bash and friends

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

Or do it on a more global level with

set -e # script will from now on exit on 1st error

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

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

if [ -f "$file" ]; then
    tail -n 100 "$file" > "$file.tmp" && mv "$file.tmp" "$file"

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


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

mkdir -p ~/tmp/stuff

Exit if var is empty

[[ -z "$url" ]] && { echo "no url for you" ; exit 1; }
# or is empty or has spaces only:
[[ -z "${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()  {
    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="$RANDOM"
    else # there is no dict file
    echo "$word"

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

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 | 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

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

awk_round "$1" "$2"

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%"${result##*[!0]}"} ;;
printf "%s\n" "$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

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

echo $PATH


# tmp dir
trap '[ -n "$tmp" ] && rm -fr "$tmp"' EXIT
mkdir -m 700 "$tmp" || { echo '!! unable to create a tmpdir' >&2; tmp=; exit 1; }

tmpdir with more advanced trap calling function

cleanup () {
(( debug )) && echo "$?"
[ -n "$tmp" ] && rm -fr "$tmp"
tput cnorm

# tmp dir
mkdir -m 700 "$tmp" || { echo '!! unable to create a tmp dir' >&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() {
# 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"
  true  # return 0
  #echo "bad"
  false # return 1


if isNumber "$ans"; then # do something
isNumber "$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
  sleep 60

Also works nicely interactively

until latoya ; do sleep 30 ; done

Bool ‘if’

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

Command in variable is a NO

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


# 1D array with 'one,two,three' fields

# 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[@]}")

Associative array expansion?


# an array here
declare -A exe
["serverOne"]="echo woot" 
["job"]="sleep 3" 

# this is the exe

[[ brackets ]] = Test


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 ))'


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


Imagine a script named ‘woot’

cd /tmp

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

. ./woot      # dotting
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"
    printf "n"

Restart something

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

restart wbar

Center text

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

centertext "woot"

printf examples

# round float
printf "%.2f\n" "a"         
printf "%.4f\n" "a"

# type protection ?
printf "%d\n" "a"
printf "%d\n" "a"

Inline block of text to variable

This is not compatible with ‘set -e’

# custom css
css=$(cat <<'CSS'
<style type="text/css">
    p       { margin-top: 0; margin-bottom: 0; vertical-align: top; font-family: 'Noto Serif', serif; font-size: large }
    a       { font-family: 'Noto Serif', serif; font-size: large }
    pre     { margin-top: 0; margin-bottom: 0; vertical-align: top; font-size: large }
echo "$css"

Bash literature

They also have a pdf version.

A collection of pure POSIX sh alternatives to external processes

Search for ‘bash’ in hacker news.

pure bash bible

A collection of pure bash alternatives to external processes.

Other magic

remove duplicated lines without sorting

awk '!x[$0]++'