run.shCourse

Shell

A variable that holds text, then quoting, then why a number is still text, a function, local scope, and the exit code.

A variable holds the drink
  1. itemtextlatte

6 lessons, in orderFree to read

Start lesson 1: Shell from zero: a variable holds text

Syllabus

Basics

The types, explained, then an object you can name and call.

  1. 01

    A shell variable holds text. item=latte stores latte, and echo prints it. There are no spaces around the equals sign.

    item=latte
echo "$item"
    latte
  2. 02

    A value with a space splits into two words if you leave the quotes off. "$name" keeps it one value. The program prints oat latte.

    Builds on lesson 1.

    name="oat latte"
echo "$name"
    oat latte

Practice

A rule that prevents a bug, then a program that uses the object.

  1. 03

    A variable that looks like a number is text. $((cents * 2)) evaluates it as arithmetic. The program stores 450 and prints 900.

    Builds on lessons 1 through 2.

    cents=450
echo $((cents * 2))
    900
  2. 04

    A function packages a command you want to repeat. label prints a drink and its price. The program prints latte $4.50.

    Builds on lessons 1 through 3.

    label() {
  printf '%s $%d.%02d\n' "$1" $(($2 / 100)) $(($2 % 100))
}
label latte 450
    latte $4.50

Further

One more idea, then a program that puts the pieces together.

  1. 05

    local makes a variable private to the function. The function prints muffin. The script then prints latte. The caller's drink is unchanged.

    Builds on lessons 1 through 4.

    name=latte
inner() {
  local name=muffin
  echo "$name"
}
inner
echo "$name"
    muffin
latte
  2. 06

    A command returns a number when it finishes. 0 means it worked. An empty drink name fails test -n. The program prints 1. Check the code before you trust the result.

    Builds on lessons 1 through 5.

    name=""
test -n "$name"
echo $?
    1

Readings

Shorter pieces in the same language. Any order is fine once the lessons feel familiar.

  1. jq: a pipeline for JSON 2 min

    GoByte Skills #24: in jq, bob does not become null. He stops existing. select emits nothing for a miss, because every jq filter takes JSON in and emits a stream of zero or more values, and | runs the next step once per value.

  2. xargs -P: parallelism in one flag 2 min

    GoByte Skills #23: xargs -P turns a serial loop into a worker pool. Twelve seconds of jobs finish in three. Also: the missing -n that makes -P do nothing at all.

  3. Pipes: the concurrency model you already use 2 min

    GoByte Skills #22: a pipeline is every stage running at once, joined by 64 KiB buffers. awk streams in 1.6 MB. sort cannot: on a 444 MB log it held 2.1 GB. So filter first.