Prompt in read -p promt interpreted as variable name

[schaetzc]

Bug Template

Bug (false negative)

• Rule Id: SC2154, SC2034
• My shellcheck version:
  shellcheck --version says 0.6.0, my package manager pacman -Qi says 0.6.0-120
• The rule's wiki page does not already cover this
• I tried on shellcheck.net and verified that this is still a problem on the latest commit

For new checks

• shellcheck.net (i.e. the latest commit) currently gives no useful warnings about this
• I searched through https://github.com/koalaman/shellcheck/issues and didn't find anything related

Here's a snippet or screenshot that shows the problem:

The following script will print the literal string a and read input to the default variable REPLY. Afterwards the unassigned variable $a is accessed.

#! /bin/bash
read -r -p a
echo "$a"

Here's what shellcheck currently says:

There is no output. Shellcheck doesn't find a problem even though there is one (false negative).

Here's what I wanted or expected to see:

echo "$a"
      ^-- SC2154: a is referenced but not assigned.

More Information

Shellcheck seems to misinterpret the prompt argument given to read's -p option as a variable name. I did some further experiments:

For the script

#! /bin/bash
read -r -p a
echo "$REPLY"

shellcheck prints a warning even though there is no problem (false positive).

read -r -p a
           ^-- SC2034: a appears unused. Verify use (or export if used externally).

I expected shellcheck to remain silent.

For the script

#! /bin/bash
read -r -p a b
echo "$a"
echo "$b"

shellcheck remains silent even though there is a problem (false negative). I expected the output

echo "$a"
      ^-- SC2154: a is referenced but not assigned.

[schaetzc]

Some further experiments showed that also other non-variable parameters are interpreted as variable names. This includes (but is not necessarily limited to) read -d a, read -i a and read -u a – in each of these cases shellcheck interprets a as a variable name even though it is a delimiter, initial text, or file descriptor.

Note that a has to be a valid variable identifier for shellcheck to give the false positive SC2034 (appears unused) and the false negative, that is, missing warning SC2154 (referenced but not assigned).

[svanzoest]

I am getting a similar error on the following (with shellcheck 0.4.6, so may already addressed in newer versions, passes fine in v0.7.1+git109.c9be7ab)

  while read -a files -r
  do
    if ! downloadFile "${files[0]}" "${files[1]}"; 
    then
      ((++downloadFailures))
      echo "downloadFailures ${downloadFailures}"
    fi

  done < "./filelist.txt"

^-- SC2154: files is referenced but not assigned.