Bash scripting

落日映苍穹つ 2023-02-20 08:23 100阅读 0赞

> title: Bash scripting  
> category: CLI  
> layout: 2017/sheet  
> tags: \[Featured\]  
> updated: 2019-10-02  
> keywords:
> 
>  *  Variables
>  *  Functions
>  *  Interpolation
>  *  Brace expansions
>  *  Loops
>  *  Conditional execution
>  *  Command substitution
> 
> 摘选自：https://github.com/rstacruz/cheatsheets项目的bash.md

![在这里插入图片描述][watermark_type_ZmFuZ3poZW5naGVpdGk_shadow_10_text_aHR0cHM6Ly9ibG9nLmNzZG4ubmV0L3UwMTM0Njk3NTM_size_16_color_FFFFFF_t_70]

## Getting started ##

\{: .-three-column\}

### Example ###

#!/usr/bin/env bash
    
    NAME="John"
    echo "Hello $NAME!"

### Variables ###

NAME="John"
    echo $NAME
    echo "$NAME"
    echo "${NAME}!"

### String quotes ###

NAME="John"
    echo "Hi $NAME"  #=> Hi John
    echo 'Hi $NAME'  #=> Hi $NAME

### Shell execution ###

echo "I'm in $(pwd)"
    echo "I'm in `pwd`"
    # Same

See [Command substitution][]

### Conditional execution ###

git commit && git push
    git commit || echo "Commit failed"

### Functions ###

\{: id=‘functions-example’\}

get_name() {
        
      echo "John"
    }
    
    echo "You are $(get_name)"

See: Functions

### Conditionals ###

\{: id=‘conditionals-example’\}

if [[ -z "$string" ]]; then
      echo "String is empty"
    elif [[ -n "$string" ]]; then
      echo "String is not empty"
    fi

See: Conditionals

### Strict mode ###

set -euo pipefail
    IFS=$'\n\t'

See: [Unofficial bash strict mode][]

### Brace expansion ###

echo {
        A,B}.js

| `{A,B}` | Same as `A B` |  
| `{A,B}.js` | Same as `A.js B.js` |  
| `{1..5}` | Same as `1 2 3 4 5` |

See: [Brace expansion][]

## Parameter expansions ##

\{: .-three-column\}

### Basics ###

name="John"
    echo ${name}
    echo ${name/J/j}    #=> "john" (substitution)
    echo ${name:0:2}    #=> "Jo" (slicing)
    echo ${name::2}     #=> "Jo" (slicing)
    echo ${name::-1}    #=> "Joh" (slicing)
    echo ${name:(-1)}   #=> "n" (slicing from right)
    echo ${name:(-2):1} #=> "h" (slicing from right)
    echo ${food:-Cake}  #=> $food or "Cake"

length=2
    echo ${name:0:length}  #=> "Jo"

See: [Parameter expansion][]

STR="/path/to/foo.cpp"
    echo ${STR%.cpp}    # /path/to/foo
    echo ${STR%.cpp}.o  # /path/to/foo.o
    echo ${STR%/*}      # /path/to
    
    echo ${
        STR##*.}     # cpp (extension)
    echo ${
        STR##*/}     # foo.cpp (basepath)
    
    echo ${
        STR#*/}      # path/to/foo.cpp
    echo ${
        STR##*/}     # foo.cpp
    
    echo ${STR/foo/bar} # /path/to/bar.cpp

STR="Hello world"
    echo ${STR:6:5}   # "world"
    echo ${STR:-5:5}  # "world"

SRC="/path/to/foo.cpp"
    BASE=${
        SRC##*/}   #=> "foo.cpp" (basepath)
    DIR=${SRC%$BASE}  #=> "/path/to/" (dirpath)

