| 1 | ---
|
| 2 | default_highlighter: oils-sh
|
| 3 | ---
|
| 4 |
|
| 5 | YSH vs. Shell Idioms
|
| 6 | ====================
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| 7 |
|
| 8 | This is an informal, lightly-organized list of recommended idioms for the
|
| 9 | [YSH]($xref) language. Each section has snippets labeled *No* and *Yes*.
|
| 10 |
|
| 11 | - Use the *Yes* style when you want to write in YSH, and don't care about
|
| 12 | compatibility with other shells.
|
| 13 | - The *No* style is discouraged in new code, but YSH will run it. The [OSH
|
| 14 | language]($xref:osh-language) is compatible with
|
| 15 | [POSIX]($xref:posix-shell-spec) and [bash]($xref).
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| 16 |
|
| 17 | [J8 Notation]: j8-notation.html
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| 18 |
|
| 19 | <!-- cmark.py expands this -->
|
| 20 | <div id="toc">
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| 21 | </div>
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| 22 |
|
| 23 | ## Use [Simple Word Evaluation](simple-word-eval.html) to Avoid "Quoting Hell"
|
| 24 |
|
| 25 | ### Substitute Variables
|
| 26 |
|
| 27 | No:
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| 28 |
|
| 29 | local x='my song.mp3'
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| 30 | ls "$x" # quotes required to avoid mangling
|
| 31 |
|
| 32 | Yes:
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| 33 |
|
| 34 | var x = 'my song.mp3'
|
| 35 | ls $x # no quotes needed
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| 36 |
|
| 37 | ### Splice Arrays
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| 38 |
|
| 39 | No:
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| 40 |
|
| 41 | local myflags=( --all --long )
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| 42 | ls "${myflags[@]}" "$@"
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| 43 |
|
| 44 | Yes:
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| 45 |
|
| 46 | var myflags = :| --all --long |
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| 47 | ls @myflags @ARGV
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| 48 |
|
| 49 | ### Explicitly Split, Glob, and Omit Empty Args
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| 50 |
|
| 51 | YSH doesn't split arguments after variable expansion.
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| 52 |
|
| 53 | No:
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| 54 |
|
| 55 | local packages='python-dev gawk'
|
| 56 | apt install $packages
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| 57 |
|
| 58 | Yes:
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| 59 |
|
| 60 | var packages = 'python-dev gawk'
|
| 61 | apt install @[split(packages)]
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| 62 |
|
| 63 | Even better:
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| 64 |
|
| 65 | var packages = :| python-dev gawk | # array literal
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| 66 | apt install @packages # splice array
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| 67 |
|
| 68 | ---
|
| 69 |
|
| 70 | YSH doesn't glob after variable expansion.
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| 71 |
|
| 72 | No:
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| 73 |
|
| 74 | local pat='*.py'
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| 75 | echo $pat
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| 76 |
|
| 77 |
|
| 78 | Yes:
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| 79 |
|
| 80 | var pat = '*.py'
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| 81 | echo @[glob(pat)] # explicit call
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| 82 |
|
| 83 | ---
|
| 84 |
|
| 85 | YSH doesn't omit unquoted words that evaluate to the empty string.
|
| 86 |
|
| 87 | No:
|
| 88 |
|
| 89 | local e=''
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| 90 | cp $e other $dest # cp gets 2 args, not 3, in sh
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| 91 |
|
| 92 | Yes:
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| 93 |
|
| 94 | var e = ''
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| 95 | cp @[maybe(e)] other $dest # explicit call
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| 96 |
|
| 97 | ### Iterate a Number of Times (Split Command Sub)
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| 98 |
|
| 99 | No:
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| 100 |
|
| 101 | local n=3
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| 102 | for x in $(seq $n); do # No implicit splitting of unquoted words in YSH
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| 103 | echo $x
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| 104 | done
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| 105 |
|
| 106 | OK:
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| 107 |
|
| 108 | var n = 3
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| 109 | for x in @(seq $n) { # Explicit splitting
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| 110 | echo $x
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| 111 | }
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| 112 |
|
| 113 | Better;
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| 114 |
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| 115 | var n = 3
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| 116 | for x in (1 .. n+1) { # Range, avoids external program
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| 117 | echo $x
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| 118 | }
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| 119 |
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| 120 | Note that `{1..3}` works in bash and YSH, but the numbers must be constant.
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| 121 |
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| 122 | ## Avoid Ad Hoc Parsing and Splitting
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| 123 |
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| 124 | In other words, avoid *groveling through backslashes and spaces* in shell.
