problem escaping quotes in script

Question

I'm constructing a command line for use with the 'mogrify' tool [part of imagemagick]. the finalised command line looks something like this :

mogrify -stroke yellow -draw 'line 0,0 0,319' -draw 'line 125,0 125,319' -draw 'line 652,0 652,319' file.png

I echo the command before executing it. If I copy/paste this echo'd line as a new command, exactly as written, directly into the shell, it works perfectly.

However, it doesn't work within the script. for some reason, mogrify looks for a files named 0,0, 0,319, etc. It's obvious that mogrify is interpreting the draw argument strings as though there were no single-quotes around them.

The final command given [i.e., the line in the script where the error occurs] is :

mogrify -stroke $C3 $args $FILE

I construct the string $args iteratively. the result of the final string is what I've put near the top of this question.

I've tried escaping the single quotes with backslash within the construction loop, double-escaping with double-quoted single quotes, swapping single and double quotes. I've also tried placing $args in double-quotes. I've even tried putting each argument into an array and then using ${args[@]} within the final call [where $args is above]. Nothing I've tried works.

[EDIT - showing further information for the array-style invocation]

for the array invocation, i prepare the array as follows :

data="99.8734 351.645 1836.05"
for p in $data; do
    x=$(echo "$W * $p / $duration" | bc)
    args+=(-draw "'line $x,$y1 $x,$y2'")
done

[nota : the $data are actually retrieved from an external source, but for purposes of this question i am assigning the data points literally here.]

when i make the call to mogrify using the array invocation, i issue the following command in the script : mogrify -stroke $C3 "${args[@]}" $FILE

in this case, neither $C3 nor $FILE have any whitespace.

the errors i get from the above look like the following : mogrify: non-conforming drawing primitive definition 'line 35,0 35,320' @ error/draw.c/RenderMVGContent/4271

so it seems the array assignment is working correctly now, but the shell is passing something inappropriate to mogrify.

when i echo the exact command line, the following is displayed : mogrify -stroke yellow -draw 'line 35,0 35,319' -draw 'line 125,0 125,319' -draw 'line 652,0 652,319' file.png

interestingly, when i change the array assignment to the following, moving the double quotes around the whole thing instead of just the line definitions : args+=("-draw 'line $x,$y1 $x,$y2'")

...then mogrify gives me the following error : mogrify: unrecognized option '-draw 'line 35,0 35,319'' @ error/mogrify.c/MogrifyImageCommand/4716

notice that the error output now correctly includes the single-quotes i had put around the line definition, where it didn't before. this gave me the idea to also try escaping the single quotes with '\'', but that doesn't work either. neither does reversing the single and double quotes [of course allowing for variable expansion inside the whole thing].

Answer 1

The double (or single) quotes are used to stop the shell processing the text in them. You can't add them as part of the command itself.

What you can do, though, is build up the command using an array, and then include that array variable in the result. Here I've used args as an array variable rather than a scalar (a string), so we can repeatedly append quoted values to its list:

C3=yellow
FILE=file.png

args=()
args+=(-draw 'line 0,0 0,319')
args+=(-draw 'line 125,0 125,319')
args+=(-draw 'line 652,0 652,319')

or

args+=(-draw "line $x,$y1 $x,$y2")

if you use variables in place of literal numbers. Note specifically that the single quotes you have shown in the your updated question are incorrect (this is wrong: args+=(-draw "'line $x,$y1 $x,$y2'") as you will be including the single quote characters in the values that are passed to mogrify).

Finally, double-quote the set of args's values so that each element is treated as a quoted value by the shell:

mogrify -stroke "$C3" "${args[@]}" "$FILE"