Appending command line arguments to a Bash array

Question

I am trying to write a Bash script that appends a string to a Bash array, where the string contains the path to a Python script together with the arguments passed into the Bash script, enclosed in double quotes.

If I call the script using ./script.sh -o "a b", I would like a CMD_COUNT of 1, but I am getting 2 instead.

script.sh:

#!/bin/bash

declare -a COMMANDS=()
COMMANDS+=("/path/to/myscript.py \"${@}\"")
CMD_COUNT=${#COMMANDS[*]}
echo $CMD_COUNT

How can I ensure that the appended string is /path/to/myscript.py "-o" "a b"?

EDIT: The full script is actually like this:

script.sh:

#!/bin/bash

declare -a COMMANDS=()
COMMANDS+=("/path/to/myscript2.py")
COMMANDS+=("/path/to/myscript.py \"${@}\"")
CMD_COUNT=${#COMMANDS[*]}
echo $CMD_COUNT

for i in ${!COMMANDS[*]}
do
    echo "${0} - command: ${COMMANDS[${i}]}"
    ${COMMANDS[${i}]}
done

Why would you want a single string /path/to/myscript.py "-o" "a b", and not an array with three strings, /path/to/myscript.py, -o, and a b? That array is in exactly the state you need to use it as a command line to invoke your program. The array with only one string... not so much. — Charles Duffy
– Charles Duffy, Commented Nov 3, 2020 at 17:34
BashFAQ #50 might be a place to start reading for background on why argument lists should be handled as arrays rather than strings containing literal quotes/whitespace/etc. — Charles Duffy
– Charles Duffy, Commented Nov 3, 2020 at 17:35
(BTW, all-caps variable names are used for variables that change or reflect behavior of the shell itself and other POSIX-standardized tools, whereas POSIX reserves variables with at least one lower-case character for application use -- see pubs.opengroup.org/onlinepubs/9699919799/basedefs/…, keeping in mind that shell variables and environment variables share a single namespace, so conventions necessarily apply to both). — Charles Duffy
– Charles Duffy, Commented Nov 3, 2020 at 17:36

Charles Duffy · Accepted Answer · 2020-11-03 17:38:20Z

1

It's a bad idea, but if it's what you really want, printf %q can be used to generate a string that, when parsed by the shell, will result in a given list of arguments. (The exact escaping might not be identical to what you'd write by hand, but the effect of evaluating it -- using eval -- will be).

#!/bin/bash

declare -a COMMANDS=( )

printf -v command '%q ' "/path/to/myscript" "$@"
COMMANDS+=( "$command" )
CMD_COUNT=${#COMMANDS[@]}
echo "$CMD_COUNT"

...but, as I said, this is all a bad idea.

answered Nov 3, 2020 at 17:38

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Charles Duffy

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9 Comments

mck Over a year ago

The appended string is /path/to/myscript.py -o a\ b, not what I wanted...

Charles Duffy Over a year ago

When you evaluate /path/to/myscript.py -o a\ b as source code in a shell, it has exactly the same behavior as /path/to/myscript.py -o "a b". As in, myscript.py can't distinguish between the two uses, because they both create the same sys.argv list.

Charles Duffy Over a year ago

@mck, ...since you're coming from a Python background: the above is like complaining that repr("""some string""") gives you 'some string' as output instead of spitting out """some string""" with the original triple-quotes. They're written different ways, but they're the exact same value.

mck Over a year ago

Sorry @CharlesDuffy, I understand what you mean. It seems it's the script that I'm having is causing some trouble with backslash escapes. I'll edit my question to reflect that.

mck Over a year ago

I have edited my question. The script doesn't work in the sense that it now got "a\" as the argument for "-o", rather than "a b". Using set -x shows that the script is called as /path/to/myscript.py -o 'a\' b

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