### Substitution ###

<table> 
 <thead> 
  <tr> 
   <th>Code</th> 
   <th>Description</th> 
  </tr> 
 </thead> 
 <tbody> 
  <tr> 
   <td><code>${FOO%suffix}</code></td> 
   <td>Remove suffix</td> 
  </tr> 
  <tr> 
   <td><code>${FOO#prefix}</code></td> 
   <td>Remove prefix</td> 
  </tr> 
  <tr> 
   <td>—</td> 
   <td>—</td> 
  </tr> 
  <tr> 
   <td><code>${FOO%%suffix}</code></td> 
   <td>Remove long suffix</td> 
  </tr> 
  <tr> 
   <td><code>${FOO##prefix}</code></td> 
   <td>Remove long prefix</td> 
  </tr> 
  <tr> 
   <td>—</td> 
   <td>—</td> 
  </tr> 
  <tr> 
   <td><code>${FOO/from/to}</code></td> 
   <td>Replace first match</td> 
  </tr> 
  <tr> 
   <td><code>${FOO//from/to}</code></td> 
   <td>Replace all</td> 
  </tr> 
  <tr> 
   <td>—</td> 
   <td>—</td> 
  </tr> 
  <tr> 
   <td><code>${FOO/%from/to}</code></td> 
   <td>Replace suffix</td> 
  </tr> 
  <tr> 
   <td><code>${FOO/#from/to}</code></td> 
   <td>Replace prefix</td> 
  </tr> 
 </tbody> 
</table>

### Comments ###

# Single line comment

: '
    This is a
    multi line
    comment
    '

### Substrings ###

<table> 
 <thead> 
  <tr> 
   <th>Expression</th> 
   <th>Description</th> 
  </tr> 
 </thead> 
 <tbody> 
  <tr> 
   <td><code>${FOO:0:3}</code></td> 
   <td>Substring <em>(position, length)</em></td> 
  </tr> 
  <tr> 
   <td><code>${FOO:(-3):3}</code></td> 
   <td>Substring from the right</td> 
  </tr> 
 </tbody> 
</table>

### Length ###

<table> 
 <thead> 
  <tr> 
   <th>Expression</th> 
   <th>Description</th> 
  </tr> 
 </thead> 
 <tbody> 
  <tr> 
   <td><code>${#FOO}</code></td> 
   <td>Length of <code>$FOO</code></td> 
  </tr> 
 </tbody> 
</table>

### Manipulation ###

STR="HELLO WORLD!"
    echo ${STR,}   #=> "hELLO WORLD!" (lowercase 1st letter)
    echo ${STR,,}  #=> "hello world!" (all lowercase)
    
    STR="hello world!"
    echo ${STR^}   #=> "Hello world!" (uppercase 1st letter)
    echo ${STR^^}  #=> "HELLO WORLD!" (all uppercase)

### Default values ###

<table> 
 <thead> 
  <tr> 
   <th>Expression</th> 
   <th>Description</th> 
  </tr> 
 </thead> 
 <tbody> 
  <tr> 
   <td><code>${FOO:-val}</code></td> 
   <td><code>$FOO</code>, or <code>val</code> if unset (or null)</td> 
  </tr> 
  <tr> 
   <td><code>${FOO:=val}</code></td> 
   <td>Set <code>$FOO</code> to <code>val</code> if unset (or null)</td> 
  </tr> 
  <tr> 
   <td><code>${FOO:+val}</code></td> 
   <td><code>val</code> if <code>$FOO</code> is set (and not null)</td> 
  </tr> 
  <tr> 
   <td><code>${FOO:?message}</code></td> 
   <td>Show error message and exit if <code>$FOO</code> is unset (or null)</td> 
  </tr> 
 </tbody> 
</table>

Omitting the `:` removes the (non)nullity checks, e.g. `${FOO-val}` expands to `val` if unset otherwise `$FOO`.

