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
  23 {: .-intro}
  24
  25 This is a quick reference to getting started with Bash scripting.
  26
  27 - [Learn bash in y minutes](https://learnxinyminutes.com/docs/bash/) _(learnxinyminutes.com)_
  28 - [Bash Guide](http://mywiki.wooledge.org/BashGuide) _(mywiki.wooledge.org)_
  29 - [Bash Hackers Wiki](https://web.archive.org/web/20230406205817/https://wiki.bash-hackers.org/) _(wiki.bash-hackers.org)_
  30
  31 ### Example
  32
  33 ```bash
  34 #!/usr/bin/env bash
  35
  36 name="John"
  37 echo "Hello $name!"
  38 ```
  39
  40 ### Variables
  41
  42 ```bash
  43 name="John"
  44 echo $name  # see below
  45 echo "$name"
  46 echo "${name}!"
  47 ```
  48
  49 Generally quote your variables unless they contain wildcards to expand or command fragments.
  50
  51 ```bash
  52 wildcard="*.txt"
  53 options="iv"
  54 cp -$options $wildcard /tmp
  55 ```
  56
  57 ### String quotes
  58
  59 ```bash
  60 name="John"
  61 echo "Hi $name"  #=> Hi John
  62 echo 'Hi $name'  #=> Hi $name
  63 ```
  64
  65 ### Shell execution
  66
  67 ```bash
  68 echo "I'm in $(pwd)"
  69 echo "I'm in `pwd`"  # obsolescent
  70 # Same
  71 ```
  72
  73 See [Command substitution](https://web.archive.org/web/20230326081741/https://wiki.bash-hackers.org/syntax/expansion/cmdsubst)
  74
  75 ### Conditional execution
  76
  77 ```bash
  78 git commit && git push
  79 git commit || echo "Commit failed"
  80 ```
  81
  82 ### Functions
  83
  84 {: id='functions-example'}
  85
  86 ```bash
  87 get_name() {
  88   echo "John"
  89 }
  90
  91 echo "You are $(get_name)"
  92 ```
  93
  94 See: [Functions](#functions)
  95
  96 ### Conditionals
  97
  98 {: id='conditionals-example'}
  99
 100 ```bash
 101 if [[ -z "$string" ]]; then
 102   echo "String is empty"
 103 elif [[ -n "$string" ]]; then
 104   echo "String is not empty"
 105 fi
 106 ```
 107
 108 See: [Conditionals](#conditionals)
 109
 110 ### Strict mode
 111
 112 ```bash
 113 set -euo pipefail
 114 IFS=$'\n\t'
 115 ```
 116
 117 See: [Unofficial bash strict mode](http://redsymbol.net/articles/unofficial-bash-strict-mode/)
 118
 119 ### Brace expansion
 120
 121 ```bash
 122 echo {A,B}.js
 123 ```
 124
 125 | Expression             | Description           |
 126 | ---------------------- | --------------------- |
 127 | `{A,B}`                | Same as `A B`         |
 128 | `{A,B}.js`             | Same as `A.js B.js`   |
 129 | `{1..5}`               | Same as `1 2 3 4 5`   |
 130 | <code>&lcub;{1..3},{7..9}}</code> | Same as `1 2 3 7 8 9` |
 131
 132 See: [Brace expansion](https://web.archive.org/web/20230207192110/https://wiki.bash-hackers.org/syntax/expansion/brace)
 133
 134 ## Parameter expansions
 135
 136 {: .-three-column}
 137
 138 ### Basics
 139
 140 ```bash
 141 name="John"
 142 echo "${name}"
 143 echo "${name/J/j}"    #=> "john" (substitution)
 144 echo "${name:0:2}"    #=> "Jo" (slicing)
 145 echo "${name::2}"     #=> "Jo" (slicing)
 146 echo "${name::-1}"    #=> "Joh" (slicing)
 147 echo "${name:(-1)}"   #=> "n" (slicing from right)
 148 echo "${name:(-2):1}" #=> "h" (slicing from right)
