bash.md

   1 ---
   2 title: Bash scripting
   3 category: CLI
   4 layout: 2017/sheet
   5 tags: [Featured]
   6 updated: 2019-03-23
   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 ### Example
  22
  23 ```bash
  24 #!/usr/bin/env bash
  25
  26 NAME="John"
  27 echo "Hello $NAME!"
  28 ```
  29
  30 ### Variables
  31
  32 ```bash
  33 NAME="John"
  34 echo $NAME
  35 echo "$NAME"
  36 echo "${NAME}!"
  37 ```
  38
  39 ### String quotes
  40
  41 ```bash
  42 NAME="John"
  43 echo "Hi $NAME"  #=> Hi John
  44 echo 'Hi $NAME'  #=> Hi $NAME
  45 ```
  46
  47 ### Shell execution
  48
  49 ```bash
  50 echo "I'm in $(pwd)"
  51 echo "I'm in `pwd`"
  52 # Same
  53 ```
  54
  55 See [Command substitution](http://wiki.bash-hackers.org/syntax/expansion/cmdsubst)
  56
  57 ### Conditional execution
  58
  59 ```bash
  60 git commit && git push
  61 git commit || echo "Commit failed"
  62 ```
  63
  64 ### Functions
  65 {: id='functions-example'}
  66
  67 ```bash
  68 get_name() {
  69   echo "John"
  70 }
  71
  72 echo "You are $(get_name)"
  73 ```
  74
  75 See: [Functions](#functions)
  76
  77 ### Conditionals
  78 {: id='conditionals-example'}
  79
  80 ```bash
  81 if [[ -z "$string" ]]; then
  82   echo "String is empty"
  83 elif [[ -n "$string" ]]; then
  84   echo "String is not empty"
  85 fi
  86 ```
  87
  88 See: [Conditionals](#conditionals)
  89
  90 ### Strict mode
  91
  92 ```bash
  93 set -euo pipefail
  94 IFS=$'\n\t'
  95 ```
  96
  97 See: [Unofficial bash strict mode](http://redsymbol.net/articles/unofficial-bash-strict-mode/)
  98
  99 ### Brace expansion
 100
 101 ```bash
 102 echo {A,B}.js
 103 ```
 104
 105 | `{A,B}` | Same as `A B` |
 106 | `{A,B}.js` | Same as `A.js B.js` |
 107 | `{1..5}` | Same as `1 2 3 4 5` |
 108
 109 See: [Brace expansion](http://wiki.bash-hackers.org/syntax/expansion/brace)
 110
 111
 112 Parameter expansions
 113 --------------------
 114 {: .-three-column}
 115
 116 ### Basics
 117
 118 ```bash
 119 name="John"
 120 echo ${name}
 121 echo ${name/J/j}    #=> "john" (substitution)
 122 echo ${name:0:2}    #=> "Jo" (slicing)
 123 echo ${name::2}     #=> "Jo" (slicing)
 124 echo ${name::-1}    #=> "Joh" (slicing)
 125 echo ${name:(-1)}   #=> "n" (slicing from right)
 126 echo ${name:(-2):1} #=> "h" (slicing from right)
 127 echo ${food:-Cake}  #=> $food or "Cake"
 128 ```
 129
 130 ```bash
 131 length=2
 132 echo ${name:0:length}  #=> "Jo"
 133 ```
 134
 135 See: [Parameter expansion](http://wiki.bash-hackers.org/syntax/pe)
 136
 137 ```bash
 138 STR="/path/to/foo.cpp"
 139 echo ${STR%.cpp}    # /path/to/foo
 140 echo ${STR%.cpp}.o  # /path/to/foo.o
 141
 142 echo ${STR##*.}     # cpp (extension)
 143 echo ${STR##*/}     # foo.cpp (basepath)
 144
 145 echo ${STR#*/}      # path/to/foo.cpp
 146 echo ${STR##*/}     # foo.cpp
 147
 148 echo ${STR/foo/bar} # /path/to/bar.cpp
 149 ```
 150
 151 ```bash
 152 STR="Hello world"
 153 echo ${STR:6:5}   # "world"
 154 echo ${STR:-5:5}  # "world"
 155 ```
 156
 157 ```bash
 158 SRC="/path/to/foo.cpp"
 159 BASE=${SRC##*/}   #=> "foo.cpp" (basepath)
 160 DIR=${SRC%$BASE}  #=> "/path/to/" (dirpath)
 161 ```
 162
 163 ### Substitution
 164
 165 | Code | Description |
 166 | --- | --- |
