accessing a positional parameter through a variable

Question

I'm trying to access a positional parameter using a variable.

i.e. if I define a variable as 1, I want to access the 1st positional parameter, if I define it as 2 I want to access the 2nd positional parameter.

example: bash script.bash arg1 arg2 arg3

 var=2
 echo $var // will only echo no. 2
 ??? // insert your command to echo the 2nd positional parameter (arg2)

can this be done?

Yes! thanks. But just to make it easier for other readers - you access it like this: ${!var}, i.e. ehco ${!var} will echo arg2 — Maverick Meerkat, Aug 01 '16 at 06:59
But that solution will make the script less portable, see the comments that follow my answer. — sjsam, Aug 01 '16 at 07:09

sjsam · Accepted Answer · 2016-08-01T07:34:27.833

2

Do

[[ "$var" -eq "1" ]] && echo "$1" #for positional parameter 1

and so on or you could do it like below :

#!/bin/sh
var=2
eval echo "\$${var}" # This will display the second positional parameter an so on

Edit

how do I use this if I actually need to access outside of echo. for example "if [ -f \$${var} ]"

The method you've pointed out it the best way:

var=2 
eval temp="\$${var}"
if [ -f "$temp" ] # Do double quote
then
#do something
fi

edited Aug 01 '16 at 07:34

answered Aug 01 '16 at 06:54

sjsam

Uh, that's not very scalable in the general case. – tripleee Aug 01 '16 at 06:55
1

Bash has `${!var}` to avoid using eval – 123 Aug 01 '16 at 07:04
@tripleee : Please see the update. – sjsam Aug 01 '16 at 07:05
@123 : Valid point, but that would make the solution bash specific,. For example `dash` doesn't support `${!var}` – sjsam Aug 01 '16 at 07:07
Question is tagged bash. – 123 Aug 01 '16 at 07:09
@123 : Question is tagged `shell` too. My solution works at the least on bash, dash, ksh, – sjsam Aug 01 '16 at 07:11
1

I added the [tag:bash] tag because the OP had that in their example, but I'm a bit hesitant. – tripleee Aug 01 '16 at 07:13
I use bash. In any case - how do I use this if I actually need to access outside of echo. for example "if [ -f \$${var} ]" ? – Maverick Meerkat Aug 01 '16 at 07:21
2

ok, I got it - you declare the variable in advance. I.e. var=2 eval var2="\$${var}" . – Maverick Meerkat Aug 01 '16 at 07:28
1

Nice, thanks for the note on portability. – Мона_Сах Aug 01 '16 at 07:42

1 Answers1