 149 echo "${food:-Cake}"  #=> $food or "Cake"
 150 ```
 151
 152 ```bash
 153 length=2
 154 echo "${name:0:length}"  #=> "Jo"
 155 ```
 156
 157 See: [Parameter expansion](https://web.archive.org/web/20230408142504/https://wiki.bash-hackers.org/syntax/pe)
 158
 159 ```bash
 160 str="/path/to/foo.cpp"
 161 echo "${str%.cpp}"    # /path/to/foo
 162 echo "${str%.cpp}.o"  # /path/to/foo.o
 163 echo "${str%/*}"      # /path/to
 164
 165 echo "${str##*.}"     # cpp (extension)
 166 echo "${str##*/}"     # foo.cpp (basepath)
 167
 168 echo "${str#*/}"      # path/to/foo.cpp
 169 echo "${str##*/}"     # foo.cpp
 170
 171 echo "${str/foo/bar}" # /path/to/bar.cpp
 172 ```
 173
 174 ```bash
 175 str="Hello world"
 176 echo "${str:6:5}"   # "world"
 177 echo "${str: -5:5}"  # "world"
 178 ```
 179
 180 ```bash
 181 src="/path/to/foo.cpp"
 182 base=${src##*/}   #=> "foo.cpp" (basepath)
 183 dir=${src%$base}  #=> "/path/to/" (dirpath)
 184 ```
 185
 186 ### Substitution
 187
 188 | Code              | Description         |
 189 | ----------------- | ------------------- |
 190 | `${foo%suffix}`   | Remove suffix       |
 191 | `${foo#prefix}`   | Remove prefix       |
 192 | ---               | ---                 |
 193 | `${foo%%suffix}`  | Remove long suffix  |
 194 | `${foo/%suffix}`  | Remove long suffix  |
 195 | `${foo##prefix}`  | Remove long prefix  |
 196 | `${foo/#prefix}`  | Remove long prefix  |
 197 | ---               | ---                 |
 198 | `${foo/from/to}`  | Replace first match |
 199 | `${foo//from/to}` | Replace all         |
 200 | ---               | ---                 |
 201 | `${foo/%from/to}` | Replace suffix      |
 202 | `${foo/#from/to}` | Replace prefix      |
 203
 204 ### Comments
 205
 206 ```bash
 207 # Single line comment
 208 ```
 209
 210 ```bash
 211 : '
 212 This is a
 213 multi line
 214 comment
 215 '
 216 ```
 217
 218 ### Substrings
 219
 220 | Expression      | Description                    |
 221 | --------------- | ------------------------------ |
 222 | `${foo:0:3}`    | Substring _(position, length)_ |
 223 | `${foo:(-3):3}` | Substring from the right       |
 224
 225 ### Length
 226
 227 | Expression | Description      |
 228 | ---------- | ---------------- |
 229 | `${#foo}`  | Length of `$foo` |
 230
 231 ### Manipulation
 232
 233 ```bash
 234 str="HELLO WORLD!"
 235 echo "${str,}"   #=> "hELLO WORLD!" (lowercase 1st letter)
 236 echo "${str,,}"  #=> "hello world!" (all lowercase)
 237
 238 str="hello world!"
 239 echo "${str^}"   #=> "Hello world!" (uppercase 1st letter)
 240 echo "${str^^}"  #=> "HELLO WORLD!" (all uppercase)
 241 ```
 242
 243 ### Default values
 244
 245 | Expression        | Description                                              |
 246 | ----------------- | -------------------------------------------------------- |
 247 | `${foo:-val}`     | `$foo`, or `val` if unset (or null)                      |
 248 | `${foo:=val}`     | Set `$foo` to `val` if unset (or null)                   |
 249 | `${foo:+val}`     | `val` if `$foo` is set (and not null)                    |
 250 | `${foo:?message}` | Show error message and exit if `$foo` is unset (or null) |