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| 125 |
|
| 126 | Instead, emit and consume [J8 Notation]($xref:j8-notation):
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| 127 |
|
| 128 | - J8 strings are [JSON]($xref) strings, with an upgrade for byte string
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| 129 | literals
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| 130 | - [JSON8]($xref) is [JSON]($xref), with this same upgrade
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| 131 | - [TSV8]($xref) is TSV with this upgrade (not yet implemented)
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| 132 |
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| 133 | Custom parsing and serializing should be limited to "the edges" of your YSH
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| 134 | programs.
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| 135 |
|
| 136 | ### More Strategies For Structured Data
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| 137 |
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| 138 | - **Wrap** and Adapt External Tools. Parse their output, and emit [J8 Notation][].
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| 139 | - These can be one-off, "bespoke" wrappers in your program, or maintained
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| 140 | programs. Use the `proc` construct and `flagspec`!
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| 141 | - Example: [uxy](https://github.com/sustrik/uxy) wrappers.
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| 142 | - TODO: Examples written in YSH and in other languages.
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| 143 | - **Patch** Existing Tools.
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| 144 | - Enhance GNU grep, etc. to emit [J8 Notation][]. Add a
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| 145 | `--j8` flag.
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| 146 | - **Write Your Own** Structured Versions.
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| 147 | - For example, you can write a structured subset of `ls` in Python with
|
| 148 | little effort.
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| 149 |
|
| 150 | <!--
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| 151 | ls -q and -Q already exist, but --j8 or --tsv8 is probably fine
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| 152 | -->
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| 153 |
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| 154 | ## The `write` Builtin Is Simpler Than `printf` and `echo`
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| 155 |
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| 156 | ### Write an Arbitrary Line
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| 157 |
|
| 158 | No:
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| 159 |
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| 160 | printf '%s\n' "$mystr"
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| 161 |
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| 162 | Yes:
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| 163 |
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| 164 | write -- $mystr
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| 165 |
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| 166 | The `write` builtin accepts `--` so it doesn't confuse flags and args.
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| 167 |
|
| 168 | ### Write Without a Newline
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| 169 |
|
| 170 | No:
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| 171 |
|
| 172 | echo -n "$mystr" # breaks if mystr is -e
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| 173 |
|
| 174 | Yes:
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| 175 |
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| 176 | write --end '' -- $mystr
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| 177 | write -n -- $mystr # -n is an alias for --end ''
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| 178 |
|
| 179 | ### Write an Array of Lines
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| 180 |
|
| 181 | var myarray = :| one two three |
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| 182 | write -- @myarray
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| 183 |
|
| 184 | ## New Long Flags on the `read` builtin
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| 185 |
|
| 186 | ### Read a Line
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| 187 |
|
| 188 | No:
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| 189 |
|
| 190 | read line # Mangles your backslashes!
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| 191 |
|
| 192 | Better:
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| 193 |
|
| 194 | read -r line # Still messes with leading and trailing whitespace
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| 195 |
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| 196 | IFS= read -r line # OK, but doesn't work in YSH
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| 197 |
|
| 198 | Yes:
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| 199 |
|
| 200 | read --raw-line # Gives you the line, without trailing \n
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| 201 |
|
| 202 | (Note that `read --raw-line` is still an unbuffered read, which means it slowly
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| 203 | reads a byte at a time. We plan to add buffered reads as well.)
|
| 204 |
|
| 205 | ### Read a Whole File
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| 206 |
|
| 207 | No:
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| 208 |
|
| 209 | read -d '' # harder to read, easy to forget -r
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| 210 |
|
| 211 | Yes:
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| 212 |
|
| 213 | read --all # sets $_reply
|
| 214 | read --all (&myvar) # sets $myvar
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| 215 |
|
| 216 | ### Read Lines of a File
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| 217 |
|
| 218 | No:
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| 219 |
|
| 220 | # The IFS= idiom doesn't work in YSH, because of dynamic scope!
