bash.md

   1 ---
   2 title: Bash scripting
   3 category: CLI
   4 layout: 2017/sheet
   5 tags: [Featured]
   6 updated: 2020-07-05
   7 keywords:
   8   - Variables
   9   - Functions
  10   - Interpolation
  11   - Brace expansions
  12   - Loops
  13   - Conditional execution
  14   - Command substitution
  15 ---
  16
  17 Getting started
  18 ---------------
  19 {: .-three-column}
  20
  21 ### Introduction
  22 {: .-intro}
  23
  24 This is a quick reference to getting started with Bash scripting.
  25
  26 - [Learn bash in y minutes](https://learnxinyminutes.com/docs/bash/) _(learnxinyminutes.com)_
  27 - [Bash Guide](http://mywiki.wooledge.org/BashGuide) _(mywiki.wooledge.org)_
  28
  29 ### Example
  30
  31 ```bash
  32 #!/usr/bin/env bash
  33
  34 NAME="John"
  35 echo "Hello $NAME!"
  36 ```
  37
  38 ### Variables
  39
  40 ```bash
  41 NAME="John"
  42 echo $NAME
  43 echo "$NAME"
  44 echo "${NAME}!"
  45 ```
  46
  47 ### String quotes
  48
  49 ```bash
  50 NAME="John"
  51 echo "Hi $NAME"  #=> Hi John
  52 echo 'Hi $NAME'  #=> Hi $NAME
  53 ```
  54
  55 ### Shell execution
  56
  57 ```bash
  58 echo "I'm in $(pwd)"
  59 echo "I'm in `pwd`"
  60 # Same
  61 ```
  62
  63 See [Command substitution](http://wiki.bash-hackers.org/syntax/expansion/cmdsubst)
  64
  65 ### Conditional execution
  66
  67 ```bash
  68 git commit && git push
  69 git commit || echo "Commit failed"
  70 ```
  71
  72 ### Functions
  73 {: id='functions-example'}
  74
  75 ```bash
  76 get_name() {
  77   echo "John"
  78 }
  79
  80 echo "You are $(get_name)"
  81 ```
  82
  83 See: [Functions](#functions)
  84
  85 ### Conditionals
  86 {: id='conditionals-example'}
  87
  88 ```bash
  89 if [[ -z "$string" ]]; then
  90   echo "String is empty"
  91 elif [[ -n "$string" ]]; then
  92   echo "String is not empty"
  93 fi
  94 ```
  95
  96 See: [Conditionals](#conditionals)
  97
  98 ### Strict mode
  99
 100 ```bash
 101 set -euo pipefail
 102 IFS=$'\n\t'
 103 ```
 104
 105 See: [Unofficial bash strict mode](http://redsymbol.net/articles/unofficial-bash-strict-mode/)
 106
 107 ### Brace expansion
 108
 109 ```bash
 110 echo {A,B}.js
 111 ```
 112
 113 | Expression | Description         |
 114 | ---------- | ------------------- |
 115 | `{A,B}`    | Same as `A B`       |
 116 | `{A,B}.js` | Same as `A.js B.js` |
 117 | `{1..5}`   | Same as `1 2 3 4 5` |
 118
 119 See: [Brace expansion](http://wiki.bash-hackers.org/syntax/expansion/brace)
 120
 121
 122 Parameter expansions
 123 --------------------
 124 {: .-three-column}
 125
 126 ### Basics
 127
 128 ```bash
 129 name="John"
 130 echo ${name}
 131 echo ${name/J/j}    #=> "john" (substitution)
 132 echo ${name:0:2}    #=> "Jo" (slicing)
 133 echo ${name::2}     #=> "Jo" (slicing)
 134 echo ${name::-1}    #=> "Joh" (slicing)
 135 echo ${name:(-1)}   #=> "n" (slicing from right)
 136 echo ${name:(-2):1} #=> "h" (slicing from right)
 137 echo ${food:-Cake}  #=> $food or "Cake"
