Reading quoted/escaped arguments correctly from a

问题:

I\'m encountering an issue passing an argument to a command in a Bash script.

poc.sh:

#!/bin/bash

ARGS=\'\"hi there\" test\'
./swap ${ARGS}

swap:

#!/bin/sh
echo \"${2}\" \"${1}\"

The current output is:

there\" \"hi

Changing only poc.sh (as I believe swap does what I want it to correctly), how do I get poc.sh to pass \"hi there\" and test as two arguments, with \"hi there\" having no quotes around it?

回答1:

A Few Introductory Words

If at all possible, don\'t use shell-quoted strings as an input format.

• It\'s hard to parse consistently: Different shells have different extensions, and different non-shell implementations implement different subsets (see the deltas between shlex and xargs below).
• It\'s hard to programmatically generate. ksh and bash have printf \'%q\', which will generate a shell-quoted string with contents of an arbitrary variable, but no equivalent exists to this in the POSIX sh standard.
• It\'s easy to parse badly. Many folks consuming this format use eval, which has substantial security concerns.

NUL-delimited streams are a far better practice, as they can accurately represent any possible shell array or argument list with no ambiguity whatsoever.

---

xargs, with bashisms

If you\'re getting your argument list from a human-generated input source using shell quoting, you might consider using xargs to parse it. Consider:

array=( )
while IFS= read -r -d \'\'; do
  array+=( \"$REPLY\" )
done < <(xargs printf \'%s\\0\' <<<\"$ARGS\")

swap \"${array[@]}\"

...will put the parsed content of $ARGS into the array array. If you wanted to read from a file instead, substitute <filename for <<<\"$ARGS\".

---

xargs, POSIX-compliant

If you\'re trying to write code compliant with POSIX sh, this gets trickier. (I\'m going to assume file input here for reduced complexity):

# This does not work with entries containing literal newlines; you need bash for that.
run_with_args() {
  while IFS= read -r entry; do
    set -- \"$@\" \"$entry\"
  done
  \"$@\"
}
xargs printf \'%s\\n\' <argfile | run_with_args ./swap

---

These approaches are safer than running xargs ./swap <argfile inasmuch as it will throw an error if there are more or longer arguments than can be accommodated, rather than running excess arguments as separate commands.

---

Python shlex -- rather than xargs -- with bashisms

If you need more accurate POSIX sh parsing than xargs implements, consider using the Python shlex module instead:

shlex_split() {
  python -c \'
import shlex, sys
for item in shlex.split(sys.stdin.read()):
    sys.stdout.write(item + \"\\0\")
\'
}
while IFS= read -r -d \'\'; do
  array+=( \"$REPLY\" )
done < <(shlex_split <<<\"$ARGS\")

回答2:

Embedded quotes do not protect whitespace; they are treated literally. Use an array in bash:

args=( \"hi there\" test)
./swap \"${args[@]}\"

In POSIX shell, you are stuck using eval (which is why most shells support arrays).

args=\'\"hi there\" test\'
eval \"./swap $args\"

As usual, be very sure you know the contents of $args and understand how the resulting string will be parsed before using eval.

回答3:

This might not be the most robust approach, but it is simple, and seems to work for your case:

## demonstration matching the question
$ ( ARGS=\'\"hi there\" test\' ; ./swap ${ARGS} )
there\" \"hi

## simple solution, using \'xargs\'
$ ( ARGS=\'\"hi there\" test\' ; echo ${ARGS} |xargs ./swap )
test hi there