bash and friends

Bash guide

https://mywiki.wooledge.org/BashGuide

They also have a pdf version.

No file for you

[[ -f /var/log/apt/history.log ]] || exit 1

Will exit if file is not there, same thing should be used if you use cd in your scripts Note: Actually just using ‘exit’ should be enough, as it returns the status value of previous command (exit equals to exit $?).

cd /some/path || exit 1

This will ensure that script doesn’t continue if dir doesn’t exist. Another way might be:

config="$HOME/bin/singularity.cfg"
test -f "$config" && source "$config" >/dev/null || { echo "$config does not exist" ; exit 1; }

Do something if directory is there

[[ -d $HOME/apps/blender ]] && mv "$HOME/apps/blender" "$HOME/apps/blender_bak_$RANDOM"

With if

file="$HOME/.pcalc.txt"
if [ -f "$file" ]; then
    tail -n 100 "$file" > "$file.tmp" && mv "$file.tmp" "$file"
fi

Note that -e would be any file/dir/socket/node, -d is dir and so on

-b filename - Block special file
-c filename - Special character file
-d directoryname - Check for directory Existence
-e filename - Check for file existence, regardless of type (node, directory, socket, etc.)
-f filename - Check for regular file existence not a directory
-G filename - Check if file exists and is owned by effective group ID
-G filename set-group-id - True if file exists and is set-group-id
-k filename - Sticky bit
-L filename - Symbolic link
-O filename - True if file exists and is owned by the effective user id
-r filename - Check if file is a readable
-S filename - Check if file is socket
-s filename - Check if file is nonzero size
-u filename - Check if file set-user-id bit is set
-w filename - Check if file is writable
-x filename - Check if file is executable

mkdir

Only if one is not already there, should also be recursive

mkdir -p ~/tmp/stuff

Exit if var is empty

[[ -z "$url" ]] && (echo "no url for you" && exit 1)

Benchmark script

time for i in $(seq 1000); do script; done

They say this is bad due to IO (disk) reads.

Remove numerals and fill some variables

echo “Uh oh. Something went really bad..” >&2 stuff=”1680x1050+2880+23” read -r h w x y «< $(echo ${stuff//[!0-9]/ })

2nd part is called special parameter expansion replacing non-numeric characters with space.

Random word

randomword()  {
    dict="/usr/share/dict/words"
    if [ -f "$dict" ]; then
        word=$(shuf -n1 "$dict" | tr -dc '[:alnum:]\n\r' | tr '[:upper:]' '[:lower:]')
        # if dict gets us null length then replace with something else
        [ -z "$word" ] && word="$RANDOM"
    else # there is no dict file
        word="$RANDOM"
    fi
    echo "$word"
}
word=$(randomword)

Checks critical and non-critical

command -v mediainfo >/dev/null 2>&1 || { >&2 echo "I need mediainfo installed." ; exit 1; }
command -v mediainfo >/dev/null 2>&1 || { >&2 echo "I'd like mediainfo installed." ; }

Dirs and files and paths breakdown

file=$(readlink -f "$1")       # quasi absolute
baseext=$(basename "${1}")     # or
baseext=${i##*/}               # file.ext
base="${baseext%.*}"           # file
ext="${file##*.}"              # ext
dir=$(dirname "${file}")       # or
dir=${i%/*}                    # directory

http://mywiki.wooledge.org/BashFAQ/073

Dump stuff from web pages

read -r _ temp _<<< $(lynx -dump http://some/link.htm | grep Cityname) && echo "$temp°C"

Lynx may be a fat solution, but it’s an easy one.

Another interesting option is combo of wget and xml2/html2

wget http://some/link.htm -O - -o /dev/null | html2 | #more filtering

grep AND

lynx -listonly -nonumbers -dump https://builder.blender.org/download/ | grep "$bits" | grep linux | grep https | grep tar | head -1

Such grep sequence will behave as logical AND, there are other shorter ways.