 138 ```
 139
 140 ```bash
 141 length=2
 142 echo ${name:0:length}  #=> "Jo"
 143 ```
 144
 145 See: [Parameter expansion](http://wiki.bash-hackers.org/syntax/pe)
 146
 147 ```bash
 148 STR="/path/to/foo.cpp"
 149 echo ${STR%.cpp}    # /path/to/foo
 150 echo ${STR%.cpp}.o  # /path/to/foo.o
 151 echo ${STR%/*}      # /path/to
 152
 153 echo ${STR##*.}     # cpp (extension)
 154 echo ${STR##*/}     # foo.cpp (basepath)
 155
 156 echo ${STR#*/}      # path/to/foo.cpp
 157 echo ${STR##*/}     # foo.cpp
 158
 159 echo ${STR/foo/bar} # /path/to/bar.cpp
 160 ```
 161
 162 ```bash
 163 STR="Hello world"
 164 echo ${STR:6:5}   # "world"
 165 echo ${STR: -5:5}  # "world"
 166 ```
 167
 168 ```bash
 169 SRC="/path/to/foo.cpp"
 170 BASE=${SRC##*/}   #=> "foo.cpp" (basepath)
 171 DIR=${SRC%$BASE}  #=> "/path/to/" (dirpath)
 172 ```
 173
 174 ### Substitution
 175
 176 | Code              | Description         |
 177 | ----------------- | ------------------- |
 178 | `${FOO%suffix}`   | Remove suffix       |
 179 | `${FOO#prefix}`   | Remove prefix       |
 180 | ---               | ---                 |
 181 | `${FOO%%suffix}`  | Remove long suffix  |
 182 | `${FOO##prefix}`  | Remove long prefix  |
 183 | ---               | ---                 |
 184 | `${FOO/from/to}`  | Replace first match |
 185 | `${FOO//from/to}` | Replace all         |
 186 | ---               | ---                 |
 187 | `${FOO/%from/to}` | Replace suffix      |
 188 | `${FOO/#from/to}` | Replace prefix      |
 189
 190 ### Comments
 191
 192 ```bash
 193 # Single line comment
 194 ```
 195
 196 ```bash
 197 : '
 198 This is a
 199 multi line
 200 comment
 201 '
 202 ```
 203
 204 ### Substrings
 205
 206 | Expression      | Description                    |
 207 | --------------- | ------------------------------ |
 208 | `${FOO:0:3}`    | Substring _(position, length)_ |
 209 | `${FOO:(-3):3}` | Substring from the right       |
 210
 211 ### Length
 212
 213 | Expression | Description      |
 214 | ---------- | ---------------- |
 215 | `${#FOO}`  | Length of `$FOO` |
 216
 217 ### Manipulation
 218
 219 ```bash
 220 STR="HELLO WORLD!"
 221 echo ${STR,}   #=> "hELLO WORLD!" (lowercase 1st letter)
 222 echo ${STR,,}  #=> "hello world!" (all lowercase)
 223
 224 STR="hello world!"
 225 echo ${STR^}   #=> "Hello world!" (uppercase 1st letter)
 226 echo ${STR^^}  #=> "HELLO WORLD!" (all uppercase)
 227 ```
 228
 229 ### Default values
 230
 231 | Expression        | Description                                              |
 232 | ----------------- | -------------------------------------------------------- |
 233 | `${FOO:-val}`     | `$FOO`, or `val` if unset (or null)                      |
 234 | `${FOO:=val}`     | Set `$FOO` to `val` if unset (or null)                   |
 235 | `${FOO:+val}`     | `val` if `$FOO` is set (and not null)                    |
 236 | `${FOO:?message}` | Show error message and exit if `$FOO` is unset (or null) |
