bash.md

   1 ---
   2 title: Bash scripting
   3 category: CLI
   4 layout: 2017/sheet
   5 tags: [Featured]
   6 updated: 2019-10-02
   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 echo ${STR%/*}      # /path/to
 142
 143 echo ${STR##*.}     # cpp (extension)
 144 echo ${STR##*/}     # foo.cpp (basepath)
 145
 146 echo ${STR#*/}      # path/to/foo.cpp
 147 echo ${STR##*/}     # foo.cpp
 148
 149 echo ${STR/foo/bar} # /path/to/bar.cpp
 150 ```
 151
 152 ```bash
 153 STR="Hello world"
 154 echo ${STR:6:5}   # "world"
 155 echo ${STR:-5:5}  # "world"
 156 ```
 157
 158 ```bash
 159 SRC="/path/to/foo.cpp"
 160 BASE=${SRC##*/}   #=> "foo.cpp" (basepath)
 161 DIR=${SRC%$BASE}  #=> "/path/to/" (dirpath)
 162 ```
 163
 164 ### Substitution
 165
 166 | Code | Description |
 167 | --- | --- |
 168 | `${FOO%suffix}` | Remove suffix |
 169 | `${FOO#prefix}` | Remove prefix |
 170 | --- | --- |
 171 | `${FOO%%suffix}` | Remove long suffix |
 172 | `${FOO##prefix}` | Remove long prefix |
 173 | --- | --- |
 174 | `${FOO/from/to}` | Replace first match |
 175 | `${FOO//from/to}` | Replace all |
 176 | --- | --- |
 177 | `${FOO/%from/to}` | Replace suffix |
 178 | `${FOO/#from/to}` | Replace prefix |
 179
 180 ### Comments
 181
 182 ```bash
 183 # Single line comment
 184 ```
 185
 186 ```bash
 187 : '
 188 This is a
 189 multi line
 190 comment
 191 '
 192 ```
 193
 194 ### Substrings
 195
 196 | Expression      | Description                    |
 197 | --------------- | ------------------------------ |
 198 | `${FOO:0:3}`    | Substring _(position, length)_ |
 199 | `${FOO:(-3):3}` | Substring from the right       |
 200
 201 ### Length
 202
 203 | Expression | Description      |
 204 | ---------- | ---------------- |
 205 | `${#FOO}`  | Length of `$FOO` |
 206
 207 ### Manipulation
 208
 209 ```bash
 210 STR="HELLO WORLD!"
 211 echo ${STR,}   #=> "hELLO WORLD!" (lowercase 1st letter)
 212 echo ${STR,,}  #=> "hello world!" (all lowercase)
 213
 214 STR="hello world!"
 215 echo ${STR^}   #=> "Hello world!" (uppercase 1st letter)
 216 echo ${STR^^}  #=> "HELLO WORLD!" (all uppercase)
 217 ```
 218
 219 ### Default values
 220
 221 | Expression        | Description                                              |
 222 | ----------------- | -------------------------------------------------------- |
 223 | `${FOO:-val}`     | `$FOO`, or `val` if unset (or null)                      |
 224 | `${FOO:=val}`     | Set `$FOO` to `val` if unset (or null)                   |
 225 | `${FOO:+val}`     | `val` if `$FOO` is set (and not null)                    |
 226 | `${FOO:?message}` | Show error message and exit if `$FOO` is unset (or null) |
