Passing shell script arguments to a subprocess

#!/bin/sh
export MY_VAR=MY_VAL
exec my_command $*

#!/bin/sh
export MY_VAR=MY_VAL
args=
for arg in "$@";
do
  args="$args '$arg'"
done
eval exec my_command $args

#!/bin/sh
export MY_VAR=MY_VAL
nargs=$#
args=
while [ $nargs -gt 0 ]
do
  args=""$$nargs" $args"
  nargs=`expr $nargs - 1`
done
eval exec my_command $args

Hm. I use:

exec my_command "$@"

which quotes it properly for you. For more complicated scenarios, I use bash arrays and "${args[@]}" – it's not usually worthwhile avoiding bash to get this all working portably.

See `man bash` in the section "Special Parameters". You want @, not *, in the individually quoted form, like Barry mentioned.

When performing basic arithmetic, at least in bash, you can do: nargs=$((nargs-1)) which should be faster than expr.

regarding the (()) syntax: yeah, but not in sh; Ubuntu's sh is a real sh and not a bash, for some reason, which breaks loads of scripts used to it being bash...

regarding "$@": it works if you don't need to fiddle with any of the arguments, which my example doesn't do (the SO example I linked to does.)

updated the text to mention "$@" – thanks!

I routinely do "sudo dpkg-reconfigure dash" on Ubuntu installations to revert to bash, too much script breakage with the default "dash" shell.

And that is why shell sucks for anything over one line. It has lots of nifty shortcuts that save your time, but they tend to get in the way when you want robust scripts.
For one-liners, though, it's perfect.

Your ending comment should come a little earlier – the general solution for this should be to use a language where the arguments are in a proper $container of strings. Only when that's not possible, invoke black quote magic.

I'd recommend relying on splicing and unsplicing (the latter may be emulated, see below) as black boxes instead of tampering with single or double quotes.

This one is pure sh (I hope it won't be messed up when posted):

#!/bin/sh

store() { r="$@"; }

r=
for arg in "$@" ; do
echo "Fiddling with $arg"
store $r "$arg"
done

echo "Args after fiddling: $r"

There once was a bug in an ancient sh, making "$@" expand incorrectly into "" (one empty argument). That's why you can meet ${1+"$@"} in some scripts, used instead of "$@", but I don't think you can stumble upon a shell where it's still necessary.

One liner

args=`seq -s' ' -f '"$%1.0f"' 1 $#`

Your workaround breaks down anyway for non-bash sh if you have more than 9 arguments.

About $(()), yes, you can do it in sh too, but you have to use the regular parameter expansion syntax inside it: $(($nargs-1)). And is POSIX, in case you wonder...
http://pubs.opengroup.org/onlinepubs/009695399/utilities/xcu_chap02.html#tag_02_06_04

In some simpler cases you can use shift and set to modify the positional parameters. For example, to munge only the first one:

first="$1"; shift
upcased="$(echo "$first" | tr a-z A-Z)"
set — "$upcased" "$@"
exec my_command "$@"

Unfortunately I don't think (off the top of my head) there's a way to generalize this to unknown numbers of parameters.

OK, so you've all convinced me that my knowledge of the shell is insufficient for most practical purposes, and that I'd probably rather keep it that way than delude myself into believing that I'd ever improve... I'll try harder to use a language giving me an argument array next time.

And, as to there being no $i beyond $9 – thanks, I'll mention that...

I'm surprised nobody's mentioned this, so I'll be that guy: use zsh.

#!/usr/bin/env zsh

Not to be snarky, but isn't this why Perl was invented? Portable, robust, and much more consistent than sh.

Here's a sample script I keep in my example sources for handy use.

It shows how to squirrel away "$@" for safe keeping and re-use.

It could work with V7 sh, I think, by using expr instead of shell arithmetic to increment the counter and of course by doing it inline instead of defining a function (and also of course losing the "local" declaration). One could probably do without expr too and just use $# and shift to decrement it, but that would lead to the confusing result in that the list_* variables would be numbered backwards.