 237
 238 Omitting the `:` removes the (non)nullity checks, e.g. `${FOO-val}` expands to `val` if unset otherwise `$FOO`.
 239
 240 Loops
 241 -----
 242 {: .-three-column}
 243
 244 ### Basic for loop
 245
 246 ```bash
 247 for i in /etc/rc.*; do
 248   echo $i
 249 done
 250 ```
 251
 252 ### C-like for loop
 253
 254 ```bash
 255 for ((i = 0 ; i < 100 ; i++)); do
 256   echo $i
 257 done
 258 ```
 259
 260 ### Ranges
 261
 262 ```bash
 263 for i in {1..5}; do
 264     echo "Welcome $i"
 265 done
 266 ```
 267
 268 #### With step size
 269
 270 ```bash
 271 for i in {5..50..5}; do
 272     echo "Welcome $i"
 273 done
 274 ```
 275
 276 ### Reading lines
 277
 278 ```bash
 279 cat file.txt | while read line; do
 280   echo $line
 281 done
 282 ```
 283
 284 ### Forever
 285
 286 ```bash
 287 while true; do
 288   ···
 289 done
 290 ```
 291
 292 Functions
 293 ---------
 294 {: .-three-column}
 295
 296 ### Defining functions
 297
 298 ```bash
 299 myfunc() {
 300     echo "hello $1"
 301 }
 302 ```
 303
 304 ```bash
 305 # Same as above (alternate syntax)
 306 function myfunc() {
 307     echo "hello $1"
 308 }
 309 ```
 310
 311 ```bash
 312 myfunc "John"
 313 ```
 314
 315 ### Returning values
 316
 317 ```bash
 318 myfunc() {
 319     local myresult='some value'
 320     echo $myresult
 321 }
 322 ```
 323
 324 ```bash
 325 result="$(myfunc)"
 326 ```
 327
 328 ### Raising errors
 329
 330 ```bash
 331 myfunc() {
 332   return 1
 333 }
 334 ```
 335
 336 ```bash
 337 if myfunc; then
 338   echo "success"
 339 else
 340   echo "failure"
 341 fi
 342 ```
 343
 344 ### Arguments
 345
 346 | Expression | Description                                      |
 347 | ---        | ---                                              |
 348 | `$#`       | Number of arguments                              |
 349 | `$*`       | All positional arguments  (as a single word)     |
 350 | `$@`       | All positional arguments (as separate strings)  |
 351 | `$1`       | First argument                                   |
 352 | `$_`       | Last argument of the previous command            |
 353
 354 **Note**: `$@` and `$*` must be quoted in order to perform as described.
 355 Otherwise, they do exactly the same thing (arguments as separate strings).
 356
 357 See [Special parameters](http://wiki.bash-hackers.org/syntax/shellvars#special_parameters_and_shell_variables).
 358
 359 Conditionals
 360 ------------
 361 {: .-three-column}
 362
 363 ### Conditions
 364
 365 Note that `[[` is actually a command/program that returns either `0` (true) or `1` (false). Any program that obeys the same logic (like all base utils, such as `grep(1)` or `ping(1)`) can be used as condition, see examples.
 366
 367 | Condition                | Description           |
 368 | ---                      | ---                   |
 369 | `[[ -z STRING ]]`        | Empty string          |
 370 | `[[ -n STRING ]]`        | Not empty string      |
 371 | `[[ STRING == STRING ]]` | Equal                 |
 372 | `[[ STRING != STRING ]]` | Not Equal             |
 373 | ---                      | ---                   |
 374 | `[[ NUM -eq NUM ]]`      | Equal                 |
 375 | `[[ NUM -ne NUM ]]`      | Not equal             |
 376 | `[[ NUM -lt NUM ]]`      | Less than             |
 377 | `[[ NUM -le NUM ]]`      | Less than or equal    |
 378 | `[[ NUM -gt NUM ]]`      | Greater than          |
 379 | `[[ NUM -ge NUM ]]`      | Greater than or equal |
 380 | ---                      | ---                   |
 381 | `[[ STRING =~ STRING ]]` | Regexp                |
 382 | ---                      | ---                   |
 383 | `(( NUM < NUM ))`        | Numeric conditions    |
 384
 385 #### More conditions
 386
 387 | Condition            | Description              |
 388 | -------------------- | ------------------------ |
 389 | `[[ -o noclobber ]]` | If OPTIONNAME is enabled |
 390 | ---                  | ---                      |
 391 | `[[ ! EXPR ]]`       | Not                      |
 392 | `[[ X && Y ]]`       | And                      |
 393 | `[[ X || Y ]]`       | Or                       |
 394
 395 ### File conditions
 396
 397 | Condition               | Description             |
 398 | ---                     | ---                     |
 399 | `[[ -e FILE ]]`         | Exists                  |
 400 | `[[ -r FILE ]]`         | Readable                |
 401 | `[[ -h FILE ]]`         | Symlink                 |
 402 | `[[ -d FILE ]]`         | Directory               |
 403 | `[[ -w FILE ]]`         | Writable                |
 404 | `[[ -s FILE ]]`         | Size is > 0 bytes       |
 405 | `[[ -f FILE ]]`         | File                    |
 406 | `[[ -x FILE ]]`         | Executable              |
 407 | ---                     | ---                     |
 408 | `[[ FILE1 -nt FILE2 ]]` | 1 is more recent than 2 |
 409 | `[[ FILE1 -ot FILE2 ]]` | 2 is more recent than 1 |
 410 | `[[ FILE1 -ef FILE2 ]]` | Same files              |
 411
 412 ### Example
 413
 414 ```bash
 415 # String
 416 if [[ -z "$string" ]]; then
 417   echo "String is empty"
 418 elif [[ -n "$string" ]]; then
 419   echo "String is not empty"
 420 else
 421   echo "This never happens"
 422 fi
 423 ```
 424
 425 ```bash
 426 # Combinations
 427 if [[ X && Y ]]; then
 428   ...
