4

I have executed the following command sequence:

$ now=$(date)
$ echo _$now_
_
$ echo _ $now _
_ Mon Sep 22 09:53:44 IST 2014 _

Why is the output of _$now_ only _?

0

4 Answers 4

11

From man bash:

DEFINITIONS
       The following definitions are used throughout the rest of this document.
       blank  A space or tab.
       word   A sequence of characters considered as a single unit by the shell.
              Also known as a token.
       name   A word consisting only of alphanumeric characters and underscores, 
              and beginning with an alphabetic character or an underscore.  Also 
              referred to as an identifier.
...
PARAMETERS
       A parameter is an entity that stores values.  It can be a name, a number, 
       or one of the special characters listed below under Special Parameters.  
       A variable is a parameter denoted by a name.

A variable can only have alphabets, numbers and underscores. And so now_ is a valid variable name, and is interpreted as such.

You can delimit the variable name in different ways:

_"$now"_
_${now}_
_$now"_"
_$now'_'

Or any combination of the above.

10

because _ is part of the variable name in your echo _$now_

use echo \_$now\_ instead.

Also you can use that just in a linear command: echo _$(date)_

2
  • 1
    or use braces to separate the variable name from the surrounding text: echo _${now}_ Commented Sep 22, 2014 at 12:27
  • @glennjackman that's covered in @muru's answer. thankx Commented Sep 22, 2014 at 12:31
7

Bear with me for a moment, this requires a bit of explaining.

First of, why is output of _ $(date) _ is _ Mon Sep 22 03:30:34 MDT 2014 _ ? Because this literally tells echo to output _ first then output $(date) then _ . Spaces separate the variables for echo.

Now try echo _$(date), note no space between _ and $(date). In this case output will be _Mon Sep 22 03:32:40 MDT 2014 . What does this do? you tell echo to concatenate underscore with output of $(date).

Try same thing with _$PWD, which will concatenate your working directory with underscore. Now try echo $PWD_ . The output will be blank. Why? because PWD_ is non-existent environment variable, and as others have mentioned _ is valid character for an environment variable, e.g. $XDG_CURRENT_DESKTOP .

So why _$PWD_ would give _ ? Because you are telling echo to concatenate _ with output of non-existent environment variable. So _ is printed, but $PWD_ output is blank, so you literally see _ concatenated with that blank output.

2
  • 1
    Then, why print _? Commented Sep 22, 2014 at 7:56
  • @ KasiyA I didn't read question first time around properly. See the edited answer Commented Sep 22, 2014 at 9:41
1
_$now_ 

is interpreted as

_${now_}

which is, in your case, obviously the string concat of '_' and ''. Use therefore

_${now}_

instead. It is much more clear to read.

You must log in to answer this question.

Not the answer you're looking for? Browse other questions tagged .