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Srobin2020S/bash.sh

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Created December 4, 2020 16:04
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Revisions

  1. @cristian-sr cristian-sr revised this gist Aug 23, 2020. No changes.
  2. @cristian-sr cristian-sr revised this gist Aug 22, 2020. No changes.
  3. @cristian-sr cristian-sr created this gist Dec 25, 2018.
    371 changes: 371 additions & 0 deletions bash.sh
    View file
    Original file line number Diff line number Diff line change
    @@ -0,0 +1,371 @@
    https://devhints.io/bash


    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
    get_name() {
    echo "John"
    }

    echo "You are $(get_name)"
    See: Functions

    Conditionals
    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
    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##*.} # 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
    ${FOO%suffix} Remove suffix
    ${FOO#prefix} Remove prefix
    ${FOO%%suffix} Remove long suffix
    ${FOO##prefix} Remove long prefix
    ${FOO/from/to} Replace first match
    ${FOO//from/to} Replace all
    ${FOO/%from/to} Replace suffix
    ${FOO/#from/to} Replace prefix
    Comments
    # Single line comment
    : '
    This is a
    multi line
    comment
    '
    Substrings
    ${FOO:0:3} Substring (position, length)
    ${FOO:-3:3} Substring from the right
    Length
    ${#FOO} Length of $FOO
    Default values
    ${FOO:-val} $FOO, or val if not set
    ${FOO:=val} Set $FOO to val if not set
    ${FOO:+val} val if $FOO is set
    ${FOO:?message} Show error message and exit if $FOO is not set
    The : is optional (eg, ${FOO=word} works)

    #Loops
    Basic for loop
    for i in /etc/rc.*; 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
    < file.txt | while read line; do
    echo $line
    done
    Forever
    while true; do
    ···
    done
    #Functions
    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
    $# Number of arguments
    $* All arguments
    $@ All arguments, starting from first
    $1 First argument
    See Special parameters.

    #Conditionals
    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.

    [[ -z STRING ]] Empty string
    [[ -n STRING ]] Not empty string
    [[ STRING == STRING ]] Equal
    [[ STRING != STRING ]] Not Equal
    [[ NUM -eq NUM ]] Equal
    [[ NUM -ne NUM ]] Not equal
    [[ NUM -lt NUM ]] Less than
    [[ NUM -le NUM ]] Less than or equal
    [[ NUM -gt NUM ]] Greater than
    [[ NUM -ge NUM ]] Greater than or equal
    [[ STRING =~ STRING ]] Regexp
    (( NUM < NUM )) Numeric conditions
    [[ -o noclobber ]] If OPTIONNAME is enabled
    [[ ! EXPR ]] Not
    [[ X ]] && [[ Y ]] And
    [[ X ]] || [[ Y ]] Or
    File conditions
    [[ -e FILE ]] Exists
    [[ -r FILE ]] Readable
    [[ -h FILE ]] Symlink
    [[ -d FILE ]] Directory
    [[ -w FILE ]] Writable
    [[ -s FILE ]] Size is > 0 bytes
    [[ -f FILE ]] File
    [[ -x FILE ]] Executable
    [[ FILE1 -nt FILE2 ]] 1 is more recent than 2
    [[ FILE1 -ot FILE2 ]] 2 is more recent than 1
    [[ FILE1 -ef FILE2 ]] Same files
    Example
    if ping -c 1 google.com; then
    echo "It appears you have a working internet connection"
    fi
    if grep -q 'foo' ~/.bash_history; then
    echo "You appear to have typed 'foo' in the past"
    fi
    # 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[@]} # 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 ${Fruits[@]:3:2} # Range (from position 3, length 2)
    Operations
    Fruits=("${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
    #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
    set -o nullglob # Non-matching globs are removed ('*.foo' => '')
    set -o failglob # Non-matching globs throw errors
    set -o nocaseglob # Case insensitive globs
    set -o globdots # Wildcards match dotfiles ("*.sh" => ".foo.sh")
    set -o 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
    history Show history
    shopt -s histverify Don’t execute expanded result immediately
    Expansions
    !$ Expand last parameter of most recent command
    !* Expand all parameters of most recent command
    !-n Expand nth most recent command
    !n Expand nth command in history
    !<command> Expand most recent invocation of command <command>
    Operations
    !! Execute last command again
    !!:s/<FROM>/<TO>/ Replace first occurrence of <FROM> to <TO> in most recent command
    !!:gs/<FROM>/<TO>/ Replace all occurrences of <FROM> to <TO> in most recent command
    !$:t Expand only basename from last parameter of most recent command
    !$:h Expand only directory from last parameter of most recent command
    !! and !$ can be replaced with any valid expansion.

    Slices
    !!:n Expand only nth token from most recent command (command is 0; first argument is 1)
    !^ Expand first argument from most recent command
    !$ Expand last token from most recent command
    !!:n-m Expand range of tokens from most recent command
    !!:n-$ Expand nth token to last from most recent command
    !! 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
    or

    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
    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
    See Special parameters.

    Go to previous directory
    pwd # /home/user/foo
    cd bar/
    pwd # /home/user/foo/bar
    cd -
    pwd # /home/user/foo