Useless
cat SH-2002
Performance
Major
2 years ago
—
2 years old
Useless cat. Consider 'cmd < file | ..' or 'cmd file | ..' instead
662echo "// +build ${GOARCH},${GOOS}"
663echo
664go tool cgo -godefs -- "$@" _const.go >_error.out
665cat _error.out | grep -vf _error.grep | grep -vf _signal.grep666echo
667echo '// Errors'
668echo 'const ('Useless cat. Consider 'cmd < file | ..' or 'cmd file | ..' instead
666echo
667echo '// Errors'
668echo 'const ('
669cat _error.out | grep -f _error.grep | sed 's/=\(.*\)/= syscall.Errno(\1)/'670echo ')'
671
672echoUseless cat. Consider 'cmd < file | ..' or 'cmd file | ..' instead
672echo
673echo '// Signals'
674echo 'const ('
675cat _error.out | grep -f _signal.grep | sed 's/=\(.*\)/= syscall.Signal(\1)/'676echo ')'
677
678# Run C program to print error and syscall strings.Useless cat. Consider 'cmd < file | ..' or 'cmd file | ..' instead
77
78 pushd "$dir"
79 go test -covermode=atomic -coverpkg=./... -coverprofile=coverage.out.tmp ./...
80 cat coverage.out.tmp | grep -v testsuite | grep -v tomltestgen | grep -v gotoml-test-decoder > coverage.out 81 go tool cover -func=coverage.out
82 popd
83Useless cat. Consider 'cmd < file | ..' or 'cmd file | ..' instead
116
117 target_diff="${output_dir}/target.diff.txt"
118 head_diff="${output_dir}/head.diff.txt"
119 cat "${target_out}" | grep -E '^github.com/pelletier/go-toml' | tr -s "\t " | cut -f 2,3 | sort > "${target_diff}"120 cat "${head_out}" | grep -E '^github.com/pelletier/go-toml' | tr -s "\t " | cut -f 2,3 | sort > "${head_diff}"
121
122 diff --side-by-side --suppress-common-lines "${target_diff}" "${head_diff}"
Description
Consider using cmd < file | .. or cmd file | .. instead.
cat is a tool for con"cat"enating files. Reading a single file as input to a program is considered a Useless Use Of Cat (UUOC).
It's more efficient and less roundabout to simply use redirection. This is especially true for programs that can benefit from seekable input, like tail or tar.
Many tools also accept optional filenames, e.g. grep -q foo file instead of cat file | grep -q foo.
Problematic code:
cat file | tr ' ' _ | nl
cat file | while IFS= read -r i; do echo "${i%?}"; done
Preferred code:
< file tr ' ' _ | nl
while IFS= read -r i; do echo "${i%?}"; done < file
Exception:
Pointing out UUOC is a long standing shell programming tradition, and removing them from a short-lived pipeline in a loop can speed it up by 2x. However, it's not necessarily a good use of time in practice, and rarely affects correctness. Ignore as you see fit.