 429 fi
 430 ```
 431
 432 ```bash
 433 # Equal
 434 if [[ "$A" == "$B" ]]
 435 ```
 436
 437 ```bash
 438 # Regex
 439 if [[ "A" =~ . ]]
 440 ```
 441
 442 ```bash
 443 if (( $a < $b )); then
 444    echo "$a is smaller than $b"
 445 fi
 446 ```
 447
 448 ```bash
 449 if [[ -e "file.txt" ]]; then
 450   echo "file exists"
 451 fi
 452 ```
 453
 454 Arrays
 455 ------
 456
 457 ### Defining arrays
 458
 459 ```bash
 460 Fruits=('Apple' 'Banana' 'Orange')
 461 ```
 462
 463 ```bash
 464 Fruits[0]="Apple"
 465 Fruits[1]="Banana"
 466 Fruits[2]="Orange"
 467 ```
 468
 469 ### Working with arrays
 470
 471 ```bash
 472 echo ${Fruits[0]}           # Element #0
 473 echo ${Fruits[-1]}          # Last element
 474 echo ${Fruits[@]}           # All elements, space-separated
 475 echo ${#Fruits[@]}          # Number of elements
 476 echo ${#Fruits}             # String length of the 1st element
 477 echo ${#Fruits[3]}          # String length of the Nth element
 478 echo ${Fruits[@]:3:2}       # Range (from position 3, length 2)
 479 echo ${!Fruits[@]}          # Keys of all elements, space-separated
 480 ```
 481
 482 ### Operations
 483
 484 ```bash
 485 Fruits=("${Fruits[@]}" "Watermelon")    # Push
 486 Fruits+=('Watermelon')                  # Also Push
 487 Fruits=( ${Fruits[@]/Ap*/} )            # Remove by regex match
 488 unset Fruits[2]                         # Remove one item
 489 Fruits=("${Fruits[@]}")                 # Duplicate
 490 Fruits=("${Fruits[@]}" "${Veggies[@]}") # Concatenate
 491 lines=(`cat "logfile"`)                 # Read from file
 492 ```
 493
 494 ### Iteration
 495
 496 ```bash
 497 for i in "${arrayName[@]}"; do
 498   echo $i
 499 done
 500 ```
 501
 502 Dictionaries
 503 ------------
 504 {: .-three-column}
 505
 506 ### Defining
 507
 508 ```bash
 509 declare -A sounds
 510 ```
 511
 512 ```bash
 513 sounds[dog]="bark"
 514 sounds[cow]="moo"
 515 sounds[bird]="tweet"
 516 sounds[wolf]="howl"
 517 ```
 518
 519 Declares `sound` as a Dictionary object (aka associative array).
 520
 521 ### Working with dictionaries
 522
 523 ```bash
 524 echo ${sounds[dog]} # Dog's sound
 525 echo ${sounds[@]}   # All values
 526 echo ${!sounds[@]}  # All keys
 527 echo ${#sounds[@]}  # Number of elements
 528 unset sounds[dog]   # Delete dog
 529 ```
 530
 531 ### Iteration
 532
 533 #### Iterate over values
 534
 535 ```bash
 536 for val in "${sounds[@]}"; do
 537   echo $val
 538 done
 539 ```
 540
 541 #### Iterate over keys
 542
 543 ```bash
 544 for key in "${!sounds[@]}"; do
 545   echo $key
 546 done
 547 ```
 548
 549 Options
 550 -------
 551
 552 ### Options
 553
 554 ```bash
 555 set -o noclobber  # Avoid overlay files (echo "hi" > foo)
 556 set -o errexit    # Used to exit upon error, avoiding cascading errors
 557 set -o pipefail   # Unveils hidden failures
 558 set -o nounset    # Exposes unset variables
 559 ```
 560
 561 ### Glob options
 562
 563 ```bash
 564 shopt -s nullglob    # Non-matching globs are removed  ('*.foo' => '')
 565 shopt -s failglob    # Non-matching globs throw errors
 566 shopt -s nocaseglob  # Case insensitive globs
 567 shopt -s dotglob     # Wildcards match dotfiles ("*.sh" => ".foo.sh")
 568 shopt -s globstar    # Allow ** for recursive matches ('lib/**/*.rb' => 'lib/a/b/c.rb')
 569 ```
 570
 571 Set `GLOBIGNORE` as a colon-separated list of patterns to be removed from glob
 572 matches.
