Demo shell script for blog post: https://janstuemmel.de/2019/08/15/shell-scripting.html

script.sh

#!/bin/sh
# USAGE: execute with e.g.: BAZ=hello ./script.sh 5 1

#
## Variables
#
FOO=foo
${BAR:-bar} # default value "bar" for BAR, can be set from outside the script

echo $FOO $BAR # prints: foo bar
echo ${FOO}baz # prints foobaz
echo ${FOO} # prints string length of "foo"

${BAZ:?variable BAZ missing} # exits script if variable BAZ is missing 

echo $BAZ

#
## Parameters
#
echo Scriptname: $0
echo Number of args: $#
echo All args: $@
echo All args expanded: "$@"
echo First arg: $1
echo Second arg: $2
echo Process number: $$
echo Last command exit code: $?
sh -stfu 2> /dev/null # silent stderr
echo Last command exit code: $?

#
## Conditionals
# 
if [ $1 -le 2 ] 
  then echo "$1 <= 2"
elif [ $1 -le 4 ]
  then echo "$1 <= 4"
else
  echo "i can't count to $1"
fi

# numbers
if [ 1 -eq "1" ]; then echo "1 == 1"; fi  # equal
if [ 1 -ne 2 ]; then echo "1 != 2"; fi    # not equal
if [ 2 -gt 1 ]; then echo "2 > 1"; fi     # greater then
if [ 2 -ge 2 ]; then echo "2 >= 2"; fi    # greater equal
if [ 1 -lt 2 ]; then echo "1 < 2"; fi     # lower then
if [ 1 -le 1 ]; then echo "1 <= 2"; fi    # lower equal

# strings
if [ "foo" = "foo" ]; then echo '"foo" == "foo"'; fi
if [ "foo" != "bar" ]; then echo '"foo" != "bar"'; fi
if [ -z "" ]; then echo '"" is empty'; fi
if [ -n "foo" ]; then echo '"foo" is not empty'; fi

# always use "$VAR" in quotes if you checking string variables
if [ "$1" = "5" ]; then echo "$1 == 5"; else echo "$1 != 5"; fi

# check folders and files
if [ -f "$0" ]; then echo "me exists"; fi
mkdir foo
if [ -d "./foo" ]; then echo "folder exists"; fi
rm -r foo
if [ ! -d "./foo" ]; then echo "folder removed"; fi

#
## Loops
# 
# while loop with break condition
i=0
while [ $i -le 10 ]
do
  echo $i
  i=$(( $i + 1 ))
done

# infinite loop 
while [ : ]
do 
  echo 1
  break # we don't want you to run infinite :)
done

# for loop with list [one, two, three]
for i in one two three
do
  echo $i
done

# for loop with list from command (all files/dirs in $PWD that end with .sh)
for i in $(ls *.sh)
do 
  echo $i
done

# loop through all file names in $PWD that end with .sh
for i in *.sh
do 
 echo $i
done

# loop through all command line arguments
for i in $@
do
  echo $i
done

# same as above, but shorter
for i
do
 echo $i 
done

#
## Functions
#
hello() {
  echo "hello $@"
}

hello world !