Check for certain extensions yourself

# do I need this
shopt -s extglob
# If no specific arguments were passed to script then it will check current dir for certain audio extensions itself
(( $# )) || set -- *.@(mp3|mp4|m4a|flac|ogg|mpc|wav|aif|opus); [[ -e $1 ]] || \
{ echo "No audio files find in this dir (mp3|mp4|m4a|flac|ogg|mpc|wav|aif|opus)"; stty sane; exit 1; }

Rounding math

# awk_round

awk_round () {
    awk 'BEGIN{printf "%."'$1'"f\n", "'$2'"}'
}

awk_round "$1" "$2"

https://brontosaurusrex.github.io/2017/06/10/bash-rounding-n-th-time/

The idea is you can do all your floating math using bc, but this will give you final rounding that one would somehow expect.

awk calc function

calca() #@ Perform arithmetic, including decimal fractions
{
local result=$(awk 'BEGIN { OFMT="%f"; print '"$*"'; exit}')
case $result in
*.*0) result=${result%"${result##*[!0]}"} ;;
esac
printf "%s\n" "$result"
}

calca 1/3

that characters special to the shell must be escaped or quoted on the command line. This applies particularly to the multiplication symbol, *.

From the book: Pro Bash Programming

Not battle-tested, not benchmarked.

bc calc function

calc () {
    echo "scale=5; $*" | bc
}

calc 1/2

Not battle-tested

or prettier,

calc2 () {
     bc <<< "scale=5; $*"
}

but slower than echo/pipe version above. That triple < is called here-string.

Add path and modify env

This may be usefull to add to scripts that are run from cron

PATH="/home/ticho/bin/":$PATH
export LC_ALL=en_US.UTF-8

or one could run firefox with a different theme

GTK_THEME=Adapta launchee firefox

see what’s out there

env
echo $PATH

tmpdir

# tmp dir http://mywiki.wooledge.org/BashFAQ/062
tmp="/tmp/$RANDOM-$$"
trap '[ -n "$tmp" ] && rm -fr "$tmp"' EXIT
mkdir -m 700 "$tmp" || { echo '!! unable to create a tmpdir' >&2; tmp=; exit 1; }

I did something audio

flite -voice rms "I did something" >/dev/null 2>&1

isNumber function

# function
shopt -s extglob
isNumber() {
var="$1"
# ˇ will fail for numbers like 4.044676226059745e-17 (so called scientific notation)
if [[ $var = @(*[0-9]*|!([+-]|)) && $var = ?([+-])*([0-9])?(.*([0-9])) ]]; then
  #echo "fine"
  ans="$var"
  true  # return 0
else
  #echo "bad"
  ans="0"
  false # return 1
fi
}

examples

ans="33.3"
if isNumber "$ans"; then # do something
ans="a"
isNumber "$ans"          # $ans will be 0 if not a number

Timer (benchmark)

This is useful for timing parts of the script

# benchmark start
START=$(date +%s.%N)

# your script here

# benchmark end
END=$(date +%s.%N)
DIFF=$(echo "$END - $START" | bc)
echo "done in $DIFF seconds"

for whole thing one could run script as

time script

Try until it works

play() {
    /usr/bin/mpv https://stream/chunklist.m3u8 --no-resume-playback
}

until play
do
  sleep 60
done

Also works nicely interactively

until latoya ; do sleep 30 ; done

Bool ‘if’

value="1"
((value)) && echo "is true"

Command in variable is a NO

http://mywiki.wooledge.org/BashFAQ/050

A simple variable string wouldn’t work because no other “expansions” happen after word splitting, and that includes recognition of keywords and control operators. Keywords and control operators expanded after word splitting are considered to be just ordinary arguments, and not part of the syntax

Quasi multidimensional array

#!/bin/bash

# 1D array with 'one,two,three' fields
data=(
one,two,three
four,five,six
seven,eight,nine
)

# actualy each line is a single field of array,
# but we can break the lines with a fancy loop:

while read -r line; do
    IFS=, read -r app config method <<< "$line"

    echo "$app $config $method"

done < <(printf '%s\n' "${data[@]}")

[[ brackets ]] = Test

(http://mywiki.wooledge.org/BashFAQ/031)[http://mywiki.wooledge.org/BashFAQ/031]

Are used for testing stuff, for example string comparision and it’s smarter brother of single [.

