[SOLVED] Quoting function calls in bash

Faki · 05-18-2022, 06:01 PM

Let me define a function as follows

Code:

time-stamp ()
 {
  if (( $# == 0)); then
    tmstamp="$(date +T%T.%N)"
  else
    tmstamp="$(date $1)"
  fi
  echo "$tmstamp"
 }

I can call the function using

Code:

stamp=$( time-stamp "+T%T.%N" )

Are there instances when one would quote the actual command with

Code:

stamp="$( ... )"

or

Code:

stamp='$( ... )'

MadeInGermany · 05-18-2022, 08:21 PM

Within " " $-expressions are evaluated, but it protects from further expansions.
You should use it in command arguments

Code:

echo "$( ... )"

You can use it in assignments

Code:

stamp="$( ... )"

but there is no effect, because in assignments there are no further expansions.

Within ' ' there are never any evaluations or expansions. (Unless you put an eval keyword in front.)

Faki · 05-19-2022, 02:56 AM

Complications arise when the call has arguments

Code:

stamp="$( date "$var" )"

pan64 · 05-19-2022, 05:08 AM

Quote:

Originally Posted by Faki

Complications arise when the call has arguments

Code:

stamp="$( date "$var" )"

what complications?

Code:

stamp="$( date "${var:-+T%T.%N}" )"

Faki · 05-19-2022, 05:37 AM

Would there not be a problem having quotes inside quotes? I was trying to quote $var. Specifically, how would I pass a variable that includes a format with spaces?

pan64 · 05-19-2022, 06:04 AM

why don't you try it yourself?
Bash recognizes $( ) and knows what is inside and outside. So it is not quotes inside quotes.

MadeInGermany · 05-19-2022, 06:29 AM

Within $( ) and ( ) there can be any shell code.
It is a "sub shell", forked from the main shell so it gets a copy of all the variables. When it terminates the main shell continues (variables from the sub shell are not copied back).

Faki · 05-19-2022, 08:07 AM

I shall use

Code:

var=$(command "$opts")

MadeInGermany · 05-21-2022, 10:24 AM

Yes perfect; this is an assignment.

In a command you should have a $-expression in "quotes", otherwise the shell attempts further expansions:

Code:

echo "$( date "$var" )"

echo is a command in the main shell where $( ) should be in quotes,
date is a command in a sub shdell where $var should be in quotes.