From 051426cfd7f8a66a78b02cc34da3f9e2e2372d13 Mon Sep 17 00:00:00 2001
From: Lars Wirzenius <lwirzenius@wikimedia.org>
Date: Fri, 15 May 2020 13:50:58 +0300
Subject: Add: talk on shell scripting

---
 shell.md | 274 +++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++
 1 file changed, 274 insertions(+)
 create mode 100644 shell.md

(limited to 'shell.md')

diff --git a/shell.md b/shell.md
new file mode 100644
index 0000000..2996d68
--- /dev/null
+++ b/shell.md
@@ -0,0 +1,274 @@
+# Introduction
+
+* Shell scripting is easy, but also risky
+* So, so easy
+* So, so risky
+* Interactive use is off-topic for this talk
+
+-----------------------------------------------------------------------------
+
+# Hashbang
+
+OK, less powerful than bash, but more portable:
+
+~~~sh
+#!/bin/sh
+~~~
+
+OK on Linux, not everywhere:
+
+~~~sh
+#!/bin/bash
+~~~
+
+Portable, but might invoke a different bash:
+
+~~~sh
+#!/usr/bin/env bash
+~~~
+
+-----------------------------------------------------------------------------
+
+# Error prevention and handling
+
+**Always, always, without exception, use this:**
+
+~~~sh
+set -eu
+~~~
+
+If it's OK for a command to fail:
+
+~~~sh
+this.may.fail || true
+~~~
+
+If using bash, optionally also use these:
+
+~~~sh
+set -o pipefail
+set -o noclobber
+~~~
+
+-----------------------------------------------------------------------------
+
+# No options in hashbang
+
+~~~sh
+#!/bin/sh
+set -eu
+~~~
+
+**Never, never, ever:**
+
+~~~sh
+#!/bin/sh -eu
+~~~
+
+Because:
+
+~~~sh
+$ sh -x foo.sh
+~~~
+
+-x is a useful debugging option for sh.
+
+
+-----------------------------------------------------------------------------
+
+# Escaping, quoting, variable expansion
+
+**Always, always** quote all variable expansions:
+
+~~~{.sh .numberLines}
+$ foo="/ home/liw"
+$ print-argv rm -f $foo
+[0]: /usr/bin/print-argv
+[1]: rm
+[2]: -f
+[3]: /
+[4]: home/liw
+$ print-argv rm -f "$foo"
+[0]: /usr/bin/print-argv
+[1]: rm
+[2]: -f
+[3]: / home/liw
+$ 
+~~~
+
+_"If your script needs to use $, switch to Python/Ruby/Perl"_&mdash;liw
+
+-----------------------------------------------------------------------------
+
+# print-argv
+
+~~~{.python .numberLines}
+#!/usr/bin/python3
+import sys
+for i, arg in enumerate(sys.argv):
+    sys.stdout.write('[%d]: %s\n' % (i, arg))
+~~~
+
+-----------------------------------------------------------------------------
+
+# Use $(...) instead of backticks. But be careful.
+
+~~~sh
+$ touch 'foo bar'
+$ print-argv foo*
+[0]: /usr/bin/print-argv
+[1]: foo bar
+$ print-argv $(ls foo*)
+[0]: /usr/bin/print-argv
+[1]: foo
+[2]: bar
+$ print-argv "$(ls foo*)"
+[0]: /usr/bin/print-argv
+[1]: foo bar
+$ 
+~~~
+
+-----------------------------------------------------------------------------
+
+# Parameter expansion: $@ vs @*
+
+~~~{.sh .numberLines}
+#!/bin/sh
+set -eu
+
+echo dollar-at
+for x in $@; do echo "<$x>"; done
+
+echo; echo dollar-at in quotes
+for x in "$@"; do echo "<$x>"; done
+
+echo; echo dollar-star
+for x in $@; do echo "<$x>"; done
+
+echo; echo dollar-star in quotes
+for x in "$@"; do echo "<$x>"; done
+~~~
+
+-----------------------------------------------------------------------------
+
+~~~{.sh .numberLines}
+$ touch 'foo bar'
+$ ./shell-param.sh foo*
+dollar-at
+<foo>
+<bar>
+
+dollar-at in quotes
+<foo bar>
+
+dollar-star
+<foo>
+<bar>
+
+dollar-star in quotes
+<foo bar>
+$ 
+~~~
+
+-----------------------------------------------------------------------------
+
+**Always, always, without exception** use `"$@"`
+
+**Never, never, ever** use `$@` or `$*` without quotes  
+exception granted if you can explain why it's safe in a specific
+case, and you do it in a comment
+
+but use Python, Ruby, or Perl instead, seriously
+
+-----------------------------------------------------------------------------
+
+# Command line option parsing in shell
+
+Don't. Use Python, Ruby, or Perl instead.
+
+
+-----------------------------------------------------------------------------
+
+# Shell functions
+
+~~~sh
+foo() {
+    local arg1
+	arg1="$1"
+}
+~~~
+
+Avoid: `local arg1="$1"`  
+because it interacts badly with pipefail.
+
+Avoid: `function foo {}`  
+because that's an unnecessary bashism.
+
+-----------------------------------------------------------------------------
+
+# Temporary files
+
+~~~{.sh .numberLines}
+#!/bin/sh
+
+set -eu
+
+cleanup() {
+    rm -rf "$tempdir"
+}
+
+
+tempdir="$(mktemp -d)"
+trap cleanup EXIT
+
+
+echo foo > "$tempdir/foo.txt"
+echo "$tempdir"
+~~~
+
+-----------------------------------------------------------------------------
+
+~~~{.sh .numberLines}
+$ mkdir t
+$ TMPDIR=t ./shell-temp.s
+t/tmp.EgiOqQ9SAU
+$ find t
+t
+$ 
+~~~
+
+-----------------------------------------------------------------------------
+
+# Summary
+
+* Know the rules.
+* The rules are intricate and hard to remember all the time.
+* You _will_ fail.
+* Give up and use a better language.
+
+-----------------------------------------------------------------------------
+
+# Further reading
+
+<https://www.davidpashley.com/articles/writing-robust-shell-scripts/>
+
+<https://www.shellcheck.net/>
+
+-----------------------------------------------------------------------------
+
+# Legalese
+
+Copyright 2020 Wikimedia Foundation
+
+This content is licensed under the Creative Commons
+Attribution-ShareAlike 4.0 International ([CC BY-SA 4.0][]) licence.
+
+[CC BY-SA 4.0]: https://creativecommons.org/licenses/by-sa/4.0/
+
+
+---
+title: "Shell scripting"
+subtitle: "some opinionated dos and dont's"
+author: "Lars Wirzenius / Wikimedia Foundation"
+date: "Version for 2020-05-18"
+...
-- 
cgit v1.2.1