mjhjmtu

This what I am trying to get in a bash shell script:

for a in "1" "2 3" "4 5 6"
do
 echo "a: $a"
done
a: 1
a: 2 3
a: 4 5 6

But the quotes are treated differently when you use a variable:

echo ""1" "2 3" "4 5 6"" > a.txt

cat a.txt
"1" "2 3" "4 5 6"

read aline < a.txt

If you double-quote the variable, you get this:

for b in "$aline"
do
 echo "b: $b"
done
b: "1" "2 3" "4 5 6"

And without quotes you get this:

for b in $aline
do
 echo "b: $b"
done
b: "1"
b: "2
b: 3"
b: "4
b: 5
b: 6"

Is there a way to get the for to process the contents of the variable as in the first example?

If you're OK with having the whole package (variable/arithmetic/command expansion, globbing, etc) on top of quote processing, this will do:

echo '"a b" "x y"' >a.txt

eval "set -- $(<a.txt)"
for b do
 printf 'b: %sn' "$b"
done

b: a b
b: x y

Change the $(<a.txt) to $(cat a.txt) if you want it to work in a regular shell, not just bash (or other advanced shells like zsh or ksh). You could only use this if you're sure you can control the content of a.txt, otherwise it will be trivial to exploit your script via splitting with ;, command expansion, etc.

However, if you just want to pass those arguments to a command, you can use xargs, which will just process the quotes, without doing any expansion:

xargs <a.txt printf 'b: %sn'

But xargs will only work with external commands, not with shell functions, will ignore backslashes inside double quotes just like inside single quotes, and won't be able to parse multiline strings.

If these latter limitations are OK, you can still use xargs as a quote parser via a wrapper or an array:

mapfile -t args < <(xargs <a.txt printf '%sn')
for b in "$args[@]"; do
 printf 'b: %sn' "$b"
done

If instead of bash you're using ksh93 (ksh93 is the shell bash tries to mimic anyway), you could use the -S option of its read builtin which is to read csv (so understands csv-style quoting):

IFS=" " read -rSA array < a.txt
printf 'a: %sn' "$array[@]"

With zsh, you could use it's z parameter expansion flag which is used to tokenise the content of a variable as if it was zsh code (and so understand quoting as zsh would, but not do the code evaluation that eval would do) along with the Q parameter expansion flag to remove one layer of quoting:

IFS= read -r line < a.txt
printf 'a: %sn' "$(Q@)$(z)line"

(the @ parameter expansion flag is to preserve empty elements, reminiscent of ksh's $array[@] behaviour).

If a.txt contains more than one line, with zsh you can replace the read command with content=$(<a.txt) and with ksh93 use read in a while loop (each read invocation potentially reading more than one line if a quoted string spans several line).