... ; esac
identical: case "* $foo $(cmd...)" in ... ; esac
Nested quotes in command substitution are fine, often necessary:
Good: echo "$(foo... "$x" "$(bar ...)")"
Nested quotes in substring ops are legal, and sometimes needed
to prevent interpretation as a pattern, but not the most readable:
Legal: foo "${x#*"$y" }"
Nested quotes in "maybe other value" subst are invalid, unnecessary:
Good: local x="${y- }"; foo "${z:+ $a }"
Bad: local x="${y-" "}"; foo "${z:+" $a "}"
Outer/inner quotes in "maybe other value" have different use cases:
"${x-$y}" always one quoted arg: "$x" if x is set, else "$y".
${x+"$x"} one quoted arg "$x" if x is set, else no arg at all.
Unquoted $x is similar to the second case, but it would get split
into few arguments if it includes any of the IFS chars.
Assignments don't need the outer quotes, and the braces delimit the
value, so nested quotes can be avoided, for readability:
a=$(foo "$x") a=${x#*"$y" } c=${y- }; bar "$a" "$b" "$c"
Signed-off-by: Avi Halachmi (:avih) <avihpit@yahoo.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>
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