summaryrefslogtreecommitdiff
path: root/shell.md
blob: 2996d68597b7db441417aecdac7fe981dba88404 (plain)
1
2
3
4
5
6
7
8
9
10
11
12
13
14
15
16
17
18
19
20
21
22
23
24
25
26
27
28
29
30
31
32
33
34
35
36
37
38
39
40
41
42
43
44
45
46
47
48
49
50
51
52
53
54
55
56
57
58
59
60
61
62
63
64
65
66
67
68
69
70
71
72
73
74
75
76
77
78
79
80
81
82
83
84
85
86
87
88
89
90
91
92
93
94
95
96
97
98
99
100
101
102
103
104
105
106
107
108
109
110
111
112
113
114
115
116
117
118
119
120
121
122
123
124
125
126
127
128
129
130
131
132
133
134
135
136
137
138
139
140
141
142
143
144
145
146
147
148
149
150
151
152
153
154
155
156
157
158
159
160
161
162
163
164
165
166
167
168
169
170
171
172
173
174
175
176
177
178
179
180
181
182
183
184
185
186
187
188
189
190
191
192
193
194
195
196
197
198
199
200
201
202
203
204
205
206
207
208
209
210
211
212
213
214
215
216
217
218
219
220
221
222
223
224
225
226
227
228
229
230
231
232
233
234
235
236
237
238
239
240
241
242
243
244
245
246
247
248
249
250
251
252
253
254
255
256
257
258
259
260
261
262
263
264
265
266
267
268
269
270
271
272
273
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"_—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"
...