Hello,

Have a little trouble with string concatenation in for loop in bash....

okay so here is my code
<code>
var1="one"
var2="two"
var3="three"
var4="four"

numbers="$var1 $var2
$var3 $var4"
echo "$numbers"
</code>

gives me the output I want

output: one two
three four

but in a for loop

<code>
var1="one"
var2="two"
var3="three"
var4="four"
numbers="$var1 $var2
$var3 $var4"

for num in $numbers
do
echo "$num"
done
</code>

I don't get my expected output

output: one
two
three
four

thanks for any help!

You need to quote $numbers in your for loop declaration. Right now it is separating parameters by whitespace and the quotes within the loop are meaningless (since there are no terms with spaces in them).

Thanks so much that worked great!

Heh, looking at this again that loop isn't doing anything...it's treating the whole expression as one term. A better construct would be the following:
which separates by line and gives the correct number of terms (2). Note that this is a bashism...you could use
to be more portable but this spawns a subshell which produces fun results when creating/modifying variables within the loop.

Unless I've misunderstood your intent, of course.

First, please use [code][/code] tags around your code, to preserve formatting and to improve readability.


Next, it's vitally important to understand the way bash handles arguments, whitespace, and word-splitting. Don't do anything else until you've finished reading this page: http://mywiki.wooledge.org/Arguments


Now that that's out of the way, a for loop works on lists of words (as defined by the shell's word-splitting operation). Each element is taken into the variable in turn, and the subsequent operations performed. So if you don't quote the input variable, then it will split it into individual words wherever there's whitespace (by default at least). But if you do quote it, it will treat the whole thing as a single block. Neither option does exactly what you want.

Therefore, when you want to operate on lines, you should use a while+read loop instead (as demonstrated by T3slider). The read command grabs whole lines of input (by default), so you don't have to worry about shell word-splitting occurring on other whitespace.

I would, however, use a here string to take input from a variable, rather than process substitution, as it doesn't require running a subshell.

Finally, a variable is designed to hold an individual string of characters. When you have multiple strings to work on (strings of strings), you should generally use an array instead. Set each array element to hold one line, and you can go back to using a for loop, and call on the array's index numbers to manipulate the individual elements.

When you have the time, read through this guide in its entirety. It will teach you pretty much all the background concepts you need to know for good bash scripting: http://mywiki.wooledge.org/BashGuide