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| 221 | while IFS= read -r line; do
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| 222 | echo $line
|
| 223 | done
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| 224 |
|
| 225 | Yes:
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| 226 |
|
| 227 | while read --raw-line {
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| 228 | echo $_reply
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| 229 | }
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| 230 | # this reads a byte at a time, unbuffered, like shell
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| 231 |
|
| 232 | Yes:
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| 233 |
|
| 234 | for line in (io.stdin) {
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| 235 | echo $line
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| 236 | }
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| 237 | # this reads buffered lines, which is much faster
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| 238 |
|
| 239 | ### Read a Number of Bytes
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| 240 |
|
| 241 | No:
|
| 242 |
|
| 243 | read -n 3 # slow because it respects -d delim
|
| 244 | # also strips whitespace
|
| 245 |
|
| 246 | Better:
|
| 247 |
|
| 248 | read -N 3 # good behavior, but easily confused with -n
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| 249 |
|
| 250 | Yes:
|
| 251 |
|
| 252 | read --num-bytes 3 # sets $_reply
|
| 253 | read --num-bytes 3 (&myvar) # sets $myvar
|
| 254 |
|
| 255 |
|
| 256 | ### Read Until `\0` (consume `find -print0`)
|
| 257 |
|
| 258 | No:
|
| 259 |
|
| 260 | # Obscure syntax that bash accepts, but not other shells
|
| 261 | read -r -d '' myvar
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| 262 |
|
| 263 | Yes:
|
| 264 |
|
| 265 | read -0 (&myvar)
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| 266 |
|
| 267 | ## YSH Enhancements to Builtins
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| 268 |
|
| 269 | ### Use `shopt` Instead of `set`
|
| 270 |
|
| 271 | Using a single builtin for all options makes scripts easier to read:
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| 272 |
|
| 273 | Discouraged:
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| 274 |
|
| 275 | set -o errexit
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| 276 | shopt -s dotglob
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| 277 |
|
| 278 | Idiomatic:
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| 279 |
|
| 280 | shopt --set errexit
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| 281 | shopt --set dotglob
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| 282 |
|
| 283 | (As always, `set` can be used when you care about compatibility with other
|
| 284 | shells.)
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| 285 |
|
| 286 | ### Use `&` When Mentioning Variable Names
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| 287 |
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| 288 | YSH uses [places](variables.html#return-by-mutating-a-place-advanced) to make
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| 289 | out-paramaters of procs more explicit.
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| 290 |
|
| 291 | No:
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| 292 |
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| 293 | read -0 record < file.bin
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| 294 | echo $record
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| 295 |
|
| 296 | Yes:
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| 297 |
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| 298 | read -0 (&myvar) < file.bin
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| 299 | echo $record
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| 300 |
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| 301 |
|
| 302 | ### Consider Using `--long-flags`
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| 303 |
|
| 304 | Easier to write:
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| 305 |
|
| 306 | test -d /tmp
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| 307 | test -d / && test -f /vmlinuz
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| 308 |
|
| 309 | shopt -u extglob
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| 310 |
|
| 311 | Easier to read:
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| 312 |
|
| 313 | test --dir /tmp
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| 314 | test --dir / && test --file /vmlinuz
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| 315 |
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| 316 | shopt --unset extglob
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| 317 |
|
| 318 | ## Use Blocks to Save and Restore Context
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| 319 |
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| 320 | ### Do Something In Another Directory
|
| 321 |
|
| 322 | No:
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| 323 |
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| 324 | ( cd /tmp; echo $PWD ) # subshell is unnecessary (and limited)
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| 325 |
|
| 326 | No:
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| 327 |
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| 328 | pushd /tmp
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| 329 | echo $PWD
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| 330 | popd
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| 331 |
|
| 332 | Yes:
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| 333 |
|
| 334 | cd /tmp {
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| 335 | echo $PWD
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| 336 | }
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| 337 |
|
| 338 | ### Batch I/O
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| 339 |
|
| 340 | No:
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| 341 |
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| 342 | echo 1 > out.txt
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| 343 | echo 2 >> out.txt # appending is less efficient
|
| 344 | # because open() and close()
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| 345 |
|
| 346 | No:
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| 347 |
|
| 348 | { echo 1
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| 349 | echo 2
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| 350 | } > out.txt
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| 351 |
|
| 352 | Yes:
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| 353 |
|
| 354 | redir > out.txt {
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| 355 | echo 1
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| 356 | echo 2
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| 357 | }
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| 358 |
|
| 359 | The `redir` builtin is syntactic sugar -- it lets you see redirects before the
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| 360 | code that uses them.