 227
 228 Omitting the `:` removes the (non)nullity checks, e.g. `${FOO-val}` expands to `val` if unset otherwise `$FOO`.
 229
 230 Loops
 231 -----
 232 {: .-three-column}
 233
 234 ### Basic for loop
 235
 236 ```bash
 237 for i in /etc/rc.*; do
 238   echo $i
 239 done
 240 ```
 241
 242 ### C-like for loop
 243
 244 ```bash
 245 for ((i = 0 ; i < 100 ; i++)); do
 246   echo $i
 247 done
 248 ```
 249
 250 ### Ranges
 251
 252 ```bash
 253 for i in {1..5}; do
 254     echo "Welcome $i"
 255 done
 256 ```
 257
 258 #### With step size
 259
 260 ```bash
 261 for i in {5..50..5}; do
 262     echo "Welcome $i"
 263 done
 264 ```
 265
 266 ### Reading lines
 267
 268 ```bash
 269 cat file.txt | while read line; do
 270   echo $line
 271 done
 272 ```
 273
 274 ### Forever
 275
 276 ```bash
 277 while true; do
 278   ···
 279 done
 280 ```
 281
 282 Functions
 283 ---------
 284 {: .-three-column}
 285
 286 ### Defining functions
 287
 288 ```bash
 289 myfunc() {
 290     echo "hello $1"
 291 }
 292 ```
 293
 294 ```bash
 295 # Same as above (alternate syntax)
 296 function myfunc() {
 297     echo "hello $1"
 298 }
 299 ```
 300
 301 ```bash
 302 myfunc "John"
 303 ```
 304
 305 ### Returning values
 306
 307 ```bash
 308 myfunc() {
 309     local myresult='some value'
 310     echo $myresult
 311 }
 312 ```
 313
 314 ```bash
 315 result="$(myfunc)"
 316 ```
 317
 318 ### Raising errors
 319
 320 ```bash
 321 myfunc() {
 322   return 1
 323 }
 324 ```
 325
 326 ```bash
 327 if myfunc; then
 328   echo "success"
 329 else
 330   echo "failure"
 331 fi
 332 ```
 333
 334 ### Arguments
 335
 336 | Expression | Description                            |
 337 | ---        | ---                                    |
 338 | `$#`       | Number of arguments                    |
 339 | `$*`       | All arguments                          |
 340 | `$@`       | All arguments, starting from first     |
 341 | `$1`       | First argument                         |
 342 | `$_`       | Last argument of the previous command  |
 343
 344 See [Special parameters](http://wiki.bash-hackers.org/syntax/shellvars#special_parameters_and_shell_variables).
 345
 346 Conditionals
 347 ------------
 348 {: .-three-column}
 349
 350 ### Conditions
 351
 352 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.
 353
 354 | Condition                | Description           |
 355 | ---                      | ---                   |
 356 | `[[ -z STRING ]]`        | Empty string          |
 357 | `[[ -n STRING ]]`        | Not empty string      |
 358 | `[[ STRING == STRING ]]` | Equal                 |
 359 | `[[ STRING != STRING ]]` | Not Equal             |
 360 | ---                      | ---                   |
 361 | `[[ NUM -eq NUM ]]`      | Equal                 |
 362 | `[[ NUM -ne NUM ]]`      | Not equal             |
 363 | `[[ NUM -lt NUM ]]`      | Less than             |
 364 | `[[ NUM -le NUM ]]`      | Less than or equal    |
 365 | `[[ NUM -gt NUM ]]`      | Greater than          |
 366 | `[[ NUM -ge NUM ]]`      | Greater than or equal |
 367 | ---                      | ---                   |
 368 | `[[ STRING =~ STRING ]]` | Regexp                |
 369 | ---                      | ---                   |
 370 | `(( NUM < NUM ))`        | Numeric conditions    |
 371
 372 #### More conditions
 373
 374 | Condition            | Description              |
 375 | -------------------- | ------------------------ |
 376 | `[[ -o noclobber ]]` | If OPTIONNAME is enabled |
 377 | ---                  | ---                      |
 378 | `[[ ! EXPR ]]`       | Not                      |
 379 | `[[ X && Y ]]`       | And                      |
 380 | `[[ X || Y ]]`       | Or                       |
 381
 382 ### File conditions
 383
 384 | Condition               | Description             |
 385 | ---                     | ---                     |
 386 | `[[ -e FILE ]]`         | Exists                  |
 387 | `[[ -r FILE ]]`         | Readable                |
 388 | `[[ -h FILE ]]`         | Symlink                 |
 389 | `[[ -d FILE ]]`         | Directory               |
 390 | `[[ -w FILE ]]`         | Writable                |
 391 | `[[ -s FILE ]]`         | Size is > 0 bytes       |
 392 | `[[ -f FILE ]]`         | File                    |
 393 | `[[ -x FILE ]]`         | Executable              |
 394 | ---                     | ---                     |
 395 | `[[ FILE1 -nt FILE2 ]]` | 1 is more recent than 2 |
 396 | `[[ FILE1 -ot FILE2 ]]` | 2 is more recent than 1 |
 397 | `[[ FILE1 -ef FILE2 ]]` | Same files              |
 398
 399 ### Example
 400
 401 ```bash
 402 # String
 403 if [[ -z "$string" ]]; then
 404   echo "String is empty"
 405 elif [[ -n "$string" ]]; then
 406   echo "String is not empty"
 407 else
 408   echo "This never happens"
 409 fi
 410 ```
 411
 412 ```bash
 413 # Combinations
 414 if [[ X && Y ]]; then
 415   ...