## Loops ##

\{: .-three-column\}

### Basic for loop ###

for i in /etc/rc.*; do
      echo $i
    done

### C-like for loop ###

for ((i = 0 ; i < 100 ; i++)); do
      echo $i
    done

### Ranges ###

for i in {
        1..5}; do
        echo "Welcome $i"
    done

#### With step size ####

for i in {
        5..50..5}; do
        echo "Welcome $i"
    done

### Reading lines ###

cat file.txt | while read line; do
      echo $line
    done

### Forever ###

while true; do
      ···
    done

## Functions ##

\{: .-three-column\}

### Defining functions ###

myfunc() {
        
        echo "hello $1"
    }

# Same as above (alternate syntax)
    function myfunc() {
        
        echo "hello $1"
    }

myfunc "John"

### Returning values ###

myfunc() {
        
        local myresult='some value'
        echo $myresult
    }

result="$(myfunc)"

### Raising errors ###

myfunc() {
        
      return 1
    }

if myfunc; then
      echo "success"
    else
      echo "failure"
    fi

### Arguments ###

<table> 
 <thead> 
  <tr> 
   <th>Expression</th> 
   <th>Description</th> 
  </tr> 
 </thead> 
 <tbody> 
  <tr> 
   <td><code>$#</code></td> 
   <td>Number of arguments</td> 
  </tr> 
  <tr> 
   <td><code>$*</code></td> 
   <td>All arguments</td> 
  </tr> 
  <tr> 
   <td><code>$@</code></td> 
   <td>All arguments, starting from first</td> 
  </tr> 
  <tr> 
   <td><code>$1</code></td> 
   <td>First argument</td> 
  </tr> 
  <tr> 
   <td><code>$_</code></td> 
   <td>Last argument of the previous command</td> 
  </tr> 
 </tbody> 
</table>

See [Special parameters][].

## Conditionals ##

\{: .-three-column\}

### Conditions ###

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.

<table> 
 <thead> 
  <tr> 
   <th>Condition</th> 
   <th>Description</th> 
  </tr> 
 </thead> 
 <tbody> 
  <tr> 
   <td><code>[[ -z STRING ]]</code></td> 
   <td>Empty string</td> 
  </tr> 
  <tr> 
   <td><code>[[ -n STRING ]]</code></td> 
   <td>Not empty string</td> 
  </tr> 
  <tr> 
   <td><code>[[ STRING == STRING ]]</code></td> 
   <td>Equal</td> 
  </tr> 
  <tr> 
   <td><code>[[ STRING != STRING ]]</code></td> 
   <td>Not Equal</td> 
  </tr> 
  <tr> 
   <td>—</td> 
   <td>—</td> 
  </tr> 
  <tr> 
   <td><code>[[ NUM -eq NUM ]]</code></td> 
   <td>Equal</td> 
  </tr> 
  <tr> 
   <td><code>[[ NUM -ne NUM ]]</code></td> 
   <td>Not equal</td> 
  </tr> 
  <tr> 
   <td><code>[[ NUM -lt NUM ]]</code></td> 
   <td>Less than</td> 
  </tr> 
  <tr> 
   <td><code>[[ NUM -le NUM ]]</code></td> 
   <td>Less than or equal</td> 
  </tr> 
  <tr> 
   <td><code>[[ NUM -gt NUM ]]</code></td> 
   <td>Greater than</td> 
  </tr> 
  <tr> 
   <td><code>[[ NUM -ge NUM ]]</code></td> 
   <td>Greater than or equal</td> 
  </tr> 
  <tr> 
   <td>—</td> 
   <td>—</td> 
  </tr> 
  <tr> 
   <td><code>[[ STRING =~ STRING ]]</code></td> 
   <td>Regexp</td> 
  </tr> 
  <tr> 
   <td>—</td> 
   <td>—</td> 
  </tr> 
  <tr> 
   <td><code>(( NUM &lt; NUM ))</code></td> 
   <td>Numeric conditions</td> 
  </tr> 
 </tbody> 
</table>

#### More conditions ####

<table> 
 <thead> 
  <tr> 
   <th>Condition</th> 
   <th>Description</th> 
  </tr> 
 </thead> 
 <tbody> 
  <tr> 
   <td><code>[[ -o noclobber ]]</code></td> 
   <td>If OPTIONNAME is enabled</td> 
  </tr> 
  <tr> 
   <td>—</td> 
   <td>—</td> 
  </tr> 
  <tr> 
   <td><code>[[ ! EXPR ]]</code></td> 
   <td>Not</td> 
  </tr> 
  <tr> 
   <td><code>[[ X &amp;&amp; Y ]]</code></td> 
   <td>And</td> 
  </tr> 
  <tr> 
   <td><code>[[ X || Y ]]</code></td> 
   <td>Or</td> 
  </tr> 
 </tbody> 
</table>