|
| 361 |
|
| 362 | ### Temporarily Set Shell Options
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| 363 |
|
| 364 | No:
|
| 365 |
|
| 366 | set +o errexit
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| 367 | myfunc # without error checking
|
| 368 | set -o errexit
|
| 369 |
|
| 370 | Yes:
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| 371 |
|
| 372 | shopt --unset errexit {
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| 373 | myfunc
|
| 374 | }
|
| 375 |
|
| 376 | ### Use the `forkwait` builtin for Subshells, not `()`
|
| 377 |
|
| 378 | No:
|
| 379 |
|
| 380 | ( cd /tmp; rm *.sh )
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| 381 |
|
| 382 | Yes:
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| 383 |
|
| 384 | forkwait {
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| 385 | cd /tmp
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| 386 | rm *.sh
|
| 387 | }
|
| 388 |
|
| 389 | Better:
|
| 390 |
|
| 391 | cd /tmp { # no process created
|
| 392 | rm *.sh
|
| 393 | }
|
| 394 |
|
| 395 | ### Use the `fork` builtin for async, not `&`
|
| 396 |
|
| 397 | No:
|
| 398 |
|
| 399 | myfunc &
|
| 400 |
|
| 401 | { sleep 1; echo one; sleep 2; } &
|
| 402 |
|
| 403 | Yes:
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| 404 |
|
| 405 | fork { myfunc }
|
| 406 |
|
| 407 | fork { sleep 1; echo one; sleep 2 }
|
| 408 |
|
| 409 | ## Use Procs (Better Shell Functions)
|
| 410 |
|
| 411 | ### Use Named Parameters Instead of `$1`, `$2`, ...
|
| 412 |
|
| 413 | No:
|
| 414 |
|
| 415 | f() {
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| 416 | local src=$1
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| 417 | local dest=${2:-/tmp}
|
| 418 |
|
| 419 | cp "$src" "$dest"
|
| 420 | }
|
| 421 |
|
| 422 | Yes:
|
| 423 |
|
| 424 | proc f(src, dest='/tmp') { # Python-like default values
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| 425 | cp $src $dest
|
| 426 | }
|
| 427 |
|
| 428 | ### Use Named Varargs Instead of `"$@"`
|
| 429 |
|
| 430 | No:
|
| 431 |
|
| 432 | f() {
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| 433 | local first=$1
|
| 434 | shift
|
| 435 |
|
| 436 | echo $first
|
| 437 | echo "$@"
|
| 438 | }
|
| 439 |
|
| 440 | Yes:
|
| 441 |
|
| 442 | proc f(first, @rest) { # @ means "the rest of the arguments"
|
| 443 | write -- $first
|
| 444 | write -- @rest # @ means "splice this array"
|
| 445 | }
|
| 446 |
|
| 447 | You can also use the implicit `ARGV` variable:
|
| 448 |
|
| 449 | proc p {
|
| 450 | cp -- @ARGV /tmp
|
| 451 | }
|
| 452 |
|
| 453 | ### Use "Out Params" instead of `declare -n`
|
| 454 |
|
| 455 | Out params are one way to "return" values from a `proc`.
|
| 456 |
|
| 457 | No:
|
| 458 |
|
| 459 | f() {
|
| 460 | local in=$1
|
| 461 | local -n out=$2
|
| 462 |
|
| 463 | out=PREFIX-$in
|
| 464 | }
|
| 465 |
|
| 466 | myvar='init'
|
| 467 | f zzz myvar # assigns myvar to 'PREFIX-zzz'
|
| 468 |
|
| 469 |
|
| 470 | Yes:
|
| 471 |
|
| 472 | proc f(in, :out) { # : is an out param, i.e. a string "reference"
|
| 473 | setref out = "PREFIX-$in"
|
| 474 | }
|
| 475 |
|
| 476 | var myvar = 'init'
|
| 477 | f zzz :myvar # assigns myvar to 'PREFIX-zzz'.
|
| 478 | # colon is required
|
| 479 |
|
| 480 | ### Note: Procs Don't Mess With Their Callers
|
| 481 |
|
| 482 | That is, [dynamic scope]($xref:dynamic-scope) is turned off when procs are
|
| 483 | invoked.
|
| 484 |
|
| 485 | Here's an example of shell functions reading variables in their caller:
|
| 486 |
|
| 487 | bar() {
|
| 488 | echo $foo_var # looks up the stack
|
| 489 | }
|
| 490 |
|
| 491 | foo() {
|
| 492 | foo_var=x
|
| 493 | bar
|
| 494 | }
|
| 495 |
|
| 496 | foo
|
| 497 |
|
| 498 | In YSH, you have to pass params explicitly:
|
| 499 |
|
| 500 | proc bar {
|
| 501 | echo $foo_var # error, not defined
|
| 502 | }
|
| 503 |
|
| 504 | Shell functions can also **mutate** variables in their caller! But procs can't
|
| 505 | do this, which makes code easier to reason about.
|
| 506 |
|
| 507 | ## Use Modules
|
| 508 |
|
| 509 | YSH has a few lightweight features that make it easier to organize code into
|
| 510 | files. It doesn't have "namespaces".
|
| 511 |
|
| 512 | ### Relative Imports
|
| 513 |
|
| 514 | Suppose we are running `bin/mytool`, and we want `BASE_DIR` to be the root of
|
| 515 | the repository so we can do a relative import of `lib/foo.sh`.
