Splitting /proc/cmdline arguments with spaces

Question

Most scripts that parse /proc/cmdline break it up into words and then filter out arguments with a case statement, example:

CMDLINE="quiet union=aufs wlan=FOO"
for x in $CMDLINE
do
»···case $x in
»···»···wlan=*)
»···»···echo "${x//wlan=}"
»···»···;;
»···esac
done

The problem is when the WLAN ESSID has spaces. Users expect to set wlan='FOO BAR' (like a shell variable) and then get the unexpected result of 'FOO with the above code, since the for loop splits on spaces.

Is there a better way of parsing the /proc/cmdline from a shell script falling short of almost evaling it?

Or is there some quoting tricks? I was thinking I could perhaps ask users to entity quote spaces and decode like so: /bin/busybox httpd -d "FOO%20BAR". Or is that a bad solution?

Answer 1

Using /proc/cmdline from a shell-script to access the command line is the wrong way. Why not use $@?

for arg in "$@"; do
    ...
done

EDIT: added quotes around $@.

EDIT: misread the question; /proc/cmdline is the kernel command line.

Answer 2

In posh:

$ f() { echo $1 - $3 - $2 - $4 
> }
$ a="quiet union=aufs wlan=FOO"
$ f $a
quiet - wlan=FOO - union=aufs -

You can define a function and give your $CMDLINE unquoted as an argument to the function. Then you'll invoke shell's parsing mechanisms. Note, that you should test this on the shell it will be working in -- zsh does some funny things with quoting ;-).

Then you can just tell the user to do quoting like in shell:

#!/bin/posh
CMDLINE="quiet union=aufs wlan=FOO"
f() {
        while test x"$1" != x 
        do      
                case $1 in
                        union=*)        echo ${1##union=}; shift;;
                        *)              shift;; 
                esac    
        done    
}       
f $CMDLINE

(posh - Policy-compliant Ordinary SHell, a shell stripped of any features beyond standard POSIX)

Answer 3

Most commonly, \0ctal escape sequences are used when spaces are unacceptable.

In Bash, printf can be used to unescape them, e.g.

CMDLINE='quiet union=aufs wlan=FOO\040BAR'
for x in $CMDLINE; do
    [[ $x = wlan=* ]] || continue
   printf '%b\n' "${x#wlan=}"
done