 251
 252 Omitting the `:` removes the (non)nullity checks, e.g. `${foo-val}` expands to `val` if unset otherwise `$foo`.
 253
 254 ## Loops
 255
 256 {: .-three-column}
 257
 258 ### Basic for loop
 259
 260 ```bash
 261 for i in /etc/rc.*; do
 262   echo "$i"
 263 done
 264 ```
 265
 266 ### C-like for loop
 267
 268 ```bash
 269 for ((i = 0 ; i < 100 ; i++)); do
 270   echo "$i"
 271 done
 272 ```
 273
 274 ### Ranges
 275
 276 ```bash
 277 for i in {1..5}; do
 278     echo "Welcome $i"
 279 done
 280 ```
 281
 282 #### With step size
 283
 284 ```bash
 285 for i in {5..50..5}; do
 286     echo "Welcome $i"
 287 done
 288 ```
 289
 290 ### Reading lines
 291
 292 ```bash
 293 while read -r line; do
 294   echo "$line"
 295 done <file.txt
 296 ```
 297
 298 ### Forever
 299
 300 ```bash
 301 while true; do
 302   ···
 303 done
 304 ```
 305
 306 ## Functions
 307
 308 {: .-three-column}
 309
 310 ### Defining functions
 311
 312 ```bash
 313 myfunc() {
 314     echo "hello $1"
 315 }
 316 ```
 317
 318 ```bash
 319 # Same as above (alternate syntax)
 320 function myfunc() {
 321     echo "hello $1"
 322 }
 323 ```
 324
 325 ```bash
 326 myfunc "John"
 327 ```
 328
 329 ### Returning values
 330
 331 ```bash
 332 myfunc() {
 333     local myresult='some value'
 334     echo "$myresult"
 335 }
 336 ```
 337
 338 ```bash
 339 result=$(myfunc)
 340 ```
 341
 342 ### Raising errors
 343
 344 ```bash
 345 myfunc() {
 346   return 1
 347 }
 348 ```
 349
 350 ```bash
 351 if myfunc; then
 352   echo "success"
 353 else
 354   echo "failure"
 355 fi
 356 ```
 357
 358 ### Arguments
 359
 360 | Expression | Description                                    |
 361 | ---------- | ---------------------------------------------- |
 362 | `$#`       | Number of arguments                            |
 363 | `$*`       | All positional arguments (as a single word)    |
 364 | `$@`       | All positional arguments (as separate strings) |
 365 | `$1`       | First argument                                 |
 366 | `$_`       | Last argument of the previous command          |
 367
 368 **Note**: `$@` and `$*` must be quoted in order to perform as described.
 369 Otherwise, they do exactly the same thing (arguments as separate strings).
 370
 371 See [Special parameters](https://web.archive.org/web/20230318164746/https://wiki.bash-hackers.org/syntax/shellvars#special_parameters_and_shell_variables).
 372
 373 ## Conditionals
 374
 375 {: .-three-column}
 376
 377 ### Conditions
 378
 379 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.
