7 SC2126

Michael Diamond edited this page 2020-04-23 12:00:01 -07:00

Table of Contents

• Consider using grep -c instead of grep | wc
• Problematic code:
• Correct code:
• For multiple files
• Rationale:
• Exceptions

Consider using grep -c instead of grep | wc

Problematic code:

grep foo | wc -l

Correct code:

grep -c foo

For multiple files

Instead of:

grep foo *.log | wc -l

You can pipe all the file contents into grep (passing the files directly to grep causes -c to print each file's count separately, rather than the total):

cat *.log | grep foo -c

Rationale:

This is purely a stylistic issue. grep can count lines without piping to wc.

Often this number is only used to see whether there are matches (i.e. == 0). In these cases it's clearer and more efficient to use grep -q and check its exit status:

if grep -q pattern file; then
  echo "The file contains the pattern"
fi

Also note that in foo | grep bar | wc -l, wc will mask the exit code of grep by default (i.e. without set -o pipefail), and always return success. If replacing with foo | grep -c bar, grep will exit non-zero when there are no matches. This is generally desirable (see above), but may require handling when used with set -e.

Exceptions

If you find piping to wc is clearer in a given situation it's fine to ignore this error.

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