 416 fi
 417 ```
 418
 419 ```bash
 420 # Equal
 421 if [[ "$A" == "$B" ]]
 422 ```
 423
 424 ```bash
 425 # Regex
 426 if [[ "A" =~ . ]]
 427 ```
 428
 429 ```bash
 430 if (( $a < $b )); then
 431    echo "$a is smaller than $b"
 432 fi
 433 ```
 434
 435 ```bash
 436 if [[ -e "file.txt" ]]; then
 437   echo "file exists"
 438 fi
 439 ```
 440
 441 Arrays
 442 ------
 443
 444 ### Defining arrays
 445
 446 ```bash
 447 Fruits=('Apple' 'Banana' 'Orange')
 448 ```
 449
 450 ```bash
 451 Fruits[0]="Apple"
 452 Fruits[1]="Banana"
 453 Fruits[2]="Orange"
 454 ```
 455
 456 ### Working with arrays
 457
 458 ```bash
 459 echo ${Fruits[0]}           # Element #0
 460 echo ${Fruits[-1]}          # Last element
 461 echo ${Fruits[@]}           # All elements, space-separated
 462 echo ${#Fruits[@]}          # Number of elements
 463 echo ${#Fruits}             # String length of the 1st element
 464 echo ${#Fruits[3]}          # String length of the Nth element
 465 echo ${Fruits[@]:3:2}       # Range (from position 3, length 2)
 466 echo ${!Fruits[@]}          # Keys of all elements, space-separated
 467 ```
 468
 469 ### Operations
 470
 471 ```bash
 472 Fruits=("${Fruits[@]}" "Watermelon")    # Push
 473 Fruits+=('Watermelon')                  # Also Push
 474 Fruits=( ${Fruits[@]/Ap*/} )            # Remove by regex match
 475 unset Fruits[2]                         # Remove one item
 476 Fruits=("${Fruits[@]}")                 # Duplicate
 477 Fruits=("${Fruits[@]}" "${Veggies[@]}") # Concatenate
 478 lines=(`cat "logfile"`)                 # Read from file
 479 ```
 480
 481 ### Iteration
 482
 483 ```bash
 484 for i in "${arrayName[@]}"; do
 485   echo $i
 486 done
 487 ```
 488
 489 Dictionaries
 490 ------------
 491 {: .-three-column}
 492
 493 ### Defining
 494
 495 ```bash
 496 declare -A sounds
 497 ```
 498
 499 ```bash
 500 sounds[dog]="bark"
 501 sounds[cow]="moo"
 502 sounds[bird]="tweet"
 503 sounds[wolf]="howl"