A bit of trivia: with V7 sh, and probably most of its earlier derivatives, one would have to use ${1+"$@"} instead of just "$@" in order to avoid passing an empty parameter when none were originally supplied. See:

http://www.in-ulm.de/~mascheck/various/bourne_args/

(AllanLane5: that was a joke right? You don't seriously think Perl is more consistent than sh, do you? LOL! Robust? ROTFL! maybe a bit more portable, but not since POSIX in, what, )

#! /bin/sh
#
# save_args — keep a list of quoted strings for later use via "$@"
#
# Note: creates a variable for each value, and uses these in another
# variable which can then be expanded with "eval" to re-reate the
# original parameters, preserving words containing whitespace
#
# See also tshquote.sh for shell_quote() which can do the same, but
# produces its result on its stdout, and which does this simply using
# the normal single-quote characters.
#
# HOWEVER, shell_quote() requires printf(1) and sed(1), while this
# variant does it all internally to the shell.
#
save_args()
{
local a c n
c=0; n=$1; shift

for a in "$@"; do
c=$(($c+1))
eval $n="$$n \"\$${n}_$c\""
eval ${n}_$c="$a"
done
}
#
# XXX Need to write free_args() to unset all the ${n}_$c variables!!!
#
# use:
#
# save_args list -foo bar "arg with spaces" "don't panic" '"Help!"'
#
# or:
#
# save_args list *.pdf # filenames with whitespace
#
# later:
#
# eval "gv ${list}"
#
# or, being a bit silly:
#
# . ./tshquote.sh
#
# eval set — ${list}
#
# for str in "$@"; do
# q="$(shell_quote "$str")"
# list2="${list2}${list2:+ }${q}"
# done

Sure /bin/sh is a real sh – but does that also mean /bin/bash is absent? Because if you are worried only about Ubuntu, then the answer is No. And if the answer is No then you can just put /bin/bash in your shebang. (Just because Ubuntu makes you actually say that you want bash and not just sh when you do want bash and not just sh does not mean you cannot use bash at all.)

Then you can copy the args to an array variable using

local ARGS=( "$@" )

whose elements you can then molest to your heart’s desire. Once done, you run your program with

exec foo "${ARGS[@]}"

Very simple.

Kartik Agaram:

I believe the suggestion to use zsh falls under Yossi’s “use another language” clause. :-)

You know, I really should stay away from the shell. My mental faculties responsible for things like ("$@") and "${ARGS[@]}" are underdeveloped, so I'm just inflicting pain upon myself.

I'd rewrite the last line as:

exec my_command ${1+"$@"};

SUSv3: "The parameter name or symbol can be enclosed in braces, which are optional except for positional parameters with more than one digit or when parameter is followed by a character that could be interpreted as part of the name."

That is, $10 is interpreted as $1 with a 0 afterwards. For positional parameters after $9, you should refer to them as ${10} etc.

Some shells (e.g. dash) fail this conformance and actually allow $10 to mean ${10}.

I'm working on a new non-POSIX command shell that cannot have this category of stupidity. Check it out: http://shok.io though note that this is EXTREMELY early, i.e. I haven't even really launched yet, and any feedback is appreciated.....

You save my day!

nfomon that is the solution

This is safer and more straightforward than using eval:

dualbus@yaqui ~ % ./args.sh foo bar baz
foo barbar bazbaz
dualbus@yaqui ~ % cat args.sh
#!/bin/sh

ENV_VAR=value;
export ENV_VAR

nargs=$#
for arg do
case $arg in
*[abc]*) arg=$arg$arg;;
esac

set "$@" "$arg"
done

shift "$nargs"

echo "$@"

# In case this doesn't show correctly here:
# http://dualbus.sdf.org/s/a7832dc9.txt

Previous samles with 'set' work fine but marke sure to use

set — "$@" "$arg"

Otherwise if the first argument is a valid one for set it will use it, e.g. -x.

Thanks! Yeah, I've seen — passed to every other command in git's shell source code... "Do one job and do it well..."

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