 380
 381 | Condition                | Description           |
 382 | ------------------------ | --------------------- |
 383 | `[[ -z STRING ]]`        | Empty string          |
 384 | `[[ -n STRING ]]`        | Not empty string      |
 385 | `[[ STRING == STRING ]]` | Equal                 |
 386 | `[[ STRING != STRING ]]` | Not Equal             |
 387 | ---                      | ---                   |
 388 | `[[ NUM -eq NUM ]]`      | Equal                 |
 389 | `[[ NUM -ne NUM ]]`      | Not equal             |
 390 | `[[ NUM -lt NUM ]]`      | Less than             |
 391 | `[[ NUM -le NUM ]]`      | Less than or equal    |
 392 | `[[ NUM -gt NUM ]]`      | Greater than          |
 393 | `[[ NUM -ge NUM ]]`      | Greater than or equal |
 394 | ---                      | ---                   |
 395 | `[[ STRING =~ STRING ]]` | Regexp                |
 396 | ---                      | ---                   |
 397 | `(( NUM < NUM ))`        | Numeric conditions    |
 398
 399 #### More conditions
 400
 401 | Condition            | Description              |
 402 | -------------------- | ------------------------ |
 403 | `[[ -o noclobber ]]` | If OPTIONNAME is enabled |
 404 | ---                  | ---                      |
 405 | `[[ ! EXPR ]]`       | Not                      |
 406 | `[[ X && Y ]]`       | And                      |
 407 | `[[ X || Y ]]`       | Or                       |
 408
 409 ### File conditions
 410
 411 | Condition               | Description             |
 412 | ----------------------- | ----------------------- |
 413 | `[[ -e FILE ]]`         | Exists                  |
 414 | `[[ -r FILE ]]`         | Readable                |
 415 | `[[ -h FILE ]]`         | Symlink                 |
 416 | `[[ -d FILE ]]`         | Directory               |
 417 | `[[ -w FILE ]]`         | Writable                |
 418 | `[[ -s FILE ]]`         | Size is > 0 bytes       |
 419 | `[[ -f FILE ]]`         | File                    |
 420 | `[[ -x FILE ]]`         | Executable              |
 421 | ---                     | ---                     |
 422 | `[[ FILE1 -nt FILE2 ]]` | 1 is more recent than 2 |
 423 | `[[ FILE1 -ot FILE2 ]]` | 2 is more recent than 1 |
 424 | `[[ FILE1 -ef FILE2 ]]` | Same files              |
 425
 426 ### Example
 427
 428 ```bash
 429 # String
 430 if [[ -z "$string" ]]; then
 431   echo "String is empty"
 432 elif [[ -n "$string" ]]; then
 433   echo "String is not empty"
 434 else
 435   echo "This never happens"
 436 fi
 437 ```
 438
 439 ```bash
 440 # Combinations
 441 if [[ X && Y ]]; then
 442   ...
 443 fi
 444 ```
 445
 446 ```bash
 447 # Equal
 448 if [[ "$A" == "$B" ]]
 449 ```
 450
 451 ```bash
 452 # Regex
 453 if [[ "A" =~ . ]]
 454 ```
 455
 456 ```bash
 457 if (( $a < $b )); then
 458    echo "$a is smaller than $b"
 459 fi
 460 ```
 461
 462 ```bash
 463 if [[ -e "file.txt" ]]; then
 464   echo "file exists"
 465 fi
 466 ```
 467
 468 ## Arrays
 469
 470 ### Defining arrays
 471
 472 ```bash
 473 Fruits=('Apple' 'Banana' 'Orange')
 474 ```
 475
 476 ```bash
 477 Fruits[0]="Apple"
 478 Fruits[1]="Banana"
 479 Fruits[2]="Orange"
 480 ```
 481
 482 ### Working with arrays
 483
 484 ```bash
 485 echo "${Fruits[0]}"           # Element #0
 486 echo "${Fruits[-1]}"          # Last element
 487 echo "${Fruits[@]}"           # All elements, space-separated
 488 echo "${#Fruits[@]}"          # Number of elements
 489 echo "${#Fruits}"             # String length of the 1st element
 490 echo "${#Fruits[3]}"          # String length of the Nth element
 491 echo "${Fruits[@]:3:2}"       # Range (from position 3, length 2)
 492 echo "${!Fruits[@]}"          # Keys of all elements, space-separated
 493 ```
 494
 495 ### Operations
 496
 497 ```bash
 498 Fruits=("${Fruits[@]}" "Watermelon")    # Push
 499 Fruits+=('Watermelon')                  # Also Push
 500 Fruits=( "${Fruits[@]/Ap*/}" )          # Remove by regex match
 501 unset Fruits[2]                         # Remove one item
 502 Fruits=("${Fruits[@]}")                 # Duplicate
 503 Fruits=("${Fruits[@]}" "${Veggies[@]}") # Concatenate
 504 lines=(`cat "logfile"`)                 # Read from file
 505 ```
 506
 507 ### Iteration
 508
 509 ```bash
 510 for i in "${arrayName[@]}"; do
 511   echo "$i"
 512 done
 513 ```
 514
 515 ## Dictionaries
 516
 517 {: .-three-column}
 518
 519 ### Defining
 520
 521 ```bash
 522 declare -A sounds
 523 ```
 524
 525 ```bash
 526 sounds[dog]="bark"
 527 sounds[cow]="moo"
 528 sounds[bird]="tweet"
 529 sounds[wolf]="howl"
