2 SC3001

Jordan Christiansen edited this page 2021-09-01 11:10:08 -05:00

Table of Contents

• In POSIX sh, process substitution is undefined.
• Problematic code:
• Correct code:
• Rationale:
• Exceptions:
• Related resources:

In POSIX sh, process substitution is undefined.

(or "In dash, ... is not supported." when using dash)

Problematic code:

#!/bin/sh
while IFS= read -r n
do
  sum=$((sum+n))
done < <(program)

Correct code:

The easiest fix is to switch to a shell that does support process substitution, by changing the shebang to #!/bin/bash or ksh.

#!/bin/bash
while IFS= read -r n
do
  sum=$((sum+n))
done < <(program)

Alternatively, process substitution can often be replaced with temporary files:

#!/bin/sh
tmp="$(mktemp)"
program > "$tmp"
while IFS= read -r n
do
  sum=$((sum+n))
done < "$tmp"
rm "$tmp"

If streaming is important, the temporary file can be a named pipe, and the producer or consumer can be run as a background job.

If the reading command accepts input from standard input, the process substitution can be replaced with a regular pipe:

#!/bin/sh
program | while IFS= read -r n
do
  sum=$((sum+n))
done

Rationale:

Process substitution is a ksh and bash extension. It does not work in sh or dash scripts.

Exceptions:

If you only intend to target shells that supports this feature, you can change the shebang to a shell that guarantees support, or ignore this warning.

You can use # shellcheck disable=SC3000-SC4000 to ignore all such compatibility warnings.

Related resources:

• Help by adding links to BashFAQ, StackOverflow, man pages, POSIX, etc!

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