 167 | `${FOO%suffix}` | Remove suffix |
 168 | `${FOO#prefix}` | Remove prefix |
 169 | --- | --- |
 170 | `${FOO%%suffix}` | Remove long suffix |
 171 | `${FOO##prefix}` | Remove long prefix |
 172 | --- | --- |
 173 | `${FOO/from/to}` | Replace first match |
 174 | `${FOO//from/to}` | Replace all |
 175 | --- | --- |
 176 | `${FOO/%from/to}` | Replace suffix |
 177 | `${FOO/#from/to}` | Replace prefix |
 178
 179 ### Comments
 180
 181 ```bash
 182 # Single line comment
 183 ```
 184
 185 ```bash
 186 : '
 187 This is a
 188 multi line
 189 comment
 190 '
 191 ```
 192
 193 ### Substrings
 194
 195 | `${FOO:0:3}`  | Substring _(position, length)_ |
 196 | `${FOO:-3:3}` | Substring from the right |
 197
 198 ### Length
 199
 200 | `${#FOO}` | Length of `$FOO` |
 201
 202 ### Default values
 203
 204 | `${FOO:-val}`        | `$FOO`, or `val` if not set |
 205 | `${FOO:=val}`        | Set `$FOO` to `val` if not set |
 206 | `${FOO:+val}`        | `val` if `$FOO` is set |
 207 | `${FOO:?message}`    | Show error message and exit if `$FOO` is not set |
 208
 209 The `:` is optional (eg, `${FOO=word}` works)
 210
 211 Loops
 212 -----
 213 {: .-three-column}
 214
 215 ### Basic for loop
 216
 217 ```bash
 218 for i in /etc/rc.*; do
 219   echo $i
 220 done
 221 ```
 222
 223 ### C-like for loop
 224
 225 ```bash
 226 for ((i = 0 ; i < 100 ; i++)); do
 227   echo $i
 228 done
 229 ```
 230
 231 ### Ranges
 232
 233 ```bash
 234 for i in {1..5}; do
 235     echo "Welcome $i"
 236 done
 237 ```
 238
 239 #### With step size
 240
 241 ```bash
 242 for i in {5..50..5}; do
 243     echo "Welcome $i"
 244 done
 245 ```
 246
 247 ### Reading lines
 248
 249 ```bash
 250 < file.txt | while read line; do
 251   echo $line
 252 done
 253 ```
 254
 255 ### Forever
 256
 257 ```bash
 258 while true; do
 259   ···
 260 done
 261 ```
 262
 263 Functions
 264 ---------
 265 {: .-three-column}
 266
 267 ### Defining functions
 268
 269 ```bash
 270 myfunc() {
 271     echo "hello $1"
 272 }
 273 ```
 274
 275 ```bash
 276 # Same as above (alternate syntax)
 277 function myfunc() {
 278     echo "hello $1"
 279 }
 280 ```
 281
 282 ```bash
 283 myfunc "John"
 284 ```
 285
 286 ### Returning values
 287
 288 ```bash
 289 myfunc() {
 290     local myresult='some value'
 291     echo $myresult
 292 }
 293 ```
 294
 295 ```bash
 296 result="$(myfunc)"
 297 ```
 298
 299 ### Raising errors
 300
 301 ```bash
 302 myfunc() {
 303   return 1
 304 }
 305 ```
 306
 307 ```bash
 308 if myfunc; then
 309   echo "success"
 310 else
 311   echo "failure"
 312 fi
 313 ```
 314
 315 ### Arguments
 316
 317 | Expression | Description                        |
 318 | ---        | ---                                |
 319 | `$#`       | Number of arguments                |
 320 | `$*`       | All arguments                      |
 321 | `$@`       | All arguments, starting from first |
 322 | `$1`       | First argument                     |
 323
 324 See [Special parameters](http://wiki.bash-hackers.org/syntax/shellvars#special_parameters_and_shell_variables).
 325
 326 Conditionals
 327 ------------
 328 {: .-three-column}
 329
 330 ### Conditions
 331
 332 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.
