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