Files and Pipelines

Redirects

Writing and Reading Files

Redirection connects command output and input to files. Scripts use it for temporary data, reports, and logs.

Program

Play the script to watch text move into a file and a line count come back out.

redirects.sh
Replay: real traced execution (multi-file project)
#!/usr/bin/env bash

file="$(mktemp)"
printf 'alpha\nbeta\n' > "$file"
lines="$(wc -l < "$file")"
rm "$file"
echo "lines=$lines"

  1. file ← /tmp/tmp.X

    3file="$(mktemp)"4printf 'alpha\nbeta\n' > "$file"
    values this step/tmp/tmp.Xfile

  2. file contents ← alpha, beta

    3file="$(mktemp)"4printf 'alpha\nbeta\n' > "$file"5lines="$(wc -l < "$file")"
    values this stepalpha, betafile contents/tmp/tmp.Xfile

  3. lines ← 2

    4printf 'alpha\nbeta\n' > "$file"5lines="$(wc -l < "$file")"6rm "$file"
    values this step2lines2 linesfile contents

  4. file ← removed

    5lines="$(wc -l < "$file")"6rm "$file"7echo "lines=$lines"
    values this step/tmp/tmp.X → removedfile

  5. echo "lines=$lines"

    6rm "$file"7echo "lines=$lines"
    outputlines=2
    values this step2lines

Write, Read, Clean Up

  1. mktemp creates a temporary file path.
  2. > writes two lines into that file.
  3. < feeds the file into wc -l.
  4. rm removes the temporary file after the count is saved.
printf stdout -> temp file
temp file -> wc stdin -> line count
redirection `>` writes stdout to a file and `<` reads stdin from a file.
temporary file `mktemp` creates a unique path for short-lived script data.

Exercise: redirects.sh

Write two lines to a temp file, read the line count with <, then remove the temp file