[[ a > b ]] || echo "a does not come after b"

( brackets ) = Subshell

Subshell is similar to child, but more info is inherited.

(cd /tmp || exit 1; date > timestamp) pwd

Cd to /tmp folder is internal to only that subshell. Also if cd would fail, it would only terminate that specific subshell and not the main process.

{ brackets; } = Command grouping

Generally speaking last command must be terminated with ;, unless } is at the start of line

{   echo "app, config, restart method"
    echo "---, ------, --------------"
    while read -r line; do
        IFS=, read -r app config method <<< "$line"

        
        echo "$app, $config, $method" 
        
      
    done < <(printf '%s\n' "${configs[@]}")
} | column -s, -t

When at the end, use ;

[[ -f $file ]] || { echo "$file not found"; exit 1; }

<() and ()> brackets = Process Substitution

A cousin of the pipe is the process substitution operator, which comes in two forms: <() and >(). It’s a convenient way to use named pipes without having to create temporary files. Whenever you think you need a temporary file to do something, process substitution might be a better way to handle things. What it does, is basically run the command inside the parentheses. With the <() operator, the command’s output is put in a something similar to named pipe.

# diff -y <(head -n 1 .dictionary) <(tail -n 1 .dictionary)

Basically diff will see two files.

(( brackets )) = Arithmetic evaluation

(( 2 == 2 )) && echo true
echo $(( 2 + 2 ))
echo "There are $(($rows * $columns)) cells"

Roll dice alias

alias dice='echo $(( RANDOM % 6 + 1 ))'

Redirects

ls > file.log 2>&1

should write both stdout and stderr to file.log.

Cron example where stdout and stderr are separated into two files

0 0 * * * bin/dropbox start >/tmp/dbstdout.log 2>/tmp/dbstderr.log

Stdout to null

ls >/dev/null  # same with space: ls > /dev/null

Stderr to null

ls 2>/dev/null # same with space: ls 2> /dev/null

Stdout and stderr to null

ls >/dev/null 2>&1
# same with space: ls > /dev/null 2>&1

Sourcing

Imagine a script named ‘woot’

#!/bin/bash
cd /tmp

If I ./woot, nothing will change (pwd will stay the same), however if I

. ./woot      # dotting
#or
source ./woot

pwd will change.

if woot is somewhere on $PATH, this will also work

. woot

Find and xz pack

All in this dir, including hidden files

find . -type f -exec xz {} +   # pack file by file
find . -type f -exec unxz {} + # unpack file by file

xz entire dir as single file (staring from one dir above)

 tar -cf- dir | xz > roboto.tar.xz

Do it maybe

rotateMaybe () {
if (( RANDOM %2 )) ; then
    mogrify -rotate 180 "$final"
    printf "r"
else
    printf "n"
fi
}

Restart something

restart() {
if pgrep -x "$1" > /dev/null
then
    (echo "$1 running, restarting"
    killall -w "$1"
    "${1}" &) &
else
    echo "$1 wasn't running"
fi
}

restart wbar

Center text

centertext(){
    columns=$(tput cols) 
    string="$1" 
    printf "%*s\n" $(((${#string}+columns)/2)) "$string"
}

centertext "woot"

printf examples

# round float
a=3.123456
printf "%.2f\n" "a"         
3.12
printf "%.4f\n" "a"
3.1235

# type protection ?
a="1"
printf "%d\n" "a"
1
a="b"
printf "%d\n" "a"
0

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