 530 ```
 531
 532 Declares `sound` as a Dictionary object (aka associative array).
 533
 534 ### Working with dictionaries
 535
 536 ```bash
 537 echo "${sounds[dog]}" # Dog's sound
 538 echo "${sounds[@]}"   # All values
 539 echo "${!sounds[@]}"  # All keys
 540 echo "${#sounds[@]}"  # Number of elements
 541 unset sounds[dog]     # Delete dog
 542 ```
 543
 544 ### Iteration
 545
 546 #### Iterate over values
 547
 548 ```bash
 549 for val in "${sounds[@]}"; do
 550   echo "$val"
 551 done
 552 ```
 553
 554 #### Iterate over keys
 555
 556 ```bash
 557 for key in "${!sounds[@]}"; do
 558   echo "$key"
 559 done
 560 ```
 561
 562 ## Options
 563
 564 ### Options
 565
 566 ```bash
 567 set -o noclobber  # Avoid overlay files (echo "hi" > foo)
 568 set -o errexit    # Used to exit upon error, avoiding cascading errors
 569 set -o pipefail   # Unveils hidden failures
 570 set -o nounset    # Exposes unset variables
 571 ```
 572
 573 ### Glob options
 574
 575 ```bash
 576 shopt -s nullglob    # Non-matching globs are removed  ('*.foo' => '')
 577 shopt -s failglob    # Non-matching globs throw errors
 578 shopt -s nocaseglob  # Case insensitive globs
 579 shopt -s dotglob     # Wildcards match dotfiles ("*.sh" => ".foo.sh")
 580 shopt -s globstar    # Allow ** for recursive matches ('lib/**/*.rb' => 'lib/a/b/c.rb')
 581 ```
 582
 583 Set `GLOBIGNORE` as a colon-separated list of patterns to be removed from glob
 584 matches.
 585
 586 ## History
 587
 588 ### Commands
 589
 590 | Command               | Description                               |
 591 | --------------------- | ----------------------------------------- |
 592 | `history`             | Show history                              |
 593 | `shopt -s histverify` | Don't execute expanded result immediately |
 594
 595 ### Expansions
 596
 597 | Expression   | Description                                          |
 598 | ------------ | ---------------------------------------------------- |
 599 | `!$`         | Expand last parameter of most recent command         |
 600 | `!*`         | Expand all parameters of most recent command         |
 601 | `!-n`        | Expand `n`th most recent command                     |
 602 | `!n`         | Expand `n`th command in history                      |
 603 | `!<command>` | Expand most recent invocation of command `<command>` |
 604
 605 ### Operations
 606
 607 | Code                 | Description                                                           |
 608 | -------------------- | --------------------------------------------------------------------- |
 609 | `!!`                 | Execute last command again                                            |
 610 | `!!:s/<FROM>/<TO>/`  | Replace first occurrence of `<FROM>` to `<TO>` in most recent command |
 611 | `!!:gs/<FROM>/<TO>/` | Replace all occurrences of `<FROM>` to `<TO>` in most recent command  |
 612 | `!$:t`               | Expand only basename from last parameter of most recent command       |
 613 | `!$:h`               | Expand only directory from last parameter of most recent command      |
 614
 615 `!!` and `!$` can be replaced with any valid expansion.