 333
 334 | Condition                | Description           |
 335 | ---                      | ---                   |
 336 | `[[ -z STRING ]]`        | Empty string          |
 337 | `[[ -n STRING ]]`        | Not empty string      |
 338 | `[[ STRING == STRING ]]` | Equal                 |
 339 | `[[ STRING != STRING ]]` | Not Equal             |
 340 | ---                      | ---                   |
 341 | `[[ NUM -eq NUM ]]`      | Equal                 |
 342 | `[[ NUM -ne NUM ]]`      | Not equal             |
 343 | `[[ NUM -lt NUM ]]`      | Less than             |
 344 | `[[ NUM -le NUM ]]`      | Less than or equal    |
 345 | `[[ NUM -gt NUM ]]`      | Greater than          |
 346 | `[[ NUM -ge NUM ]]`      | Greater than or equal |
 347 | ---                      | ---                   |
 348 | `[[ STRING =~ STRING ]]` | Regexp                |
 349 | ---                      | ---                   |
 350 | `(( NUM < NUM ))`        | Numeric conditions    |
 351
 352 | Condition              | Description              |
 353 | ---                    | ---                      |
 354 | `[[ -o noclobber ]]`   | If OPTIONNAME is enabled |
 355 | ---                    | ---                      |
 356 | `[[ ! EXPR ]]`         | Not                      |
 357 | `[[ X ]] && [[ Y ]]`   | And                      |
 358 | `[[ X ]] || [[ Y ]]`   | Or                       |
 359
 360 ### File conditions
 361
 362 | Condition               | Description             |
 363 | ---                     | ---                     |
 364 | `[[ -e FILE ]]`         | Exists                  |
 365 | `[[ -r FILE ]]`         | Readable                |
 366 | `[[ -h FILE ]]`         | Symlink                 |
 367 | `[[ -d FILE ]]`         | Directory               |
 368 | `[[ -w FILE ]]`         | Writable                |
 369 | `[[ -s FILE ]]`         | Size is > 0 bytes       |
 370 | `[[ -f FILE ]]`         | File                    |
 371 | `[[ -x FILE ]]`         | Executable              |
 372 | ---                     | ---                     |
 373 | `[[ FILE1 -nt FILE2 ]]` | 1 is more recent than 2 |
 374 | `[[ FILE1 -ot FILE2 ]]` | 2 is more recent than 1 |
 375 | `[[ FILE1 -ef FILE2 ]]` | Same files              |
 376
 377 ### Example
 378
 379 ```bash
 380 if ping -c 1 google.com; then
 381   echo "It appears you have a working internet connection"
 382 fi
 383 ````
 384
 385 ```bash
 386 if grep -q 'foo' ~/.bash_history; then
 387   echo "You appear to have typed 'foo' in the past"
 388 fi
 389 ```
 390
 391 ```bash
 392 # String
 393 if [[ -z "$string" ]]; then
 394   echo "String is empty"
 395 elif [[ -n "$string" ]]; then
 396   echo "String is not empty"
 397 fi
 398 ```
 399
 400 ```bash
 401 # Combinations
 402 if [[ X ]] && [[ Y ]]; then
 403   ...
 404 fi
 405 ```
 406
 407 ```bash
 408 # Equal
 409 if [[ "$A" == "$B" ]]
 410 ```
 411
 412 ```bash
 413 # Regex
 414 if [[ "A" =~ "." ]]
 415 ```
 416
 417 ```bash
 418 if (( $a < $b )); then
 419    echo "$a is smaller than $b"
 420 fi
 421 ```
 422
 423 ```bash
 424 if [[ -e "file.txt" ]]; then
 425   echo "file exists"
 426 fi
 427 ```
 428
 429 Arrays
 430 ------
 431
 432 ### Defining arrays
 433
 434 ```bash
 435 Fruits=('Apple' 'Banana' 'Orange')
 436 ```
 437
 438 ```bash
 439 Fruits[0]="Apple"
 440 Fruits[1]="Banana"
 441 Fruits[2]="Orange"
 442 ```
 443
 444 ### Working with arrays
 445
 446 ```bash
 447 echo ${Fruits[0]}           # Element #0
 448 echo ${Fruits[@]}           # All elements, space-separated
 449 echo ${#Fruits[@]}          # Number of elements
 450 echo ${#Fruits}             # String length of the 1st element
 451 echo ${#Fruits[3]}          # String length of the Nth element
 452 echo ${Fruits[@]:3:2}       # Range (from position 3, length 2)
 453 ```
 454
 455 ### Operations
 456
 457 ```bash
 458 Fruits=("${Fruits[@]}" "Watermelon")    # Push
 459 Fruits+=('Watermelon')                  # Also Push
 460 Fruits=( ${Fruits[@]/Ap*/} )            # Remove by regex match
 461 unset Fruits[2]                         # Remove one item
 462 Fruits=("${Fruits[@]}")                 # Duplicate
 463 Fruits=("${Fruits[@]}" "${Veggies[@]}") # Concatenate
 464 lines=(`cat "logfile"`)                 # Read from file
 465 ```
 466
 467 ### Iteration
 468
 469 ```bash
 470 for i in "${arrayName[@]}"; do
 471   echo $i
 472 done
 473 ```
 474
 475 Dictionaries
 476 ------------
 477 {: .-three-column}
 478
 479 ### Defining
 480
 481 ```bash
 482 declare -A sounds
 483 ```
 484
 485 ```bash
 486 sounds[dog]="bark"
 487 sounds[cow]="moo"
 488 sounds[bird]="tweet"
 489 sounds[wolf]="howl"
