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!

6 thoughts on - Special Characters In Bash Strings

  • 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).

  • 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.

  • Thank you, I read up on bash expansion of tilde and realized substituting $HOME for ~ would be the best. Once I had done that the script worked.

    The reason it worked in some places may have been that I had set the script to POSIX compliant using:

    set +o posix

  • Thank you, I read up on bash expansion of tilde and realized substituting $HOME for ~ would be the best and would avoid any other unforeseen complications.

    Once I had done that the script worked.