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