wc

Count lines, words, and bytes

$ wc [file]

$ wc -l [file]

$ wc -w [file]

$ wc -m [file]

$ wc -c [file]

$ [command] | wc -l

$ ls | wc -l

$ wc -L [file]

wc [options] [file...]

wc (word count) prints newline, word, and byte counts for each file. With multiple files, it also prints totals. With no files or stdin, it reads standard input.
A word is defined as a sequence of non-whitespace characters delimited by whitespace or line boundaries. This matches most intuitive definitions of "word."
The default output shows lines, words, and bytes in that order. Options select which counts to display; if none are specified, all three are shown.
wc -l is extremely common for counting items in pipelines, such as counting files, matches, or log entries.

-l, --lines

Print line count

Print word count

Print byte count

Print character count

Print length of longest line

Read filenames from file F (NUL-terminated)

When to print total (auto, always, only, never)

Byte count (-c) and character count (-m) differ for multi-byte encodings like UTF-8. Use -m for actual character counts.
For counting files, ls | wc -l fails if filenames contain newlines. Use find -print0 | wc -l --files0-from=- for robustness.
Empty files have 0 lines, 0 words, and 0 bytes. A file with just "hello" (no newline) has 0 lines but 1 word.

sort(1), uniq(1), cut(1), awk(1)