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Bash Argument Parsing and Quoting

October 4th, 2016
bash, tech  [html]
There are many ways in which bash is an awkward language, and handling of arguments is certainly one of them. Here are two things you might like to do in a shell:
  • quoting: turn an array into a string, using shell-style escaping
  • unquoting: turn a string into an array, interpreting shell-style escaping
For example, consider the following set of strings, separated by line breaks:
argument1
argument two
argument  three
argument='four   and'
There are multiple ways you could quote these as arguments, so here's one example:
argument1 "argument two" "argument  three" "argument='four   and'"
Basically, I want Python's shlex, with its quote and split, but I want it in bash. Unfortunately, bash can't do this easily. What can you do?

Quoting you have to do yourself, but it's pretty straight-forward. Replace \ with \\, ' with \', and wrap each argument in 's if it contains anything suspicious. Here's something that does this:

function quote() {
  first=true
  for arg in "$@"; do
    if $first; then first=false; else echo -n " "; fi
    if echo "$arg" | grep -q '[^a-zA-Z0-9./_=-]'; then
      arg="'$(echo "$arg" | sed -e 's~\\~\\\\~g' -e "s~'~\\\\'~g")'"
    fi
    echo -n "$arg"
  done
  echo
}

Going the other way is both simpler and kind of evil. It turns out that the way you do this is:

eval unquoted=("$quoted")
A warning though: if quoted contains things like $(echo foo) then they will be run. In many cases this isn't a problem, but it's something to be aware of.

(I needed both of these when writing the automated installation script for ngx_pagespeed. You need to be able to give it arguments to pass through to nginx's ./configure, and I wanted people to be able to enter them exactly as they would on the command line. This means I need to parse them into an array first. On the other hand, if you're just using the script to set up ngx_pagespeed it needs to print out a string that you should paste as ./configure arguments, which means I need quoting as well.)


[1] Which would actually need to work like read and be passed the name of a variable to put the array in, since you can't return arrays in bash.

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