Shell Problems
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Build text pipelines around explicit record rules
Shell problems reward choosing the right record separator and composing small tools. Word Frequency is a counting task only after deciding what counts as a word. If the input defines words as whitespace-separated tokens, convert whitespace runs into newlines, sort tokens, then count adjacent equal lines.
When to use it
Use line-oriented tools for line tasks and field-aware tools for columns. Word Frequency, Tenth Line, and Transpose File have different record models; a pipeline that works for whitespace tokens is not automatically a CSV parser.
Worked example
Two lines of input: red blue red blue red
- Normalize whitespace to one token per line and discard empty lines. The tokens are red, blue, red, blue, red.
- Sort the tokens, then run uniq -c. Sorting brings both blue tokens together and all three red tokens together.
- Sort the counts numerically in descending order. Use a secondary word key if ties need deterministic lexical order.
The frequency output is 3 red, then 2 blue. uniq -c alone would count separate runs instead of all occurrences.
Time and space
For n tokens, a comparison-sort pipeline generally requires O(n log n) comparisons, with additional cost for reading and comparing characters. External sort may spill to disk. A tenth-line task instead needs to read at most ten lines and can stop immediately afterward.
Mistakes to check
- uniq only combines adjacent equal lines. Sorting first changes order, so do not use that pipeline when original record order must be preserved.
- Choose numeric sorting for counts; lexical sorting places 10 before 2. Locale can also change the order of words.
- Quote file paths and preserve backslashes when reading raw lines. Test empty input, fewer than ten lines, and a final line without a newline.
Further reading: GNU Coreutils: uniq and adjacent repeated lines
Guide reviewed .