|
| 516 |
|
| 517 | No:
|
| 518 |
|
| 519 | # All of these are common idioms, with caveats
|
| 520 | BASE_DIR=$(dirname $0)/..
|
| 521 |
|
| 522 | BASE_DIR=$(dirname ${BASH_SOURCE[0]})/..
|
| 523 |
|
| 524 | BASE_DIR=$(cd $($dirname $0)/.. && pwd)
|
| 525 |
|
| 526 | BASE_DIR=$(dirname (dirname $(readlink -f $0)))
|
| 527 |
|
| 528 | source $BASE_DIR/lib/foo.sh
|
| 529 |
|
| 530 | Yes:
|
| 531 |
|
| 532 | const BASE_DIR = "$this_dir/.."
|
| 533 |
|
| 534 | source $BASE_DIR/lib/foo.sh
|
| 535 |
|
| 536 | # Or simply:
|
| 537 | source $_this_dir/../lib/foo.sh
|
| 538 |
|
| 539 | The value of `_this_dir` is the directory that contains the currently executing
|
| 540 | file.
|
| 541 |
|
| 542 | ### Include Guards
|
| 543 |
|
| 544 | No:
|
| 545 |
|
| 546 | # libfoo.sh
|
| 547 | if test -z "$__LIBFOO_SH"; then
|
| 548 | return
|
| 549 | fi
|
| 550 | __LIBFOO_SH=1
|
| 551 |
|
| 552 | Yes:
|
| 553 |
|
| 554 | # libfoo.sh
|
| 555 | module libfoo.sh || return 0
|
| 556 |
|
| 557 | ### Taskfile Pattern
|
| 558 |
|
| 559 | No:
|
| 560 |
|
| 561 | deploy() {
|
| 562 | echo ...
|
| 563 | }
|
| 564 | "$@"
|
| 565 |
|
| 566 | Yes
|
| 567 |
|
| 568 | proc deploy() {
|
| 569 | echo ...
|
| 570 | }
|
| 571 | runproc @ARGV # gives better error messages
|
| 572 |
|
| 573 | ## Error Handling
|
| 574 |
|
| 575 | [YSH Fixes Shell's Error Handling (`errexit`)](error-handling.html) once and
|
| 576 | for all! Here's a comprehensive list of error handling idioms.
|
| 577 |
|
| 578 | ### Don't Use `&&` Outside of `if` / `while`
|
| 579 |
|
| 580 | It's implicit because `errexit` is on in YSH.
|
| 581 |
|
| 582 | No:
|
| 583 |
|
| 584 | mkdir /tmp/dest && cp foo /tmp/dest
|
| 585 |
|
| 586 | Yes:
|
| 587 |
|
| 588 | mkdir /tmp/dest
|
| 589 | cp foo /tmp/dest
|
| 590 |
|
| 591 | It also avoids the *Trailing `&&` Pitfall* mentioned at the end of the [error
|
| 592 | handling](error-handling.html) doc.
|
| 593 |
|
| 594 | ### Ignore an Error
|
| 595 |
|
| 596 | No:
|
| 597 |
|
| 598 | ls /bad || true # OK because ls is external
|
| 599 | myfunc || true # suffers from the "Disabled errexit Quirk"
|
| 600 |
|
| 601 | Yes:
|
| 602 |
|
| 603 | try { ls /bad }
|
| 604 | try { myfunc }
|
| 605 |
|
| 606 | ### Retrieve A Command's Status When `errexit` is On
|
| 607 |
|
| 608 | No:
|
| 609 |
|
| 610 | # set -e is enabled earlier
|
| 611 |
|
| 612 | set +e
|
| 613 | mycommand # this ignores errors when mycommand is a function
|
| 614 | status=$? # save it before it changes
|
| 615 | set -e
|
| 616 |
|
| 617 | echo $status
|
| 618 |
|
| 619 | Yes:
|
| 620 |
|
| 621 | try {
|
| 622 | mycommand
|
| 623 | }
|
| 624 | echo $[_error.code]
|
| 625 |
|
| 626 | ### Does a Builtin Or External Command Succeed?
|
| 627 |
|
| 628 | These idioms are OK in both shell and YSH:
|
| 629 |
|
| 630 | if ! cp foo /tmp {
|
| 631 | echo 'error copying' # any non-zero status
|
| 632 | }
|
| 633 |
|
| 634 | if ! test -d /bin {
|
| 635 | echo 'not a directory'
|
| 636 | }
|
| 637 |
|
| 638 | To be consistent with the idioms below, you can also write them like this:
|
| 639 |
|
| 640 | try {
|
| 641 | cp foo /tmp
|
| 642 | }
|
| 643 | if failed { # shortcut for (_error.code !== 0)
|
| 644 | echo 'error copying'
|
| 645 | }
|
| 646 |
|
| 647 | ### Does a Function Succeed?
