CentOS - Jul 2022 - Special characters in bash strings

I have run into a bash variable string problem that I think I have nailed down
to the variable string containing a tilde (~). Not sure if my conclusion is
correct and could use some help.

To make a long(er) story short, an associative array variable was created:

p[work_path]="~/projects/test/"

and referenced in the following format in the shell script:

"${p[work_path]}"

To my consternation this worked fine in some places but not in others. I tried
to use the above construct when piping output, as part of a file reference when
calling psql from the command line and when referencing an xslt file with
xsltproc.

In some places it worked, in others it did not but when I substituted the
variable reference above with the path in clear text itself it then worked.

It looks like there are some nuances on variable substitution that I have yet to
learn, perhaps tied to the use of the tilde since using the variable
p[work_path]="/home/user/projects/test/" seemed to work in all places.

Pointers welcome!

On 7/6/22 18:41, H wrote:
> To my consternation this worked fine in some places but not in others.

It might be easier to explain if you had an example of where it worked.? 
The bash man page has a section titled "EXPANSION" that details the 
order in which expansions happen.? Since tilde expansion happens before 
variable expansion, the case you're discussing shouldn't work in any 
context (other than an eval or equivalent).

On Wed, Jul 06, 2022 at 09:41:14PM -0400, H wrote:
>I have run into a bash variable string problem that I think I have nailed
down to the variable string containing a tilde (~). Not sure if my conclusion is
correct and could use some help.
>
>To make a long(er) story short, an associative array variable was created:
>
>p[work_path]="~/projects/test/"
>
>and referenced in the following format in the shell script:
>
>"${p[work_path]}"
Is there a reason you desire "delayed" evaluation of the tilde?

If no, then evaluate the tilde in the above assignment by not
quoting it.

If yes, then where tilde evaluation IS wanted, you will likely
need to do a second round of shell evaluation of the command
line by using the keyword "eval".

$ x="~/foo"
$ y=~"/foo"         # y contains the tilde evaluated version

$ echo "$x $y"
~/foo /home/jon/foo

$ echo $x $y        # quotes don't matter here
~/foo /home/jon/foo

$ eval echo "$x $y"
/home/jon/foo /home/jon/foo


Use of eval could introduce other unexpected/unwanted
evaluations and is discouraged unless required.

-- 
Jon H. LaBadie                  jcu at labadie.us

Instead of using the ?~? in the file path, try using the ?${HOME}? variable
instead. If you must use the ?~?, you will likely need to escape it in many
places. Another thing to note, if you use single quotes ( ? ) instead of double
quotes ( ? ) , most places in bash scripts won?t try to expand the variables
inside them. This is also helpful if you have a script calling SSH and want to
pass a command to the remote server which contains a remote variable reference:

[bob at localhost bob]# ssh user at host ?echo ${HOME}; exit?
[bob at localhost bob]# /home/user
Vs:
[bob at localhost bob]# ssh user at host ?echo ${HOME}; exit?
[bob at localhost bob]# /home/bob

Greg

From: CentOS <centos-bounces at centos.org> on behalf of H <agents at
meddatainc.com>
Date: Wednesday, July 6, 2022 at 21:41
To: Centos Mailing List <centos at centos.org>
Subject: [CentOS] Special characters in bash strings
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I have run into a bash variable string problem that I think I have nailed down
to the variable string containing a tilde (~). Not sure if my conclusion is
correct and could use some help.

To make a long(er) story short, an associative array variable was created:

p[work_path]="~/projects/test/"

and referenced in the following format in the shell script:

"${p[work_path]}"

To my consternation this worked fine in some places but not in others. I tried
to use the above construct when piping output, as part of a file reference when
calling psql from the command line and when referencing an xslt file with
xsltproc.

In some places it worked, in others it did not but when I substituted the
variable reference above with the path in clear text itself it then worked.

It looks like there are some nuances on variable substitution that I have yet to
learn, perhaps tied to the use of the tilde since using the variable
p[work_path]="/home/user/projects/test/" seemed to work in all places.

Pointers welcome!

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