 504 ```
 505
 506 Declares `sound` as a Dictionary object (aka associative array).
 507
 508 ### Working with dictionaries
 509
 510 ```bash
 511 echo ${sounds[dog]} # Dog's sound
 512 echo ${sounds[@]}   # All values
 513 echo ${!sounds[@]}  # All keys
 514 echo ${#sounds[@]}  # Number of elements
 515 unset sounds[dog]   # Delete dog
 516 ```
 517
 518 ### Iteration
 519
 520 #### Iterate over values
 521
 522 ```bash
 523 for val in "${sounds[@]}"; do
 524   echo $val
 525 done
 526 ```
 527
 528 #### Iterate over keys
 529
 530 ```bash
 531 for key in "${!sounds[@]}"; do
 532   echo $key
 533 done
 534 ```
 535
 536 Options
 537 -------
 538
 539 ### Options
 540
 541 ```bash
 542 set -o noclobber  # Avoid overlay files (echo "hi" > foo)
 543 set -o errexit    # Used to exit upon error, avoiding cascading errors
 544 set -o pipefail   # Unveils hidden failures
 545 set -o nounset    # Exposes unset variables
 546 ```
 547
 548 ### Glob options
 549
 550 ```bash
 551 shopt -s nullglob    # Non-matching globs are removed  ('*.foo' => '')
 552 shopt -s failglob    # Non-matching globs throw errors
 553 shopt -s nocaseglob  # Case insensitive globs
 554 shopt -s dotglob     # Wildcards match dotfiles ("*.sh" => ".foo.sh")
 555 shopt -s globstar    # Allow ** for recursive matches ('lib/**/*.rb' => 'lib/a/b/c.rb')
 556 ```
 557
 558 Set `GLOBIGNORE` as a colon-separated list of patterns to be removed from glob
 559 matches.
 560
 561 History
 562 -------
 563
 564 ### Commands
 565
 566 | `history` | Show history |
 567 | `shopt -s histverify` | Don't execute expanded result immediately |
 568
 569 ### Expansions
 570
 571 | `!$` | Expand last parameter of most recent command |
 572 | `!*` | Expand all parameters of most recent command |
 573 | `!-n` | Expand `n`th most recent command |
 574 | `!n` | Expand `n`th command in history |
 575 | `!<command>` | Expand most recent invocation of command `<command>` |
 576
 577 ### Operations
 578
 579 | Code                 | Description                                                           |
 580 | -------------------- | --------------------------------------------------------------------- |
 581 | `!!`                 | Execute last command again                                            |
 582 | `!!:s/<FROM>/<TO>/`  | Replace first occurrence of `<FROM>` to `<TO>` in most recent command |
 583 | `!!:gs/<FROM>/<TO>/` | Replace all occurrences of `<FROM>` to `<TO>` in most recent command  |
 584 | `!$:t`               | Expand only basename from last parameter of most recent command       |
 585 | `!$:h`               | Expand only directory from last parameter of most recent command      |
 586
 587 `!!` and `!$` can be replaced with any valid expansion.
 588
 589 ### Slices
 590
 591 | Code     | Description                                                                              |
 592 | -------- | ---------------------------------------------------------------------------------------- |
 593 | `!!:n`   | Expand only `n`th token from most recent command (command is `0`; first argument is `1`) |
 594 | `!^`     | Expand first argument from most recent command                                           |
 595 | `!$`     | Expand last token from most recent command                                               |
 596 | `!!:n-m` | Expand range of tokens from most recent command                                          |
 597 | `!!:n-$` | Expand `n`th token to last from most recent command                                      |
 598
 599 `!!` can be replaced with any valid expansion i.e. `!cat`, `!-2`, `!42`, etc.
