OT: bash help

[Mike Wright]

Hi all,

I'm trying to write a simple script that if provided an argument, uses 
that, or if nothing is provided, uses a predefined string.

if [ -n $# ]
then
     WORDS=$1
else
     WORDS="these are some words"
fi
echo $WORDS;

The second case is always comes back "".

But if I write

WORDS='these are some words'
echo $WORDS

I get the assigned string.

Why doesn't the assignment work when inside an if/then?  How do I make 
it work?  What's the difference between the case where the assignment is 
inside the if/then and outside the if/then?

TIA,
Mike Wright

[Mike Wright]

08/16/2014 04:14 PM, Mark C. Allman wrote:
>  On Sun, 2014-08-17 at 00:42 +0200, Suvayu Ali wrote:
> > On Sat, Aug 16, 2014 at 02:44:14PM -0700, Mike Wright wrote:
> >> Hi all,
> >>
> >> I'm trying to write a simple script that if provided an argument, uses that,
> >> or if nothing is provided, uses a predefined string.
> >>
> >> if [ -n $# ]
> >
> > This will always be true.  -n tests if a string is empty or not.  0
> > counts as non-empty.  You should use one of the comparison operators.
> > Try something like this:
> >
> >    [ $# -gt 0 ]
> >
> > Hope this helps,
> >
> > --
> > Suvayu
> >
> > Open source is the future. It sets us free.
>
>  Are you looking for behavior that the following test script
>  demonstrates?
>
>       #!/bin/bash
>
>       theArg=${1:-The Default Value}
>
>       echo $theArg 
Thanx Mark,

Very elegant.  I like it.

I was just going through the section on "variable substitution" in "Unix 
in a Nutshell" and saw that.

[Suvayu Ali]

On Sat, Aug 16, 2014 at 02:44:14PM -0700, Mike Wright wrote:
>  Hi all,
>  
>  I'm trying to write a simple script that if provided an argument, uses that,
>  or if nothing is provided, uses a predefined string.
>  
>  if [ -n $# ] 
This will always be true.  -n tests if a string is empty or not.  0
counts as non-empty.  You should use one of the comparison operators.
Try something like this:

  [ $# -gt 0 ]

Hope this helps,

-- 
Suvayu

Open source is the future. It sets us free.

[Mark C. Allman]

On Sun, 2014-08-17 at 00:42 +0200, Suvayu Ali wrote:
>  On Sat, Aug 16, 2014 at 02:44:14PM -0700, Mike Wright wrote:
>  > Hi all,
>  > 
>  > I'm trying to write a simple script that if provided an argument, uses that,
>  > or if nothing is provided, uses a predefined string.
>  > 
>  > if [ -n $# ]
>  
>  This will always be true.  -n tests if a string is empty or not.  0
>  counts as non-empty.  You should use one of the comparison operators.
>  Try something like this:
>  
>    [ $# -gt 0 ]
>  
>  Hope this helps,
>  
>  -- 
>  Suvayu
>  
>  Open source is the future. It sets us free. 
Are you looking for behavior that the following test script
demonstrates?

    #!/bin/bash

    theArg=${1:-The Default Value}

    echo $theArg

-- 
Mark C. Allman, PMP, CSM
Founder, See How You Ski, www.seehowyouski.com
617-947-4263, Twitter:  @allmanpc

[Joe Zeff]

On 08/16/2014 02:44 PM, Mike Wright wrote:
>  I'm trying to write a simple script that if provided an argument,
> uses
>  that, or if nothing is provided, uses a predefined string. 
I found this example that might help: 
http://tldp.org/LDP/abs/html/comparison-ops.html#STRTEST

[Fred Smith]

On Sat, Aug 16, 2014 at 02:44:14PM -0700, Mike Wright wrote:
>  Hi all,
>  
>  I'm trying to write a simple script that if provided an argument,
>  uses that, or if nothing is provided, uses a predefined string.
>  
>  if [ -n $# ]
>  then
>      WORDS=$1
>  else
>      WORDS="these are some words"
>  fi
>  echo $WORDS;
>  
>  The second case is always comes back "".
>  
>  But if I write
>  
>  WORDS='these are some words'
>  echo $WORDS
>  
>  I get the assigned string.
>  
>  Why doesn't the assignment work when inside an if/then?  How do I
>  make it work?  What's the difference between the case where the
>  assignment is inside the if/then and outside the if/then? 
This works:

	#!/bin/sh
	if [ $# -ne 0 ]
	then
	    WORDS=$1
	else
	    WORDS="these are some words"
	fi
	echo $WORDS

the hashbang line should be used, but doesn't actually change the
output of this script.