 573
 574 History
 575 -------
 576
 577 ### Commands
 578
 579 | Command               | Description                               |
 580 | --------------------- | ----------------------------------------- |
 581 | `history`             | Show history                              |
 582 | `shopt -s histverify` | Don't execute expanded result immediately |
 583
 584 ### Expansions
 585
 586 | Expression   | Description                                          |
 587 | ------------ | ---------------------------------------------------- |
 588 | `!$`         | Expand last parameter of most recent command         |
 589 | `!*`         | Expand all parameters of most recent command         |
 590 | `!-n`        | Expand `n`th most recent command                     |
 591 | `!n`         | Expand `n`th command in history                      |
 592 | `!<command>` | Expand most recent invocation of command `<command>` |
 593
 594 ### Operations
 595
 596 | Code                 | Description                                                           |
 597 | -------------------- | --------------------------------------------------------------------- |
 598 | `!!`                 | Execute last command again                                            |
 599 | `!!:s/<FROM>/<TO>/`  | Replace first occurrence of `<FROM>` to `<TO>` in most recent command |
 600 | `!!:gs/<FROM>/<TO>/` | Replace all occurrences of `<FROM>` to `<TO>` in most recent command  |
 601 | `!$:t`               | Expand only basename from last parameter of most recent command       |
 602 | `!$:h`               | Expand only directory from last parameter of most recent command      |
 603
 604 `!!` and `!$` can be replaced with any valid expansion.
 605
 606 ### Slices
 607
 608 | Code     | Description                                                                              |
 609 | -------- | ---------------------------------------------------------------------------------------- |
 610 | `!!:n`   | Expand only `n`th token from most recent command (command is `0`; first argument is `1`) |
 611 | `!^`     | Expand first argument from most recent command                                           |
 612 | `!$`     | Expand last token from most recent command                                               |
 613 | `!!:n-m` | Expand range of tokens from most recent command                                          |
 614 | `!!:n-$` | Expand `n`th token to last from most recent command                                      |
 615
 616 `!!` can be replaced with any valid expansion i.e. `!cat`, `!-2`, `!42`, etc.
 617
 618
 619 Miscellaneous
 620 -------------
 621
 622 ### Numeric calculations
 623
 624 ```bash
 625 $((a + 200))      # Add 200 to $a
 626 ```
 627
 628 ```bash
 629 $(($RANDOM%200))  # Random number 0..199
 630 ```
 631
 632 ### Subshells
 633
 634 ```bash
 635 (cd somedir; echo "I'm now in $PWD")
 636 pwd # still in first directory
 637 ```
 638
 639 ### Redirection
 640
 641 ```bash
 642 python hello.py > output.txt   # stdout to (file)
 643 python hello.py >> output.txt  # stdout to (file), append
 644 python hello.py 2> error.log   # stderr to (file)
 645 python hello.py 2>&1           # stderr to stdout
 646 python hello.py 2>/dev/null    # stderr to (null)
 647 python hello.py &>/dev/null    # stdout and stderr to (null)
 648 ```
 649
 650 ```bash
 651 python hello.py < foo.txt      # feed foo.txt to stdin for python
 652 diff <(ls -r) <(ls)            # Compare two stdout without files
 653 ```
 654
 655 ### Inspecting commands
 656
 657 ```bash
 658 command -V cd
 659 #=> "cd is a function/alias/whatever"
 660 ```
 661
 662 ### Trap errors
 663
 664 ```bash
 665 trap 'echo Error at about $LINENO' ERR
 666 ```
 667
 668 or
 669
 670 ```bash
 671 traperr() {
 672   echo "ERROR: ${BASH_SOURCE[1]} at about ${BASH_LINENO[0]}"
 673 }
 674
 675 set -o errtrace
 676 trap traperr ERR
 677 ```
 678
 679 ### Case/switch
 680
