cat $file 2>/dev/null and <$file 2>/dev/null are not equivalent if file does not exist

[KaspervdHeijden]

For bugs

• Rule Id (if any, e.g. SC1000): SC2002
• My shellcheck version (shellcheck --version or "online"): 0.9.0
• [ x] The rule's wiki page does not already cover this (e.g. https://shellcheck.net/wiki/SC2086)
• [x ] I tried on https://www.shellcheck.net/ and verified that this is still a problem on the latest commit

For new checks and feature suggestions

• [ x] https://www.shellcheck.net/ (i.e. the latest commit) currently gives no useful warnings about this
• [x ] 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:

#!/urs/bin/env sh

file="somefile.txt"

cat "${file}" 2>/dev/null | tr '\n' ' '

Here's what shellcheck currently says:

cat "${file}" 2>/dev/null | tr '\n' ' ' ^-- SC2002 (style): Useless cat. Consider 'cmd < file | ..' or 'cmd file | ..' instead.

Here's what I wanted or expected to see:

Nothing

• tr does not accept a file name argument; so the cmd "${file}" is not possible
• cat "${file}" 2>/dev/null and <"${file}" 2>/dev/null are not equivalent; the former does not output an errormessage when the file does not exist, whilst the latter does because it's the shell complaining, not an external tool.

[slycordinator]

You can suppress the output of <"${file}" for a non-existent file.

Depending on your needs, you could use: { <"${file}"; } 2>/dev/null or ( <"${file}"; ) 2>/dev/null

[KaspervdHeijden]

Nice trick! Today I learned. Would it be possible to let shellcheck suggest this in this specific situation?

For what it's worth, I guess we're now in the territory that it's just easier to use cat anyway, making it a false positive (we are still creating a subshell). But that's just my option.

[slycordinator]

"we are still creating a subshell"

The option with {} doesn't create a stubshell.

On Fri, Dec 29, 2023, 7:18 PM Kasper van der Heijden < @.***> wrote:

Nice trick! Today I learned. Would it be possible to let shellcheck suggest this in this specific situation?

For what it's worth, I guess we're now in the territory that it's just easier to use cat anyway, making it a false positive (we are still creating a subshell). But that's just my option.

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[KaspervdHeijden]

Note: If I use { <"${file}"; } 2>/dev/null shellcheck still does not like it:

^-- SC2188 (warning): This redirection doesn't have a command. Move to its command (or use 'true' as no-op).