Replacing bash arrays and C-style for loops in POSIX sh

Question

I want to convert following bash code in pure shell script (sh) language so that it should run by other script language mode i.e dash script.

arguments=("$@")
for (( i=0; i<$#; i++ )); do
  case "${arguments[$i]}" in
    -foo)
    let "i = i + 1"
    echo "${arguments[$i]}"
      ;;
    *)                                                                                        
      break
  esac
done

above code finely run in bash mode but through an error on dash mode.

2: ./orig.sh: Syntax error: "(" unexpected

Now If I change line 2 to get rid the error as following line

arguments="$@"

But now I got another error about loop

 3: ./orig.sh: Syntax error: Bad for loop variable

you can see that error just starting a script file with #!/bin/dash — Equation Solver, Oct 23 '14 at 13:33
If I had a machine with dash handy I'm sure I could do that. I don't. You do and you are asking the question. — Etan Reisner, Oct 23 '14 at 13:34
If a run that script on dash I got '2: ./orig.sh: Syntax error: "(" unexpected' — Equation Solver, Oct 23 '14 at 13:37
We aren't a free code translation service. If you asked how to convert from bash to Python, you'd get downvoted and closed for that too. — Charles Duffy, Oct 23 '14 at 13:42
...that said, I've tried to edit the question to be a bit more on-topic. — Charles Duffy, Oct 23 '14 at 13:47
The code you actually posted could be replaced with `[ "$1" = "-foo" ] && echo "$2"`. No loop needed at all, since the loop terminates as soon as it sees the first non-`-foo` argument. — chepner, Oct 23 '14 at 13:57

Charles Duffy · Answer 1 · 2014-10-23T14:50:13.107

4

POSIX sh doesn't support arrays or C-style for loops.

i=0
while [ $(( ( i += 1 ) <= 10 )) -ne 0 ]; do
  eval "val=\$$i"
  case "$val" in
    -foo)
      i=$(( i + 1 ))
      eval "val=\$$i"
      echo "$val"
      ;;
    *)                                                                                        
      break
  esac
done

edited Oct 23 '14 at 14:50

answered Oct 23 '14 at 13:45

Charles Duffy

280,126
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The `echo` should output the argument following `-foo`, not `-foo` itself. – chepner Oct 23 '14 at 13:59
Thanks (and for the reassurance as well); should now be fixed. – Charles Duffy Oct 23 '14 at 14:50

chepner · Answer 2 · 2014-10-26T15:18:04.490

3

The POSIX way to handle the arguments is to consume them using shift while $# is non-zero.

while [ $# -gt 0 ]; do
    case $1 in
      -foo)
        echo "$2"
        shift; shift
        ;;
      *)
        break ;;
    esac
done

edited Oct 26 '14 at 15:18

answered Oct 23 '14 at 13:58

chepner

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I suddenly feel very silly for doing a literal translation rather than looking at intent. – Charles Duffy Oct 23 '14 at 14:40
I debated posting this or just commenting on your answer. Yours is non-destructive and preserves the value of i where processing left off, so I think both are useful in different contexts. – chepner Oct 23 '14 at 14:42
Fair enough; retracted my self-deletion. :) – Charles Duffy Oct 23 '14 at 14:51
1

One minor correction; the `> 0` should be `-gt 0`. (Quoting the echo wouldn't hurt either). – Charles Duffy Oct 23 '14 at 16:27

score -2 · Answer 3 · answered Oct 23 '14 at 16:17

-2

Now I have a Answer of my question, something like following works for me.

arguments="$@"
for i in $@; do
  case "$1" in
    -foo)
    shift 2
    echo "$1"
      ;;
    *)                                                                                        
      break
  esac
done

answered Oct 23 '14 at 16:17

Equation Solver

545
7
18

Don't ever do `arguments="$@"` in a POSIX shell -- it won't behave the way you expect. Compare the way it works when your script is run with `-foo "hello world"` and the way it works when your script is called with `-foo hello world`; the original bash script handled the former case, in particular, correctly. – Charles Duffy Oct 23 '14 at 16:25
also, using `$@` unquoted -- as you do here -- makes it behave the same way as unquoted `$*`, with all the pitfalls and caveats of that usage (stemming, principally, from string-splitting and glob expansion). Moreover, you don't even _need_ a for loop at all -- `while [ $# -gt 0 ]` would be more correct! – Charles Duffy Oct 23 '14 at 16:26
A good test, by the way, would be to see whether `./yourscript -foo "hello * world"` prints `hello * world` as it should -- that would catch both string splitting and glob expansion behavior. – Charles Duffy Oct 23 '14 at 16:29
This will also fail if you forget the second argument; `shift 2` will do nothing if there aren't two arguments to shift. – chepner Oct 23 '14 at 21:23
@CharlesDuffy, Hi, then what is the POSIX shell way to write the expression `arguments="$@"` – Equation Solver Oct 27 '14 at 12:42
@EquationSolver, there *is* no POSIX shell way to write that expression, since POSIX has no way to store an array (a list of separate arguments) in a single variable while preserving the original delimiters. – Charles Duffy Oct 27 '14 at 14:31
...this shouldn't be surprising -- if POSIX sh were as expressive as bash, there would have been no need for bash to have created extensions in the first place. – Charles Duffy Oct 27 '14 at 23:26

Replacing bash arrays and C-style for loops in POSIX sh

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