 616
 617 ### Slices
 618
 619 | Code     | Description                                                                              |
 620 | -------- | ---------------------------------------------------------------------------------------- |
 621 | `!!:n`   | Expand only `n`th token from most recent command (command is `0`; first argument is `1`) |
 622 | `!^`     | Expand first argument from most recent command                                           |
 623 | `!$`     | Expand last token from most recent command                                               |
 624 | `!!:n-m` | Expand range of tokens from most recent command                                          |
 625 | `!!:n-$` | Expand `n`th token to last from most recent command                                      |
 626
 627 `!!` can be replaced with any valid expansion i.e. `!cat`, `!-2`, `!42`, etc.
 628
 629 ## Miscellaneous
 630
 631 ### Numeric calculations
 632
 633 ```bash
 634 $((a + 200))      # Add 200 to $a
 635 ```
 636
 637 ```bash
 638 $(($RANDOM%200))  # Random number 0..199
 639 ```
 640
 641 ```bash
 642 declare -i count  # Declare as type integer
 643 count+=1          # Increment
 644 ```
 645
 646 ### Subshells
 647
 648 ```bash
 649 (cd somedir; echo "I'm now in $PWD")
 650 pwd # still in first directory
 651 ```
 652
 653 ### Redirection
 654
 655 ```bash
 656 python hello.py > output.txt            # stdout to (file)
 657 python hello.py >> output.txt           # stdout to (file), append
 658 python hello.py 2> error.log            # stderr to (file)
 659 python hello.py 2>&1                    # stderr to stdout
 660 python hello.py 2>/dev/null             # stderr to (null)
 661 python hello.py >output.txt 2>&1        # stdout and stderr to (file), equivalent to &>
 662 python hello.py &>/dev/null             # stdout and stderr to (null)
 663 echo "$0: warning: too many users" >&2  # print diagnostic message to stderr
 664 ```
 665
 666 ```bash
 667 python hello.py < foo.txt      # feed foo.txt to stdin for python
 668 diff <(ls -r) <(ls)            # Compare two stdout without files
 669 ```
 670
 671 ### Inspecting commands
 672
 673 ```bash
 674 command -V cd
 675 #=> "cd is a function/alias/whatever"
 676 ```
 677
 678 ### Trap errors
 679
 680 ```bash
 681 trap 'echo Error at about $LINENO' ERR
 682 ```
 683
 684 or
 685
 686 ```bash
 687 traperr() {
 688   echo "ERROR: ${BASH_SOURCE[1]} at about ${BASH_LINENO[0]}"
 689 }
 690
 691 set -o errtrace
 692 trap traperr ERR
 693 ```
 694
 695 ### Case/switch
 696
 697 ```bash
 698 case "$1" in
 699   start | up)
 700     vagrant up
 701     ;;
 702
 703   *)
 704     echo "Usage: $0 {start|stop|ssh}"
 705     ;;
 706 esac
 707 ```
 708
 709 ### Source relative
 710
 711 ```bash
 712 source "${0%/*}/../share/foo.sh"
 713 ```
 714
 715 ### printf
 716
 717 ```bash
 718 printf "Hello %s, I'm %s" Sven Olga
 719 #=> "Hello Sven, I'm Olga
 720
 721 printf "1 + 1 = %d" 2
 722 #=> "1 + 1 = 2"
 723
 724 printf "This is how you print a float: %f" 2
 725 #=> "This is how you print a float: 2.000000"
 726
 727 printf '%s\n' '#!/bin/bash' 'echo hello' >file
 728 # format string is applied to each group of arguments
 729 printf '%i+%i=%i\n' 1 2 3  4 5 9
 730 ```
 731
 732 ### Transform strings
 733
 734 | Command option | Description                                         |
 735 | -------------- | --------------------------------------------------- |
 736 | `-c`           | Operations apply to characters not in the given set |