|
| 648 |
|
| 649 | When the command is a shell function, you shouldn't use `if myfunc` directly.
|
| 650 | This is because shell has the *Disabled `errexit` Quirk*, which is detected by
|
| 651 | YSH `strict_errexit`.
|
| 652 |
|
| 653 | **No**:
|
| 654 |
|
| 655 | if myfunc; then # errors not checked in body of myfunc
|
| 656 | echo 'success'
|
| 657 | fi
|
| 658 |
|
| 659 | **Yes**. The *`$0` Dispatch Pattern* is a workaround that works in all shells.
|
| 660 |
|
| 661 | if $0 myfunc; then # invoke a new shell
|
| 662 | echo 'success'
|
| 663 | fi
|
| 664 |
|
| 665 | "$@" # Run the function $1 with args $2, $3, ...
|
| 666 |
|
| 667 | **Yes**. The YSH `try` builtin sets the special `_error` variable and returns
|
| 668 | `0`.
|
| 669 |
|
| 670 | try {
|
| 671 | myfunc # doesn't abort
|
| 672 | }
|
| 673 | if failed {
|
| 674 | echo 'success'
|
| 675 | }
|
| 676 |
|
| 677 | ### Does a Pipeline Succeed?
|
| 678 |
|
| 679 | No:
|
| 680 |
|
| 681 | if ps | grep python; then
|
| 682 | echo 'found'
|
| 683 | fi
|
| 684 |
|
| 685 | This is technically correct when `pipefail` is on, but it's impossible for
|
| 686 | YSH `strict_errexit` to distinguish it from `if myfunc | grep python` ahead
|
| 687 | of time (the ["meta" pitfall](error-handling.html#the-meta-pitfall)). If you
|
| 688 | know what you're doing, you can disable `strict_errexit`.
|
| 689 |
|
| 690 | Yes:
|
| 691 |
|
| 692 | try {
|
| 693 | ps | grep python
|
| 694 | }
|
| 695 | if failed {
|
| 696 | echo 'found'
|
| 697 | }
|
| 698 |
|
| 699 | # You can also examine the status of each part of the pipeline
|
| 700 | if (_pipeline_status[0] !== 0) {
|
| 701 | echo 'ps failed'
|
| 702 | }
|
| 703 |
|
| 704 | ### Does a Command With Process Subs Succeed?
|
| 705 |
|
| 706 | Similar to the pipeline example above:
|
| 707 |
|
| 708 | No:
|
| 709 |
|
| 710 | if ! comm <(sort left.txt) <(sort right.txt); then
|
| 711 | echo 'error'
|
| 712 | fi
|
| 713 |
|
| 714 | Yes:
|
| 715 |
|
| 716 | try {
|
| 717 | comm <(sort left.txt) <(sort right.txt)
|
| 718 | }
|
| 719 | if failed {
|
| 720 | echo 'error'
|
| 721 | }
|
| 722 |
|
| 723 | # You can also examine the status of each process sub
|
| 724 | if (_process_sub_status[0] !== 0) {
|
| 725 | echo 'first process sub failed'
|
| 726 | }
|
| 727 |
|
| 728 | (I used `comm` in this example because it doesn't have a true / false / error
|
| 729 | status like `diff`.)
|
| 730 |
|
| 731 | ### Handle Errors in YSH Expressions
|
| 732 |
|
| 733 | try {
|
| 734 | var x = 42 / 0
|
| 735 | echo "result is $[42 / 0]"
|
| 736 | }
|
| 737 | if failed {
|
| 738 | echo 'divide by zero'
|
| 739 | }
|
| 740 |
|
| 741 | ### Test Boolean Statuses, like `grep`, `diff`, `test`
|
| 742 |
|
| 743 | The YSH `boolstatus` builtin distinguishes **error** from **false**.
|
| 744 |
|
| 745 | **No**, this is subtly wrong. `grep` has 3 different return values.
|
| 746 |
|
| 747 | if grep 'class' *.py {
|
| 748 | echo 'found' # status 0 means found
|
| 749 | } else {
|
| 750 | echo 'not found OR ERROR' # any non-zero status
|
| 751 | }
|
| 752 |
|
| 753 | **Yes**. `boolstatus` aborts the program if `egrep` doesn't return 0 or 1.
