Validating bash script argument by flag name and value

Question

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?

Use a getopts loop to parse the option and assign it to a variable. See "How do I parse command line arguments in Bash?" and BashFAQ #35: "How can I handle command-line options and arguments in my script easily?" — Gordon Davisson
– Gordon Davisson, Commented Jan 26, 2021 at 3:42

user1934428 · Accepted Answer · 2021-01-26 10:45:35Z

1

[ -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

edited Jan 26, 2021 at 10:45

answered Jan 26, 2021 at 7:38

user1934428

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4 Comments

hotmeatballsoup Over a year ago

Thanks @user1934428 (+1), just curious, why nest -z $mypassword inside two sets of braces ("[[" and "]]"), instead of just one set?

user1934428 Over a year ago

[[ 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.

hotmeatballsoup Over a year ago

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?

user1934428 Over a year ago

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.

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