### File conditions ###

<table> 
 <thead> 
  <tr> 
   <th>Condition</th> 
   <th>Description</th> 
  </tr> 
 </thead> 
 <tbody> 
  <tr> 
   <td><code>[[ -e FILE ]]</code></td> 
   <td>Exists</td> 
  </tr> 
  <tr> 
   <td><code>[[ -r FILE ]]</code></td> 
   <td>Readable</td> 
  </tr> 
  <tr> 
   <td><code>[[ -h FILE ]]</code></td> 
   <td>Symlink</td> 
  </tr> 
  <tr> 
   <td><code>[[ -d FILE ]]</code></td> 
   <td>Directory</td> 
  </tr> 
  <tr> 
   <td><code>[[ -w FILE ]]</code></td> 
   <td>Writable</td> 
  </tr> 
  <tr> 
   <td><code>[[ -s FILE ]]</code></td> 
   <td>Size is &gt; 0 bytes</td> 
  </tr> 
  <tr> 
   <td><code>[[ -f FILE ]]</code></td> 
   <td>File</td> 
  </tr> 
  <tr> 
   <td><code>[[ -x FILE ]]</code></td> 
   <td>Executable</td> 
  </tr> 
  <tr> 
   <td>—</td> 
   <td>—</td> 
  </tr> 
  <tr> 
   <td><code>[[ FILE1 -nt FILE2 ]]</code></td> 
   <td>1 is more recent than 2</td> 
  </tr> 
  <tr> 
   <td><code>[[ FILE1 -ot FILE2 ]]</code></td> 
   <td>2 is more recent than 1</td> 
  </tr> 
  <tr> 
   <td><code>[[ FILE1 -ef FILE2 ]]</code></td> 
   <td>Same files</td> 
  </tr> 
 </tbody> 
</table>

### Example ###

# String
    if [[ -z "$string" ]]; then
      echo "String is empty"
    elif [[ -n "$string" ]]; then
      echo "String is not empty"
    fi

# Combinations
    if [[ X && Y ]]; then
      ...
    fi

# Equal
    if [[ "$A" == "$B" ]]

# Regex
    if [[ "A" =~ . ]]

if (( $a < $b )); then
       echo "$a is smaller than $b"
    fi

if [[ -e "file.txt" ]]; then
      echo "file exists"
    fi

## Arrays ##

### Defining arrays ###

Fruits=('Apple' 'Banana' 'Orange')

Fruits[0]="Apple"
    Fruits[1]="Banana"
    Fruits[2]="Orange"

### Working with arrays ###

echo ${Fruits[0]}           # Element #0
    echo ${Fruits[-1]}          # Last element
    echo ${Fruits[@]}           # All elements, space-separated
    echo ${#Fruits[@]}          # Number of elements
    echo ${#Fruits}             # String length of the 1st element
    echo ${#Fruits[3]}          # String length of the Nth element
    echo ${Fruits[@]:3:2}       # Range (from position 3, length 2)
    echo ${!Fruits[@]}          # Keys of all elements, space-separated

### Operations ###

Fruits=("${Fruits[@]}" "Watermelon")    # Push
    Fruits+=('Watermelon')                  # Also Push
    Fruits=( ${Fruits[@]/Ap*/} )            # Remove by regex match
    unset Fruits[2]                         # Remove one item
    Fruits=("${Fruits[@]}")                 # Duplicate
    Fruits=("${Fruits[@]}" "${Veggies[@]}") # Concatenate
    lines=(`cat "logfile"`)                 # Read from file

### Iteration ###

for i in "${arrayName[@]}"; do
      echo $i
    done

## Dictionaries ##

\{: .-three-column\}

### Defining ###

declare -A sounds

sounds[dog]="bark"
    sounds[cow]="moo"
    sounds[bird]="tweet"
    sounds[wolf]="howl"

Declares `sound` as a Dictionary object (aka associative array).