|
| 754 |
|
| 755 | if boolstatus grep 'class' *.py { # may abort
|
| 756 | echo 'found' # status 0 means found
|
| 757 | } else {
|
| 758 | echo 'not found' # status 1 means not found
|
| 759 | }
|
| 760 |
|
| 761 | More flexible style:
|
| 762 |
|
| 763 | try {
|
| 764 | grep 'class' *.py
|
| 765 | }
|
| 766 | case (_error.code) {
|
| 767 | (0) { echo 'found' }
|
| 768 | (1) { echo 'not found' }
|
| 769 | (else) { echo 'fatal' }
|
| 770 | }
|
| 771 |
|
| 772 | ## Use YSH Expressions, Initializations, and Assignments (var, setvar)
|
| 773 |
|
| 774 | ### Set an Environment Variable Globally
|
| 775 |
|
| 776 | No:
|
| 777 |
|
| 778 | export PYTHONPATH=. # export is disabled in YSH
|
| 779 |
|
| 780 | Yes:
|
| 781 |
|
| 782 | setglobal ENV.PYTHONPATH = '.'
|
| 783 |
|
| 784 | That is, enviroments use the [ENV][] object/namespace, rather than being global
|
| 785 | variables.
|
| 786 |
|
| 787 | [ENV]: ref/chap-special-var.html#ENV
|
| 788 |
|
| 789 | Note: the idiom for setting an env var locally is unchanged:
|
| 790 |
|
| 791 | PYTHONPATH=. myscript.py
|
| 792 |
|
| 793 | ### Initialize and Assign Strings and Integers
|
| 794 |
|
| 795 | No:
|
| 796 |
|
| 797 | local mystr=foo
|
| 798 | mystr='new value'
|
| 799 |
|
| 800 | local myint=42 # still a string in shell
|
| 801 |
|
| 802 | Yes:
|
| 803 |
|
| 804 | var mystr = 'foo'
|
| 805 | setvar mystr = 'new value'
|
| 806 |
|
| 807 | var myint = 42 # a real integer
|
| 808 |
|
| 809 | ### Expressions on Integers
|
| 810 |
|
| 811 | No:
|
| 812 |
|
| 813 | x=$(( 1 + 2*3 ))
|
| 814 | (( x = 1 + 2*3 ))
|
| 815 |
|
| 816 | Yes:
|
| 817 |
|
| 818 | setvar x = 1 + 2*3
|
| 819 |
|
| 820 | ### Mutate Integers
|
| 821 |
|
| 822 | No:
|
| 823 |
|
| 824 | (( i++ )) # interacts poorly with errexit
|
| 825 | i=$(( i+1 ))
|
| 826 |
|
| 827 | Yes:
|
| 828 |
|
| 829 | setvar i += 1 # like Python, with a keyword
|
| 830 |
|
| 831 | ### Initialize and Assign Arrays
|
| 832 |
|
| 833 | Arrays in YSH look like `:| my array |` and `['my', 'array']`.
|
| 834 |
|
| 835 | No:
|
| 836 |
|
| 837 | local -a myarray=(one two three)
|
| 838 | myarray[3]='THREE'
|
| 839 |
|
| 840 | Yes:
|
| 841 |
|
| 842 | var myarray = :| one two three |
|
| 843 | setvar myarray[3] = 'THREE'
|
| 844 |
|
| 845 | var same = ['one', 'two', 'three']
|
| 846 | var typed = [1, 2, true, false, null]
|
| 847 |
|
| 848 |
|
| 849 | ### Initialize and Assign Dicts
|
| 850 |
|
| 851 | Dicts in YSH look like `{key: 'value'}`.