"$#" is an integer, not a string, and that it should therefore be tested
as an integer and not a string, hence the if statement being:

	if [ $# -ne 0 ]

and you don't NEED the quotes around the literal string, though
it doesn't harm to put them there, perhaps as documentation to make
clear what you intention is.


-- 
-------------------------------------------------------------------------------
 .----    Fred Smith   /              
( /__  ,__.   __   __ /  __   : /     
 /    /  /   /__) /  /  /__) .+'           Home: fredex(a)fcshome.stoneham.ma.us 
/    /  (__ (___ (__(_ (___ / :__                                 781-438-5471 
-------------------------------- Jude 1:24,25 ---------------------------------

[Joe Zeff]

On 08/16/2014 02:44 PM, Mike Wright wrote:
>  if [ -n $# ]
>  then
>       WORDS=$1
>  else
>       WORDS="these are some words"
>  fi
>  echo $WORDS;
>
>  The second case is always comes back "".
>
>  But if I write
>
>  WORDS='these are some words'
>  echo $WORDS
>
>  I get the assigned string. 
In your first example, you use full quotes (") but in your second, you 
use single (').  Try using single quotes and see if it makes a difference.

[Ian Malone]

On 17 August 2014 08:36, Cameron Simpson <cs(a)zip.com.au&gt; wrote:
>  On 16Aug2014 14:44, Mike Wright <mike.wright(a)mailinator.com&gt;
> wrote:
> >
> > I'm trying to write a simple script that if provided an argument, uses
> > that, or if nothing is provided, uses a predefined string.
>
>
>  This is general shell stuff, not bash specific.
>
> > if [ -n $# ]
>
>
>  This is always true. Even "0" is a nonempty string.
>
>  Test [ $# -gt 0 ] instead.
>
>  
Though it's a matter of preference I'd prefer [ $# = 0 ] or  [ $# != 0
] tests unless you really need one of the numeric comparisons. The
reason (and it doesn't actually apply for $#, hence matter of
preference) is that if you have somehow ended up with a string rather
than a number (e.g. an error from wc or something) then you have to
think about what your comparison is going to do for that case, whereas
the =! and == comparisons will do what you expect. If you do need -gt,
-lt and the rest then (except for $#) it's best to check what you've
got is a number first.


-- 
imalone
http://ibmalone.blogspot.co.uk

[Cameron Simpson]

On 16Aug2014 14:44, Mike Wright <mike.wright(a)mailinator.com&gt; wrote:
> I'm trying to write a simple script that if provided an argument,
> uses that, or if nothing is provided, uses a predefined string. 
This is general shell stuff, not bash specific.

> if [ -n $# ] 
This is always true. Even "0" is a nonempty string.

Test [ $# -gt 0 ] instead.

>     WORDS=$1
> else
>     WORDS="these are some words"
> fi 
I write this stuff like this:

   # very near the top of the script
   words="these are some words"

   [... command line parsing...]
   if [ $# -gt 0 ]
   then
     words=$1
     shift
   fi

Tip: run your script with "-x":

   sh -x my_script

It will show the actual commands executed.

Finally:

Never use $UPPERCASE names for variables that are local to your script. Use 
lower case for script-local variables.

Why?

- exported variables are by convention names with upper case, so using lower 
case makes it obvious that this variable is for your script versus general use 
(like $PATH)

- if a variable _is_ in the exported environment, and your script uses that 
name, the changed value will get exported to all the commands your script runs 
even if you don't export it yourself: it came in from the extrernal environment 
and it will automatically go out with the environment given to subcommands

- you can't expected (or be expected) to know every exported name that can 
possibly be used, nor even those commonly in use; by adopting the use of lower 
case names for script-local variable you entirely avoid needing omniscience 
about exported names.

You're writing the script with $WORDS probably because there are many many 
example scripts like it. They've been written by people who have never thought 
this through.

Cheers,
Cameron Simpson <cs(a)zip.com.au&gt;

Fatal error!  Hit any user to continue...
         - Phillip Coles <Phillip.Coles(a)tas.for.csiro.au&gt;