Shell scripting - cheatsheet

#!/usr/bin/env bash

#!/bin/sh

#!/usr/bin/env zsh

echo "suppressed" 2>&1 > /dev/null

# [[...]] would be non POSIX, even if it is preferred for bash/zsh!
if [ -d "${DIR}" ]; then
  echo "${DIR} exists"
elif [ -d "${DIR2}" ]; then
  echo "${DIR2} exists" 
else
  echo "${DIR} and ${DIR2} missing"
fi

# short version
[ -d "${DIR}" ] && ( echo "${DIR} exists" ) || ( echo "${DIR} missing" );

# if NOT (negation)
[ ! -d "${DIR}" ] && ( echo "${DIR} missing" );

day=$(date +%w) 
case "${day}" in
  0|\
  7) echo "Weekend - yay!" 
    ;;
  6) echo "Almost made it!" 
    ;;
  *)   
    echo "Oh no..."   
    ;;
esac

for i in 1 2 3 4 5; do
   echo "$i"
done

i=0
while true; do 
  i=$((i+1))
  if [ $i -lt 2 ] && (continue)
  echo $i
  [ $i -gt 3 ] && (break)
done

function append { 
  echo "${1}${2}" # returning strings only with echo
  return 0 # real return value stored in $?
}
VALUE=$(append "123" "456")
echo $?

#truncate file (empty file is kept)
> /tmp/test.txt

# overwrite existing contents
echo "test-overwrite" > /tmp/file.txt

# append content
echo "test-append" >> /tmp/file.txt

# append multiple lines
cat <<EOF >> /tmp/file.txt
multiple
lines
EOF

# overwrite contents with multiple lines and sudo permissions
sudo tee /tmp/file.txt > /dev/null <<EOT
multiple
lines
EOT

# append multiple lines with sudo permissions
sudo tee -a /tmp/file.txt > /dev/null <<EOT
multiple
lines
EOT

# search for pattern in file
grep 'pattern' file

# search for pattern recursively in my_directory
grep -R 'pattern' my_directory

# search for multiple patterns
grep 'pattern1\|pattern2' file

# search for NON matches (invert search)
grep -v 'pattern' file

# search without printing found matches
grep -q 'pattern' file && (echo "pattern found")

# comment out matching line (dtparam=audio=on => #dtparam=audio=on)
FILE_TO_REPLACE="/boot/config.txt"
sed -i 's/^dtparam=audio=on/#dtparam=audio=on/g' "${FILE_TO_REPLACE}"

# replace matching line (value=hello => value=replaced)
FILE_TO_REPLACE="/tmp/test.txt"
sed -i 's/^value=.*/value=replaced/g' "${FILE_TO_REPLACE}"

# append line AFTER match (dtoverlay=hifiberry-dac after #dtparam=audio=on) 
FILE_TO_REPLACE="/boot/config.txt"
sed -i '/^#dtparam=audio=on/a dtoverlay=hifiberry-dac/' "${FILE_TO_REPLACE}"

# insert a line BEFORE match (dtoverlay=hifiberry-dac before #dtparam=audio=on)
FILE_TO_REPLACE="/boot/config.txt"
sed -i '/^#dtparam=audio=on/i dtoverlay=hifiberry-dac/' "${FILE_TO_REPLACE}"

# insert multiline before a match (using `\n`)
FILE_TO_REPLACE="/boot/config.txt"
sed -i '/^#dtparam=audio=on/i dtoverlay=hifiberry-dac\n#another=value/' "${FILE_TO_REPLACE}"

[ -d "${DIR}" ] || (mkdir -p "${DIR}")

cd /tmp/ && touch "testfile.txt" && cd -

SAVEIFS=$IFS
IFS=$(echo -en "\n\b")
FILES=/data/*
for f in $FILES; do
  echo "$f"
done
IFS=$SAVEIFS # restore $IFS

find . -print0 | while read -d $'\0' file; do
  echo -v "$file"
done

# linux only (unix uniq works different)
# skip 0 byte files (! -empty)
# only files, not dirs (-type f)
# create md5 checksum (md5sum {})
# sort by md5 checksum
# compare first 32 chars (only hash, exclude filename) of only duplicate lines and all of them (uniq -w32 -dD)
find "." ! -empty -type f -exec md5sum {} + | sort | uniq -w32 -dD

# simple operations (only integers)
i=0
i=$((i+1))
i=$((i-1))

# more complex (including floats)
var=$(echo "1+2"|bc)

ASTRING="a-string-with-$VARIABLE"

cat <<EOF
testing
EOF

(
cat <<EOF
testing
EOF
) | grep 'testing'

# (result: 1 for match, 0 for no match)
grep -oc 'yes' <<< "Available: yes"

tr -s ' ' <<< "this  is   a    test"

PADLEN=15
echo "to_pad" | sed -e :a -e "s/^.\{1,$PADLEN\}\$/ &/;ta" ; 

ls -l *.jpg | wc -l

export -f my_function
find . -name '*.jpg' -exec bash -c 'myfunction "$0"' {} \;

find . -iregex '.*\.\(jpg\|svg\|gif\|png\|jpeg\)$' 

find . -name '*' -type f -not -path '*.mp3'