String concatenation does not work for comma

the most likely explanation is that you have set to $IFS

,

The easiest way is double-quote$C

, in this case echo

the value is passed unmodified:

echo "$C"

      

        
        
        
      

    

^{Also note that you don't need semicolons to break your commands, given that each command is on its own line.}

To print the current value$IFS

in an unambiguous manner, use

printf '%q\n' "$IFS"  # the default value will print as $' \t\n' - space, tab, newline

      

        
        
        
      

    

As for why it ,

disappeared :

• When you use an unquoted reference, for example $C
  
  , the shell applies different shell extension values to the value.
• Specifically, word splitting is applied, which means that the value is split into tokens using any of the characters contained in the special variable $IFS
  
  as the delimiter ("IFS" stands for "Internal Field Separator").
• $IFS
  
  Contains space, tab and newline by default , effectively separating spaces.
• In your case $IFS
  
  , it most likely contained ,
  
  , resulting in Hello,World
  
  split into Hello
  
  and World
  
  , which are then passed echo
  
  in as separate arguments. As stated, double quoted links do not allow this behavior.
• echo
  
  when multiple arguments are given, a single space is always used to allocate them on output.

Tips for setting $IFS

:

Since it is a global variable , it is recommended to restore its previous value after changing it: $IFS

prevIFS=$IFS IFS=',' # save old value, set desired value (',', in this example)
# ... perform operations with custom $IFS in effect
IFS=$prevIFS         # restore old value

      

        
        
        
      

    

However, there are methods that localize the change , which means you don't need to explicitly save and restore its previous value:

If a custom value $IFS

is only required for a single command based on an external utility or builtin - usually read

- prepend IFS=...

for the command ; eg:.

IFS=/ read -r var1 var2 <<<'a/b'  # -> $var1 == 'a', $var2 == 'b'

      

        
        
        
      

    

This causes the changed to be $IFS

effective only for the called command.

Caution . This does NOT work in situations where the changed IFS value needs to take effect BEFORE invoking the embedded / executable, such as with a shell extension; eg:.

 # !! Does NOT work as intended, because $var is expanded BEFORE `IFS=/` takes effect.
var='a/b'; IFS=/ set -- $var

      

        
        
        
      

    

Inside a wrapper function , if you want to change $IFS

for the whole function, but only for that function , use a local $IFS

variable that shadows the global $IFS

:

foo() {
    local IFS=/ var1 var2 # $IFS change confined to this function due to `local`
    read -r  var1 var2 <<<"$1"
    echo "[$var1] [$var2]"
 }

 foo "a/b" # -> '[a] [b]'

      

        
        
        
      

    

If possible, enclose the command list in a subshell :

(arr=(a b); IFS=/; echo "${arr[*]}") # -> 'a/b'

      

        
        
        
      

    

The modification $IFS

is only visible to the subshell.

Caveat : Variables modified or created in the subshell are not visible to the current shell (which is actually what this method relies on).