3

SC2005

Vidar Holen edited this page 2021-09-23 10:25:11 -07:00

Table of Contents

• Useless echo? Instead of echo $(cmd), just use cmd
• Problematic code:
• Correct code:
• Rationale
• Exceptions

Useless echo? Instead of echo $(cmd), just use cmd

Problematic code:

echo "$(whoami)"

Correct code:

whoami

Rationale

ShellCheck found the unnecessary construct echo "$(somecommand here)".

This is generally due to a misunderstanding about what echo does. It has no role in "showing on screen" or similar, but simply writes a string to standard output. This is also how all other programs output data.

echo "$(somecommand)" will capture the output somecommand writes to standard output and write it to standard output, where it was already going. At best this is a no-op, but it may have several other negative effects:

• It disables parallel processing in pipelines, such as echo "$(find . -name '*.iso')" | xargs sha1sum which does not allow iterating files and checksumming at the same time. Similarly, users don't see incremental updates as programs run.
• It introduces shell and echo related pitfalls like being unable to output the string -n, stripping NUL bytes and trailing linefeeds, and expanding escape sequences in some shells but not others.
• It suppresses the exit code of the command, so that echo "$(grep '^user:' /etc/passwd)" no longer returns with failure when the user is not found.
• It does not allow programs to tailor their output for terminals, such as ls vs echo "$(ls)" where the former outputs columns and colors according to user preferences, while the latter doesn't.
• It uses unnecessary memory to buffer up the data before writing it where it was already going.

To avoid all this, simply replace echo "$(somecommand)" with somecommand as in the example. It's shorter, faster, and more correct.

Exceptions

If you are relying on one of the otherwise detrimental effects for correctness, you can consider one of:

# Suppress exit code without the other negative effects
cmd || true

# Disable tty specific output without the other negative effects
cmd | cat

# Buffer up potentially large output without using more memory or modifying the content in any way
cmd > file.tmp
cat file.tmp

# Exactly like `echo "$(cmd)"`, but allows output like `-n` and works the same across shells
printf '%s\n' "$(cmd)"

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