In a Bash script, how to use a variable to set multiple command options?

Question

I want to store options to arbitrary commands as strings in bash so that I can do e.g.

presets_A='-A'
presets_B='-A -l -F'
ls $presets_A
ls $presets_B

The first one works, the socond gives ls: invalid option -- ' '.

The same happens when I try to store the entire command in a string variable (as opposed to a function or an alias, which is not what I want):

presets_A='ls -A'
presets_B='ls -A -l -F'
$presets_A
$presets_B

This gives ls -A: command not found. Not good. Obviously, I haven't yet found the correct arbitrary mixture of $%"(@]}" quotes and parens that Bash is so famous for. ${!presets_A} also didn't work but chances are I got confused and messed up.

EDIT for clarification, I do know how to use a function to encapsulate setting options and subsuming a bunch of parametrized calls under a single command. What I'm looking for is the equivalent of (Bash) foo "$@" or (Python) foo( *P, **Q ) or JS foo( ...P ) such that I get a single serializable and transportable value that comprises the arguments to a call in a single place.

Obligatory link mywiki.wooledge.org/BashFAQ/050 - and it's not an arbitrary mixture of those symbols that you're looking for. — Tom Fenech
– Tom Fenech, Commented Mar 12, 2018 at 13:10
I'm unable to reproduce this with your code as posted. Are you sure you're not quoting the variables? Have you modified IFS? — that other guy
– that other guy, Commented Mar 12, 2018 at 13:39
as @thatotherguy said, there's nothing actually wrong with presets_B='-A -l -F' followed by ls $presets_B - could you expand on your question so that we can reproduce your issue? — Tom Fenech
– Tom Fenech, Commented Mar 12, 2018 at 14:27
Why don't you just use an alias like alias lB="ls -A -l -F" — user unknown
– user unknown, Commented Mar 13, 2018 at 12:08

Tom Fenech · Accepted Answer · 2018-03-12 13:14:15Z

4

If functions really don't do what you want, then you can use an array to separate the arguments:

presets_A=( -A )
presets_B=( "${presets_A[@]}" -l -F ) # or just -A -l -F

ls "${presets_A[@]}" # ls -A
ls "${presets_B[@]}" # ls -A -l -F

Use "${array[@]}" to expand the array into a list of arguments, separated by the field separator (a space, by default).

But I would consider just defining two functions

la () {
  ls -A
}

lalf () {
  ls -A -l -F
}

answered Mar 12, 2018 at 13:14

Tom Fenech

75.1k13 gold badges119 silver badges154 bronze badges

Sign up to request clarification or add additional context in comments.

1 Comment

Gabe Gates Over a year ago

Worked like a charm for my script to preload common database options, thanks!

John Frazer · Accepted Answer · 2018-03-13 12:16:22Z

I've since found the source for my troubles; I had tried to use a variable for options in a script with this header:

#!/usr/bin/env bash
set -euo pipefail; IFS=$'\n\t'

presets_A='-A'
presets_B='-A -l -F'
ls $presets_A
ls $presets_B

It does work as intended when I remove the Internal Field Separator setting, and I therefore think the correct answer to my question is:

"Storing multiple command arguments in a variable is OK in Bash and should work as long as IFS has not been reset to something other than a space."

I can affirm that

#!/usr/bin/env bash
set -euo pipefail; IFS=$' '

presets_A='-A'
presets_B='-A -l -F'
command_C='ls -A -l -F'
ls $presets_A
echo '----------------------------------'
ls $presets_B
echo '----------------------------------'
$command_C

does work as expected; thanks go to @TomFenech for pointing out that much (note the explicit IFS=' ' near the top).

FWIW the reason i had re-set IFS is that there are quite a few recommendations to do so out there; they call it 'Bash strict mode'. Turns out it makes your scripts incompatible in subtle ways.

Collectives™ on Stack Overflow

In a Bash script, how to use a variable to set multiple command options?

2 Answers 2

1 Comment

Comments

Your Answer

Hot Network Questions

Collectives™ on Stack Overflow

2 Answers 2

1 Comment

Comments

Your Answer

Sign up or log in

Post as a guest

Related