 681 ```bash
 682 case "$1" in
 683   start | up)
 684     vagrant up
 685     ;;
 686
 687   *)
 688     echo "Usage: $0 {start|stop|ssh}"
 689     ;;
 690 esac
 691 ```
 692
 693 ### Source relative
 694
 695 ```bash
 696 source "${0%/*}/../share/foo.sh"
 697 ```
 698
 699 ### printf
 700
 701 ```bash
 702 printf "Hello %s, I'm %s" Sven Olga
 703 #=> "Hello Sven, I'm Olga
 704
 705 printf "1 + 1 = %d" 2
 706 #=> "1 + 1 = 2"
 707
 708 printf "This is how you print a float: %f" 2
 709 #=> "This is how you print a float: 2.000000"
 710 ```
 711
 712 ### Transform strings
 713
 714 | Command option     | Description                                         |
 715 | ------------------ | --------------------------------------------------- |
 716 | `-c`               | Operations apply to characters not in the given set |
 717 | `-d`               | Delete characters                                   |
 718 | `-s`               | Replaces repeated characters with single occurrence |
 719 | `-t`               | Truncates                                           |
 720 | `[:upper:]`        | All upper case letters                              |
 721 | `[:lower:]`        | All lower case letters                              |
 722 | `[:digit:]`        | All digits                                          |
 723 | `[:space:]`        | All whitespace                                      |
 724 | `[:alpha:]`        | All letters                                         |
 725 | `[:alnum:]`        | All letters and digits                              |
 726
 727 #### Example
 728
 729 ```bash
 730 echo "Welcome To Devhints" | tr [:lower:] [:upper:]
 731 WELCOME TO DEVHINTS
 732 ```
 733
 734 ### Directory of script
 735
 736 ```bash
 737 DIR="${0%/*}"
 738 ```
 739
 740 ### Getting options
 741
 742 ```bash
 743 while [[ "$1" =~ ^- && ! "$1" == "--" ]]; do case $1 in
 744   -V | --version )
 745     echo $version
 746     exit
 747     ;;
 748   -s | --string )
 749     shift; string=$1
 750     ;;
 751   -f | --flag )
 752     flag=1
 753     ;;
 754 esac; shift; done
 755 if [[ "$1" == '--' ]]; then shift; fi
 756 ```
 757
 758 ### Heredoc
 759
 760 ```sh
 761 cat <<END
 762 hello world
 763 END
 764 ```
 765
 766 ### Reading input
 767
 768 ```bash
 769 echo -n "Proceed? [y/n]: "
 770 read ans
 771 echo $ans
 772 ```
 773
 774 ```bash
 775 read -n 1 ans    # Just one character
 776 ```
 777
 778 ### Special variables
 779
 780 | Expression | Description                            |
 781 | ---------- | -------------------------------------- |
 782 | `$?`       | Exit status of last task               |
 783 | `$!`       | PID of last background task            |
 784 | `$$`       | PID of shell                           |
 785 | `$0`       | Filename of the shell script           |
 786 | `$_`       | Last argument of the previous command  |
 787
 788 See [Special parameters](http://wiki.bash-hackers.org/syntax/shellvars#special_parameters_and_shell_variables).
 789
 790 ### Go to previous directory
 791
 792 ```bash
 793 pwd # /home/user/foo
 794 cd bar/
 795 pwd # /home/user/foo/bar
 796 cd -
 797 pwd # /home/user/foo
 798 ```
 799
 800 ### Check for command's result
 801
 802 ```bash
 803 if ping -c 1 google.com; then
 804   echo "It appears you have a working internet connection"
 805 fi
 806 ```
 807
 808 ### Grep check
 809
 810 ```bash
 811 if grep -q 'foo' ~/.bash_history; then
 812   echo "You appear to have typed 'foo' in the past"
 813 fi
 814 ```
 815
 816 ## Also see
 817 {: .-one-column}
 818
 819 * [Bash-hackers wiki](http://wiki.bash-hackers.org/) _(bash-hackers.org)_
 820 * [Shell vars](http://wiki.bash-hackers.org/syntax/shellvars) _(bash-hackers.org)_
 821 * [Learn bash in y minutes](https://learnxinyminutes.com/docs/bash/) _(learnxinyminutes.com)_
 822 * [Bash Guide](http://mywiki.wooledge.org/BashGuide) _(mywiki.wooledge.org)_
 823 * [ShellCheck](https://www.shellcheck.net/) _(shellcheck.net)_