String concatenation doesn't work for comma character

问题

String concatenation on bash script doesn't work on comma "," character.

A="Hello";
B=",World";
C=$A$B
echo $C;

It prints the output as

Hello World

Bash version is:

GNU bash, version 4.1.2(1)-release (x86_64-redhat-linux-gnu)

The same code seems to work in here

回答1:

The most likely explanation is that you have $IFS set to ,

The simplest way around this is to double-quote $C, in which case echo is passed the value unmodified:

echo "$C"

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

To print the current value of $IFS in unambiguous form, use

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

As for why the , disappeared:

When you use an unquoted variable reference such as $C, the shell applies various shell expansions to the value.
Notably, word splitting is applied, which means that the value is split into tokens using any of the characters contained in the special $IFS variable as a separator ("IFS" stands for "Internal Field Separator").
By default, $IFS contains a space, a tab, and a newline, effectively splitting by whitespace.
In your case, $IFS likely contained , resulting in Hello,World getting split into Hello and World, which are then passed to echo as separate arguments. As stated, double-quoting variable references prevents this behavior.
echo, when given multiple arguments, always uses a single space to separate them on output.

Tips for setting $IFS:

Since $IFS is a global variable, it's good practice to restore it to its previous value after changing it:

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 techniques that localize the change, which means you do not have to explicitly save and restore its previous value:

If a custom $IFS value is only needed for a single command based on an external utility or builtin -- typically read -- prepend IFS=... to the command; e.g.:

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

This makes the changed $IFS take effect only for the command invoked.

Caveat: This does NOT work in situations where the changed IFS value must take effect BEFORE invoking the builtin / executable, such as with shell expansions; e.g.:

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

Inside a shell function, if you want to change $IFS for the entire 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 feasible, enclose a list of commands in a subshell:

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

The $IFS modification is only visible to subshell.

Caveat: variables modified or created in the subshell are not visible to the current shell (which is, in fact, what this technique relies on).

来源：https://stackoverflow.com/questions/29493353/string-concatenation-doesnt-work-for-comma-character

标签

bash

string-concatenation