 600
 601
 602 Miscellaneous
 603 -------------
 604
 605 ### Numeric calculations
 606
 607 ```bash
 608 $((a + 200))      # Add 200 to $a
 609 ```
 610
 611 ```bash
 612 $((RANDOM%=200))  # Random number 0..200
 613 ```
 614
 615 ### Subshells
 616
 617 ```bash
 618 (cd somedir; echo "I'm now in $PWD")
 619 pwd # still in first directory
 620 ```
 621
 622 ### Redirection
 623
 624 ```bash
 625 python hello.py > output.txt   # stdout to (file)
 626 python hello.py >> output.txt  # stdout to (file), append
 627 python hello.py 2> error.log   # stderr to (file)
 628 python hello.py 2>&1           # stderr to stdout
 629 python hello.py 2>/dev/null    # stderr to (null)
 630 python hello.py &>/dev/null    # stdout and stderr to (null)
 631 ```
 632
 633 ```bash
 634 python hello.py < foo.txt      # feed foo.txt to stdin for python
 635 ```
 636
 637 ### Inspecting commands
 638
 639 ```bash
 640 command -V cd
 641 #=> "cd is a function/alias/whatever"
 642 ```
 643
 644 ### Trap errors
 645
 646 ```bash
 647 trap 'echo Error at about $LINENO' ERR
 648 ```
 649
 650 or
 651
 652 ```bash
 653 traperr() {
 654   echo "ERROR: ${BASH_SOURCE[1]} at about ${BASH_LINENO[0]}"
 655 }
 656
 657 set -o errtrace
 658 trap traperr ERR
 659 ```
 660
 661 ### Case/switch
 662
 663 ```bash
 664 case "$1" in
 665   start | up)
 666     vagrant up
 667     ;;
 668
 669   *)
 670     echo "Usage: $0 {start|stop|ssh}"
 671     ;;
 672 esac
 673 ```
 674
 675 ### Source relative
 676
 677 ```bash
 678 source "${0%/*}/../share/foo.sh"
 679 ```
 680
 681 ### printf
 682
 683 ```bash
 684 printf "Hello %s, I'm %s" Sven Olga
 685 #=> "Hello Sven, I'm Olga
 686
 687 printf "1 + 1 = %d" 2
 688 #=> "1 + 1 = 2"
 689
 690 printf "This is how you print a float: %f" 2
 691 #=> "This is how you print a float: 2.000000"
 692 ```
 693
 694 ### Directory of script
 695
 696 ```bash
 697 DIR="${0%/*}"
 698 ```
 699
 700 ### Getting options
 701
 702 ```bash
 703 while [[ "$1" =~ ^- && ! "$1" == "--" ]]; do case $1 in
 704   -V | --version )
 705     echo $version
 706     exit
 707     ;;
 708   -s | --string )
 709     shift; string=$1
 710     ;;
 711   -f | --flag )
 712     flag=1
 713     ;;
 714 esac; shift; done
 715 if [[ "$1" == '--' ]]; then shift; fi
 716 ```
 717
 718 ### Heredoc
 719
 720 ```sh
 721 cat <<END
 722 hello world
 723 END
 724 ```
 725
 726 ### Reading input
 727
 728 ```bash
 729 echo -n "Proceed? [y/n]: "
 730 read ans
 731 echo $ans
 732 ```
 733
 734 ```bash
 735 read -n 1 ans    # Just one character
 736 ```
 737
 738 ### Special variables
 739
 740 | `$?` | Exit status of last task |
 741 | `$!` | PID of last background task |
 742 | `$$` | PID of shell |
 743 | `$0` | Filename of the shell script |
 744
 745 See [Special parameters](http://wiki.bash-hackers.org/syntax/shellvars#special_parameters_and_shell_variables).
 746
 747 ### Go to previous directory
 748
 749 ```bash
 750 pwd # /home/user/foo
 751 cd bar/
 752 pwd # /home/user/foo/bar
 753 cd -
 754 pwd # /home/user/foo
 755 ```
 756
 757 ### Check for command's result
 758
 759 ```bash
 760 if ping -c 1 google.com; then
 761   echo "It appears you have a working internet connection"
 762 fi
 763 ```
 764
 765 ### Grep check
 766
 767 ```bash
 768 if grep -q 'foo' ~/.bash_history; then
 769   echo "You appear to have typed 'foo' in the past"
 770 fi
 771 ```
 772
 773 ## Also see
 774 {: .-one-column}
 775
 776 * [Bash-hackers wiki](http://wiki.bash-hackers.org/) _(bash-hackers.org)_
 777 * [Shell vars](http://wiki.bash-hackers.org/syntax/shellvars) _(bash-hackers.org)_
 778 * [Learn bash in y minutes](https://learnxinyminutes.com/docs/bash/) _(learnxinyminutes.com)_
 779 * [Bash Guide](http://mywiki.wooledge.org/BashGuide) _(mywiki.wooledge.org)_
 780 * [ShellCheck](https://www.shellcheck.net/) _(shellcheck.net)_