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Git & Command Line Tutorial · Command Line Basics · Lesson 5 of 20

Pipes and Redirection

Connect commands with | so the output of one feeds the next, and send output to files with >, >> and 2>. The real power of the command line.

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Pipes

A pipe | sends one command's output into the next command's input. Small tools combined this way solve big problems:

grep "ERROR" app.log | cut -d' ' -f4- | sort | uniq -c | sort -rn | head -3

Handy pipeline tools: sort (-n numbers, -r reverse), uniq -c (count repeats; needs sorted input), cut -d, -f2 (columns), tr (translate characters), awk '{print $1}' (fields), sed 's/old/new/' (replace), xargs (turn lines into arguments).

Redirection

Syntax Meaning
> file Write output to a file (replaces it)
>> file Append to a file
< file Read input from a file
2> errors.txt Write errors (stderr) to a file
> out.txt 2>&1 Send both output and errors to one file
> /dev/null Throw output away
sort | uniq -c | sort -rn, and >, >> and 2>
printf "apple\nbanana\napple\ncherry\nbanana\napple\n" > fruit.txt

echo "--- top fruits:"
sort fruit.txt | uniq -c | sort -rn

echo "--- as a CSV header:"
head -n 3 fruit.txt | tr '\n' ',' | sed 's/,$/\n/'

echo "first run" > log.txt          # > replaces
echo "second run" >> log.txt        # >> appends
cat log.txt

ls missing-file 2> errors.txt || echo "--- the error went to errors.txt:"
cat errors.txt

echo "--- word lengths:"
echo "pipes connect small tools" | tr ' ' '\n' | awk '{ print length($0), $0 }'

Output

--- top fruits:
      3 apple
      2 banana
      1 cherry
--- as a CSV header:
apple,banana,apple
first run
second run
--- the error went to errors.txt:
ls: cannot access 'missing-file': No such file or directory
--- word lengths:
5 pipes
7 connect
5 small
5 tools

Run this on your computer — see “Get Started” for setup.

Test yourself

What does >> do?

  1. Replaces the file
  2. Appends output to the end of a file
  3. Reads from a file
  4. Pipes to another command
Show answer

Answer: Appends output to the end of a file

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