### Working with dictionaries ###

echo ${sounds[dog]} # Dog's sound
    echo ${sounds[@]}   # All values
    echo ${!sounds[@]}  # All keys
    echo ${#sounds[@]}  # Number of elements
    unset sounds[dog]   # Delete dog

### Iteration ###

#### Iterate over values ####

for val in "${sounds[@]}"; do
      echo $val
    done

#### Iterate over keys ####

for key in "${!sounds[@]}"; do
      echo $key
    done

## Options ##

### Options ###

set -o noclobber  # Avoid overlay files (echo "hi" > foo)
    set -o errexit    # Used to exit upon error, avoiding cascading errors
    set -o pipefail   # Unveils hidden failures
    set -o nounset    # Exposes unset variables

### Glob options ###

shopt -s nullglob    # Non-matching globs are removed  ('*.foo' => '')
    shopt -s failglob    # Non-matching globs throw errors
    shopt -s nocaseglob  # Case insensitive globs
    shopt -s dotglob     # Wildcards match dotfiles ("*.sh" => ".foo.sh")
    shopt -s globstar    # Allow ** for recursive matches ('lib/**/*.rb' => 'lib/a/b/c.rb')

Set `GLOBIGNORE` as a colon-separated list of patterns to be removed from glob  
matches.

## History ##

### Commands ###

### Expansions ###

### Operations ###

<table> 
 <thead> 
  <tr> 
   <th>Code</th> 
   <th>Description</th> 
  </tr> 
 </thead> 
 <tbody> 
  <tr> 
   <td><code>!!</code></td> 
   <td>Execute last command again</td> 
  </tr> 
  <tr> 
   <td><code>!!:s/&lt;FROM&gt;/&lt;TO&gt;/</code></td> 
   <td>Replace first occurrence of <code>&lt;FROM&gt;</code> to <code>&lt;TO&gt;</code> in most recent command</td> 
  </tr> 
  <tr> 
   <td><code>!!:gs/&lt;FROM&gt;/&lt;TO&gt;/</code></td> 
   <td>Replace all occurrences of <code>&lt;FROM&gt;</code> to <code>&lt;TO&gt;</code> in most recent command</td> 
  </tr> 
  <tr> 
   <td><code>!$:t</code></td> 
   <td>Expand only basename from last parameter of most recent command</td> 
  </tr> 
  <tr> 
   <td><code>!$:h</code></td> 
   <td>Expand only directory from last parameter of most recent command</td> 
  </tr> 
 </tbody> 
</table>

`!!` and `!$` can be replaced with any valid expansion.

### Slices ###

<table> 
 <thead> 
  <tr> 
   <th>Code</th> 
   <th>Description</th> 
  </tr> 
 </thead> 
 <tbody> 
  <tr> 
   <td><code>!!:n</code></td> 
   <td>Expand only <code>n</code>th token from most recent command (command is <code>0</code>; first argument is <code>1</code>)</td> 
  </tr> 
  <tr> 
   <td><code>!^</code></td> 
   <td>Expand first argument from most recent command</td> 
  </tr> 
  <tr> 
   <td><code>!$</code></td> 
   <td>Expand last token from most recent command</td> 
  </tr> 
  <tr> 
   <td><code>!!:n-m</code></td> 
   <td>Expand range of tokens from most recent command</td> 
  </tr> 
  <tr> 
   <td><code>!!:n-$</code></td> 
   <td>Expand <code>n</code>th token to last from most recent command</td> 
  </tr> 
 </tbody> 
</table>

`!!` can be replaced with any valid expansion i.e. `!cat`, `!-2`, `!42`, etc.