 490 ```
 491
 492 Declares `sound` as a Dictionary object (aka associative array).
 493
 494 ### Working with dictionaries
 495
 496 ```bash
 497 echo ${sounds[dog]} # Dog's sound
 498 echo ${sounds[@]}   # All values
 499 echo ${!sounds[@]}  # All keys
 500 echo ${#sounds[@]}  # Number of elements
 501 unset sounds[dog]   # Delete dog
 502 ```
 503
 504 ### Iteration
 505
 506 #### Iterate over values
 507
 508 ```bash
 509 for val in "${sounds[@]}"; do
 510   echo $val
 511 done
 512 ```
 513
 514 #### Iterate over keys
 515
 516 ```bash
 517 for key in "${!sounds[@]}"; do
 518   echo $key
 519 done
 520 ```
 521
 522 Options
 523 -------
 524
 525 ### Options
 526
 527 ```bash
 528 set -o noclobber  # Avoid overlay files (echo "hi" > foo)
 529 set -o errexit    # Used to exit upon error, avoiding cascading errors
 530 set -o pipefail   # Unveils hidden failures
 531 set -o nounset    # Exposes unset variables
 532 ```
 533
 534 ### Glob options
 535
 536 ```bash
 537 set -o nullglob    # Non-matching globs are removed  ('*.foo' => '')
 538 set -o failglob    # Non-matching globs throw errors
 539 set -o nocaseglob  # Case insensitive globs
 540 set -o globdots    # Wildcards match dotfiles ("*.sh" => ".foo.sh")
 541 set -o globstar    # Allow ** for recursive matches ('lib/**/*.rb' => 'lib/a/b/c.rb')
 542 ```
 543
 544 Set `GLOBIGNORE` as a colon-separated list of patterns to be removed from glob
 545 matches.
 546
 547 History
 548 -------
 549
 550 ### Commands
 551
 552 | `history` | Show history |
 553 | `shopt -s histverify` | Don't execute expanded result immediately |
 554
 555 ### Expansions
 556
 557 | `!$` | Expand last parameter of most recent command |
 558 | `!*` | Expand all parameters of most recent command |
 559 | `!-n` | Expand `n`th most recent command |
 560 | `!n` | Expand `n`th command in history |
 561 | `!<command>` | Expand most recent invocation of command `<command>` |
 562
 563 ### Operations
 564
 565 | `!!` | Execute last command again |
 566 | `!!:s/<FROM>/<TO>/` | Replace first occurrence of `<FROM>` to `<TO>` in most recent command |
 567 | `!!:gs/<FROM>/<TO>/` | Replace all occurrences of `<FROM>` to `<TO>` in most recent command |
 568 | `!$:t` | Expand only basename from last parameter of most recent command |
 569 | `!$:h` | Expand only directory from last parameter of most recent command |
 570
 571 `!!` and `!$` can be replaced with any valid expansion.
 572
 573 ### Slices
 574
 575 | `!!:n` | Expand only `n`th token from most recent command (command is `0`; first argument is `1`) |
 576 | `!^` | Expand first argument from most recent command |
 577 | `!$` | Expand last token from most recent command |
 578 | `!!:n-m` | Expand range of tokens from most recent command |
 579 | `!!:n-$` | Expand `n`th token to last from most recent command |
 580
 581 `!!` can be replaced with any valid expansion i.e. `!cat`, `!-2`, `!42`, etc.