|
| 852 |
|
| 853 | No:
|
| 854 |
|
| 855 | local -A myassoc=(['key']=value ['k2']=v2)
|
| 856 | myassoc['key']=V
|
| 857 |
|
| 858 |
|
| 859 | Yes:
|
| 860 |
|
| 861 | # keys don't need to be quoted
|
| 862 | var myassoc = {key: 'value', k2: 'v2'}
|
| 863 | setvar myassoc['key'] = 'V'
|
| 864 |
|
| 865 | ### Get Values From Arrays and Dicts
|
| 866 |
|
| 867 | No:
|
| 868 |
|
| 869 | local x=${a[i-1]}
|
| 870 | x=${a[i]}
|
| 871 |
|
| 872 | local y=${A['key']}
|
| 873 |
|
| 874 | Yes:
|
| 875 |
|
| 876 | var x = a[i-1]
|
| 877 | setvar x = a[i]
|
| 878 |
|
| 879 | var y = A['key']
|
| 880 |
|
| 881 | ### Conditions and Comparisons
|
| 882 |
|
| 883 | No:
|
| 884 |
|
| 885 | if (( x > 0 )); then
|
| 886 | echo 'positive'
|
| 887 | fi
|
| 888 |
|
| 889 | Yes:
|
| 890 |
|
| 891 | if (x > 0) {
|
| 892 | echo 'positive'
|
| 893 | }
|
| 894 |
|
| 895 | ### Substituting Expressions in Words
|
| 896 |
|
| 897 | No:
|
| 898 |
|
| 899 | echo flag=$((1 + a[i] * 3)) # C-like arithmetic
|
| 900 |
|
| 901 | Yes:
|
| 902 |
|
| 903 | echo flag=$[1 + a[i] * 3] # Arbitrary YSH expressions
|
| 904 |
|
| 905 | # Possible, but a local var might be more readable
|
| 906 | echo flag=$['1' if x else '0']
|
| 907 |
|
| 908 |
|
| 909 | ## Use [Egg Expressions](eggex.html) instead of Regexes
|
| 910 |
|
| 911 | ### Test for a Match
|
| 912 |
|
| 913 | No:
|
| 914 |
|
| 915 | local pat='[[:digit:]]+'
|
| 916 | if [[ $x =~ $pat ]]; then
|
| 917 | echo 'number'
|
| 918 | fi
|
| 919 |
|
| 920 | Yes:
|
| 921 |
|
| 922 | if (x ~ /digit+/) {
|
| 923 | echo 'number'
|
| 924 | }
|
| 925 |
|
| 926 | Or extract the pattern:
|
| 927 |
|
| 928 | var pat = / digit+ /
|
| 929 | if (x ~ pat) {
|
| 930 | echo 'number'
|
| 931 | }
|
| 932 |
|
| 933 | ### Extract Submatches
|
| 934 |
|
| 935 | No:
|
| 936 |
|
| 937 | if [[ $x =~ foo-([[:digit:]]+) ]] {
|
| 938 | echo "${BASH_REMATCH[1]}" # first submatch
|
| 939 | }
|
| 940 |
|
| 941 | Yes:
|
| 942 |
|
| 943 | if (x ~ / 'foo-' <capture d+> /) { # <> is capture
|
| 944 | echo $[_group(1)] # first submatch
|
| 945 | }
|
| 946 |
|
| 947 | ## Glob Matching
|
| 948 |
|
| 949 | No:
|
| 950 |
|
| 951 | if [[ $x == *.py ]]; then
|
| 952 | echo 'Python'
|
| 953 | fi
|
| 954 |
|
| 955 | Yes:
|
| 956 |
|
| 957 | if (x ~~ '*.py') {
|
| 958 | echo 'Python'
|
| 959 | }
|
| 960 |
|
| 961 |
|
| 962 | No:
|
| 963 |
|
| 964 | case $x in
|
| 965 | *.py)
|
| 966 | echo Python
|
| 967 | ;;
|
| 968 | *.sh)
|
| 969 | echo Shell
|
| 970 | ;;
|
| 971 | esac
|
| 972 |
|
| 973 | Yes (purely a style preference):
|
| 974 |
|
| 975 | case $x { # curly braces
|
| 976 | (*.py) # balanced parens
|
| 977 | echo 'Python'
|
| 978 | ;;
|
| 979 | (*.sh)
|
| 980 | echo 'Shell'
|
| 981 | ;;
|
| 982 | }
|
| 983 |
|
| 984 | ## TODO
|
| 985 |
|
| 986 | ### Distinguish Between Variables and Functions
|
| 987 |
|
| 988 | - `$RANDOM` vs. `random()`
|
| 989 | - `LANG=C` vs. `shopt --setattr LANG=C`
|
| 990 |
|
| 991 | ## Related Documents
|
| 992 |
|
| 993 | - [Shell Language Idioms](shell-idioms.html). This advice applies to shells
|
| 994 | other than YSH.
|
| 995 | - [What Breaks When You Upgrade to YSH](upgrade-breakage.html). Shell constructs that YSH
|
| 996 | users should avoid.
|
| 997 | - [YSH Fixes Shell's Error Handling (`errexit`)](error-handling.html). YSH fixes the
|
| 998 | flaky error handling in POSIX shell and bash.
|
| 999 | - TODO: Go through more of the [Pure Bash
|
| 1000 | Bible](https://github.com/dylanaraps/pure-bash-bible). YSH provides
|
| 1001 | alternatives for such quirky syntax.
|
| 1002 |
|