## Miscellaneous ##

### Numeric calculations ###

$((a + 200))      # Add 200 to $a

$((RANDOM%=200))  # Random number 0..200

### Subshells ###

(cd somedir; echo "I'm now in $PWD")
    pwd # still in first directory

### Redirection ###

python hello.py > output.txt   # stdout to (file)
    python hello.py >> output.txt  # stdout to (file), append
    python hello.py 2> error.log   # stderr to (file)
    python hello.py 2>&1           # stderr to stdout
    python hello.py 2>/dev/null    # stderr to (null)
    python hello.py &>/dev/null    # stdout and stderr to (null)

python hello.py < foo.txt      # feed foo.txt to stdin for python

### Inspecting commands ###

command -V cd
    #=> "cd is a function/alias/whatever"

### Trap errors ###

trap 'echo Error at about $LINENO' ERR

traperr() {
        
      echo "ERROR: ${BASH_SOURCE[1]} at about ${BASH_LINENO[0]}"
    }
    
    set -o errtrace
    trap traperr ERR

### Case/switch ###

case "$1" in
      start | up)
        vagrant up
        ;;
    
      *)
        echo "Usage: $0 {start|stop|ssh}"
        ;;
    esac

### Source relative ###

source "${0%/*}/../share/foo.sh"

### printf ###

printf "Hello %s, I'm %s" Sven Olga
    #=> "Hello Sven, I'm Olga
    
    printf "1 + 1 = %d" 2
    #=> "1 + 1 = 2"
    
    printf "This is how you print a float: %f" 2
    #=> "This is how you print a float: 2.000000"

### Directory of script ###

DIR="${0%/*}"

### Getting options ###

while [[ "$1" =~ ^- && ! "$1" == "--" ]]; do case $1 in
      -V | --version )
        echo $version
        exit
        ;;
      -s | --string )
        shift; string=$1
        ;;
      -f | --flag )
        flag=1
        ;;
    esac; shift; done
    if [[ "$1" == '--' ]]; then shift; fi

### Heredoc ###

cat <<END
    hello world
    END

### Reading input ###

echo -n "Proceed? [y/n]: "
    read ans
    echo $ans

read -n 1 ans    # Just one character

### Special variables ###

| `$?` | Exit status of last task |  
| `$!` | PID of last background task |  
| `$$` | PID of shell |  
| `$0` | Filename of the shell script |

See [Special parameters][].

### Go to previous directory ###

pwd # /home/user/foo
    cd bar/
    pwd # /home/user/foo/bar
    cd -
    pwd # /home/user/foo

### Check for command’s result ###

if ping -c 1 google.com; then
      echo "It appears you have a working internet connection"
    fi

### Grep check ###

if grep -q 'foo' ~/.bash_history; then
      echo "You appear to have typed 'foo' in the past"
    fi

## Also see ##

\{: .-one-column\}

*  [Bash-hackers wiki][] *(bash-hackers.org)*
 *  [Shell vars][] *(bash-hackers.org)*
 *  [Learn bash in y minutes][] *(learnxinyminutes.com)*
 *  [Bash Guide][] *(mywiki.wooledge.org)*
 *  [ShellCheck][] *(shellcheck.net)*

[watermark_type_ZmFuZ3poZW5naGVpdGk_shadow_10_text_aHR0cHM6Ly9ibG9nLmNzZG4ubmV0L3UwMTM0Njk3NTM_size_16_color_FFFFFF_t_70]: https://img-blog.csdnimg.cn/20200625190227844.png?x-oss-process=image/watermark,type_ZmFuZ3poZW5naGVpdGk,shadow_10,text_aHR0cHM6Ly9ibG9nLmNzZG4ubmV0L3UwMTM0Njk3NTM=,size_16,color_FFFFFF,t_70
[Command substitution]: http://wiki.bash-hackers.org/syntax/expansion/cmdsubst
[Unofficial bash strict mode]: http://redsymbol.net/articles/unofficial-bash-strict-mode/
[Brace expansion]: http://wiki.bash-hackers.org/syntax/expansion/brace
[Parameter expansion]: http://wiki.bash-hackers.org/syntax/pe
[Special parameters]: http://wiki.bash-hackers.org/syntax/shellvars#special_parameters_and_shell_variables
[Bash-hackers wiki]: http://wiki.bash-hackers.org/
[Shell vars]: http://wiki.bash-hackers.org/syntax/shellvars
[Learn bash in y minutes]: https://learnxinyminutes.com/docs/bash/
[Bash Guide]: http://mywiki.wooledge.org/BashGuide
[ShellCheck]: https://www.shellcheck.net/