 582
 583
 584 Miscellaneous
 585 -------------
 586
 587 ### Numeric calculations
 588
 589 ```bash
 590 $((a + 200))      # Add 200 to $a
 591 ```
 592
 593 ```bash
 594 $((RANDOM%=200))  # Random number 0..200
 595 ```
 596
 597 ### Subshells
 598
 599 ```bash
 600 (cd somedir; echo "I'm now in $PWD")
 601 pwd # still in first directory
 602 ```
 603
 604 ### Redirection
 605
 606 ```bash
 607 python hello.py > output.txt   # stdout to (file)
 608 python hello.py >> output.txt  # stdout to (file), append
 609 python hello.py 2> error.log   # stderr to (file)
 610 python hello.py 2>&1           # stderr to stdout
 611 python hello.py 2>/dev/null    # stderr to (null)
 612 python hello.py &>/dev/null    # stdout and stderr to (null)
 613 ```
 614
 615 ```bash
 616 python hello.py < foo.txt      # feed foo.txt to stdin for python
 617 ```
 618
 619 ### Inspecting commands
 620
 621 ```bash
 622 command -V cd
 623 #=> "cd is a function/alias/whatever"
 624 ```
 625
 626 ### Trap errors
 627
 628 ```bash
 629 trap 'echo Error at about $LINENO' ERR
 630 ```
 631
 632 or
 633
 634 ```bash
 635 traperr() {
 636   echo "ERROR: ${BASH_SOURCE[1]} at about ${BASH_LINENO[0]}"
 637 }
 638
 639 set -o errtrace
 640 trap traperr ERR
 641 ```
 642
 643 ### Case/switch
 644
 645 ```bash
 646 case "$1" in
 647   start | up)
 648     vagrant up
 649     ;;
 650
 651   *)
 652     echo "Usage: $0 {start|stop|ssh}"
 653     ;;
 654 esac
 655 ```
 656
 657 ### Source relative
 658
 659 ```bash
 660 source "${0%/*}/../share/foo.sh"
 661 ```
 662
 663 ### printf
 664
 665 ```bash
 666 printf "Hello %s, I'm %s" Sven Olga
 667 #=> "Hello Sven, I'm Olga
 668 ```
 669
 670 ### Directory of script
 671
 672 ```bash
 673 DIR="${0%/*}"
 674 ```
 675
 676 ### Getting options
 677
 678 ```bash
 679 while [[ "$1" =~ ^- && ! "$1" == "--" ]]; do case $1 in
 680   -V | --version )
 681     echo $version
 682     exit
 683     ;;
 684   -s | --string )
 685     shift; string=$1
 686     ;;
 687   -f | --flag )
 688     flag=1
 689     ;;
 690 esac; shift; done
 691 if [[ "$1" == '--' ]]; then shift; fi
 692 ```
 693
 694 ### Heredoc
 695
 696 ```sh
 697 cat <<END
 698 hello world
 699 END
 700 ```
 701
 702 ### Reading input
 703
 704 ```bash
 705 echo -n "Proceed? [y/n]: "
 706 read ans
 707 echo $ans
 708 ```
 709
 710 ```bash
 711 read -n 1 ans    # Just one character
 712 ```
 713
 714 ### Special variables
 715
 716 | `$?` | Exit status of last task |
 717 | `$!` | PID of last background task |
 718 | `$$` | PID of shell |
 719
 720 See [Special parameters](http://wiki.bash-hackers.org/syntax/shellvars#special_parameters_and_shell_variables).
 721
 722 ### Go to previous directory
 723
 724 ```bash
 725 pwd # /home/user/foo
 726 cd bar/
 727 pwd # /home/user/foo/bar
 728 cd -
 729 pwd # /home/user/foo
 730 ```
 731
 732 ## Also see
 733 {: .-one-column}
 734
 735 * [Bash-hackers wiki](http://wiki.bash-hackers.org/) _(bash-hackers.org)_
 736 * [Shell vars](http://wiki.bash-hackers.org/syntax/shellvars) _(bash-hackers.org)_
 737 * [Learn bash in y minutes](https://learnxinyminutes.com/docs/bash/) _(learnxinyminutes.com)_
 738 * [Bash Guide](http://mywiki.wooledge.org/BashGuide) _(mywiki.wooledge.org)_
 739 * [ShellCheck](https://www.shellcheck.net/) _(shellcheck.net)_