 737 | `-d`           | Delete characters                                   |
 738 | `-s`           | Replaces repeated characters with single occurrence |
 739 | `-t`           | Truncates                                           |
 740 | `[:upper:]`    | All upper case letters                              |
 741 | `[:lower:]`    | All lower case letters                              |
 742 | `[:digit:]`    | All digits                                          |
 743 | `[:space:]`    | All whitespace                                      |
 744 | `[:alpha:]`    | All letters                                         |
 745 | `[:alnum:]`    | All letters and digits                              |
 746
 747 #### Example
 748
 749 ```bash
 750 echo "Welcome To Devhints" | tr '[:lower:]' '[:upper:]'
 751 WELCOME TO DEVHINTS
 752 ```
 753
 754 ### Directory of script
 755
 756 ```bash
 757 dir=${0%/*}
 758 ```
 759
 760 ### Getting options
 761
 762 ```bash
 763 while [[ "$1" =~ ^- && ! "$1" == "--" ]]; do case $1 in
 764   -V | --version )
 765     echo "$version"
 766     exit
 767     ;;
 768   -s | --string )
 769     shift; string=$1
 770     ;;
 771   -f | --flag )
 772     flag=1
 773     ;;
 774 esac; shift; done
 775 if [[ "$1" == '--' ]]; then shift; fi
 776 ```
 777
 778 ### Heredoc
 779
 780 ```sh
 781 cat <<END
 782 hello world
 783 END
 784 ```
 785
 786 ### Reading input
 787
 788 ```bash
 789 echo -n "Proceed? [y/n]: "
 790 read -r ans
 791 echo "$ans"
 792 ```
 793
 794 The `-r` option disables a peculiar legacy behavior with backslashes.
 795
 796 ```bash
 797 read -n 1 ans    # Just one character
 798 ```
 799
 800 ### Special variables
 801
 802 | Expression         | Description                            |
 803 | ------------------ | -------------------------------------- |
 804 | `$?`               | Exit status of last task               |
 805 | `$!`               | PID of last background task            |
 806 | `$$`               | PID of shell                           |
 807 | `$0`               | Filename of the shell script           |
 808 | `$_`               | Last argument of the previous command  |
 809 | `${PIPESTATUS[n]}` | return value of piped commands (array) |
 810
 811 See [Special parameters](https://web.archive.org/web/20230318164746/https://wiki.bash-hackers.org/syntax/shellvars#special_parameters_and_shell_variables).
 812
 813 ### Go to previous directory
 814
 815 ```bash
 816 pwd # /home/user/foo
 817 cd bar/
 818 pwd # /home/user/foo/bar
 819 cd -
 820 pwd # /home/user/foo
 821 ```
 822
 823 ### Check for command's result
 824
 825 ```bash
 826 if ping -c 1 google.com; then
 827   echo "It appears you have a working internet connection"
 828 fi
 829 ```
 830
 831 ### Grep check
 832
 833 ```bash
 834 if grep -q 'foo' ~/.bash_history; then
 835   echo "You appear to have typed 'foo' in the past"
 836 fi
 837 ```
 838
 839 ## Also see
 840
 841 {: .-one-column}
 842
 843 - [Bash-hackers wiki](https://web.archive.org/web/20230406205817/https://wiki.bash-hackers.org/) _(bash-hackers.org)_
 844 - [Shell vars](https://web.archive.org/web/20230318164746/https://wiki.bash-hackers.org/syntax/shellvars) _(bash-hackers.org)_
 845 - [Learn bash in y minutes](https://learnxinyminutes.com/docs/bash/) _(learnxinyminutes.com)_
 846 - [Bash Guide](http://mywiki.wooledge.org/BashGuide) _(mywiki.wooledge.org)_
 847 - [ShellCheck](https://www.shellcheck.net/) _(shellcheck.net)_