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I have a bash script that I want invoked using a require -p argument that is of type string/alphanumeric. For example:

sh myscript.sh -p abc123

My best attempt thus far:

#!/usr/bin/bash
mypassword=${p:}
echo $mypassword
if [ -z mypassword ]
then
  echo "error"
fi

Since this is a required argument, I don't have a good meaningful default to apply (hence my attempt with using ${p:} to set mypassword to empty/null, and then attempt the null check conditional right afterwards).

When I run it via sh myscript.sh -p abc123 I get:

myscript.sh: line 46: ${p:}: bad substitution

All I'm trying to accomplish is this:

If -p wasn't specified, or if it was specified, but no value for it was given (e.g. -p abc123), I want to echo "error". In other words, -p must not only be provided, but a value for it must be supplied as well.

Any ideas as to how I could accomplish this?

hotmeatballsoup
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    Use a `getopts` loop to parse the option and assign it to a variable. See ["How do I parse command line arguments in Bash?"](https://stackoverflow.com/questions/192249/how-do-i-parse-command-line-arguments-in-bash/14203146#14203146) and [BashFAQ #35: "How can I handle command-line options and arguments in my script easily?"](http://mywiki.wooledge.org/BashFAQ/035#getopts) – Gordon Davisson Jan 26 '21 at 03:42

1 Answers1

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[ -z mypassword ] will always evaluate to false, because the word mypassword has a length greater than zero. You could test with, for instance, [[ -z $mypassword ]].

Also, the term ${p:} does not make sense. Perhaps you meant ${p:=} ?

UPDATE:

One more note: The way you are invoking your script would not work, even after those fixed, becaues mypassword will always be empty. For your script to work, p must be an environment variable holding the password. You would have to call your script (assuming it is in your working directory) like this:

# To run it as sh-script (POSIX shell)
p=THIS_IS_MY_PASSWORD sh myscript.sh 

# To run it as bash-script, if the script is executable
p=THIS_IS_MY_PASSWORD ./myscript.sh

# To run it as bash-script, if the script is not executable
p=THIS_IS_MY_PASSWORD bash myscript.sh
user1934428
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  • Thanks @user1934428 (+1), just curious, why nest `-z $mypassword` inside **two** sets of braces ("`[[`" and "`]]`"), instead of just one set? – hotmeatballsoup Jan 26 '21 at 10:36
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    `[[` is a syntactic construct, while `[` is a command, even though bash implements it as an internal command. Íf you are only doing a `-z` and nothing else, `[[` saves you from quoting the parameter expansion, while with `[` you would have to write `[ -z "$mypassword" ]` to catch the case where _mypassword_ contains a space. – user1934428 Jan 26 '21 at 10:39
  • Thanks again @user1934428 (+1), any chance you could give me a working code example that would allow me to invoke my script using the provided (desired) method? Such as `sh ./myscript.sh -p 12345`? – hotmeatballsoup Jan 26 '21 at 18:20
  • Gordon Davisson already did this in his comment, which also points to useful examples. I suggest that you try to follow his advice, and if you really get stuck, ask a new question and post your approach to use `getopts`. – user1934428 Jan 27 '21 at 07:52