by: Linux/Unix Raspberry Pi

This is a quick reference guide for creating automation scripts and using bash features to issue efficient and reliable commands.

Some entries include an example; the most straightforward ones do not.

BASIC USAGE OF VARIABLES, STRINGS, AND NUMBERS

echo [string]
[variable]=[string]
echo $[variable]
$(( [expression] ))

E.g. echo $((4*5))

Operators available:

  • +
    • Addition
  • –
    • Subtraction
  • *
    • Multiplication
  • /
    • Division
  • %
    • Remainder of a division
  • **
    • Exponentiation
$( bc -l <<< [expression] )

E.g. $( bc -l <<< 10/3 )

let [variable]=[expression]
declare -i [variable]
  • MAKING A VARIABLE ACCESSIBLE TO A CHILD PROCESS (LIKE ENVIRONMENT VARIABLES)
export [variable name]=[value]
  • EXECUTING A COMMAND IN THE BACKGROUND
[command] &
  • LISTING BACKGROUND PROCESSES
jobs
  • BRINGING A BACKGROUND PROCESS TO THE FOREGROUND
fg [job number]
  • RUN A SECOND COMMAND IF THE FIRST SUCCEEDS
[command1] && [command2]
  • RUN A SECOND COMMAND IF THE FIRST FAILS
[command1] || [command2]

CONDITIONAL STATEMENTS IN A SINGLE LINE

  • IF
if [condition]; then [command]; fi
if [condition]; then [command1]; else [command2]; fi
if [condition1]; then [command1]; elif [condition2]; then [command2]; else [command3]; fi
  • WHILE LOOP
while [condition]; do [command]; done
  • UNTIL LOOP (opposite of WHILE)
until [condition]; do [command]; done
  • FOR LOOP
for [variable] in [list]; do [command]; done

E.g. for name in John Mike Sarah; do echo "Hello, $name"; done

E.g. for X in {1..10}; do echo $X' >> output.txt; done

E.g. for X in $(cat list.txt); do echo $ID >> output.txt; done

for (( [initialisation]; [condition]; [increment] )); do [command]; done

E.g. for ((i=0; i<5; i++)); do echo $i; done

Note: For any of the loops above, consider using the following commands where applicable:

  1. “break” to exit a loop early.
  2. “continue” to skip the current iteration and move to the next one.

The loops above run one iteration at a time. The example below parallelizes execution, keeping a defined number of simultaneous processes running.

cat [source_file] | xargs -P [parallel] -I {} sh -c '{} >> [output_file]'

E.g. cat commands.list | xargs -P 10 -I {} sh -c '{} >> output.txt'

  • CASE
case [variable] in [pattern1]) [command1];; [pattern2]) [command2];; esac

E.g. case $option in "a"|"A") echo "a or A";; "b"|"B") echo "b or B";; *) echo "Not a valid option."; esac

  • SELECT
select [variable] in [list]; do [command]; done

E.g. select X in pwd uptime whoami; do $X; break; done

PROCESSING ARGUMENTS

  • GETOPTS – a built-in Bash utility for parsing short options (like -a, -b, -c) and their optional arguments from the command line.
getopts [options] [variable]

E.g.

#!/bin/bash
while getopts "a:b:c" opt; do
  case $opt in
    a) echo "Option -a, argument: $OPTARG" ;;
    b) echo "Option -b, argument: $OPTARG" ;;
    c) echo "Option -c, no argument" ;;
    ?) echo "Usage: $0 [-a arg] [-b arg] [-c]" ;;
  esac
done
  • SHIFT – shifts positional parameters to the left, removing N arguments and shifting all remaining arguments down by one.
shift N

E.g.

#!/bin/bash
while [ "$#" -gt 0 ]; do
  echo "Argument: $1"
  shift # Shift the arguments by one
done

SIGNALS AND STATUS

  • TRAP
trap [command] [signal]

E.g.

  • EXIT
exit [status]

E.g.

  • RETURN
return [status]

E.g.

SETTING PARAMETERS FOR THE SCRIPT EXECUTION ENVIRONMENT

  • SET/UNSET to exit the script if any command returns a non-zero exit status.
set -e
set +e
  • SET/UNSET to exit the script if any variable is accessed without being set.
set -u
set +u
  • SET/UNSET to print each command before it is executed.
set -x
set +x

REDIRECTING OUTPUTS

  • STANDARD OUTPUT (STDOUT)
[command] > [somewhere]
  • STANDARD ERROR (STDERR)
[command] 2> [somewhere]
  • REDIRECT STDERR INTO STDOUT
[command] > [somewhere] 2>&1
  • DISCARD BOTH OUTPUTS
[command] &> /dev/null
  • PIPING
[command] 2>&1 | tee [file]
  • WRITING TO A FILE
[command] > [file]

SCRIPTING AND AUTOMATION

  • SET THE INTERPRETER TO BASH
#!/bin/bash
  • MAKE THE SCRIPT EXECUTABLE
chmod +x [script name]
  • EXECUTE A SCRIPT
./[script name]
  • READ USER INPUT
read [options] [variable]

Available options (all optional):

  • -p
    • Display a prompt message.
  • -s
    • Hide the entered characters (e.g. for passwords).
  • -t
    • Time out after a specified number of seconds.
  • -d
    • Set a custom delimiter instead of the default newline.
  • -r
    • Allow special characters such as backslash.

E.g. read -p "Enter your name: " name

E.g. read -s -p "Enter your password: " password

E.g. read -t 10 -p "Enter your input within 10 seconds: " input

E.g. read -d '|' -p "Enter your name and age separated by '|': " name age

COPY PASTE TIPS

  • QUICK FORMATTING SNIPPET
BLACK=`tput setaf 0`
RED=`tput setaf 1`
GREEN=`tput setaf 2`
YELLOW=`tput setaf 3`
BLUE=`tput setaf 4`
MAGENTA=`tput setaf 5`
CYAN=`tput setaf 6`
WHITE=`tput setaf 7`

BOLD=`tput bold`
RESET=`tput sgr0`

echo -e "hello ${RED}some red text${RESET} world"
  • DIALOGUE BOXES
#!/bin/bash
function box_out() {
  input_char=$(echo "$@" | wc -c)
  line=$(for i in `seq 0 $input_char`; do printf "─"; done)
  tput bold
  space=${line//─/ }
  echo " ┌${line}┐"
  printf ' │ ' ; echo -n "$@"; printf "%s\n" ' │';
  echo " └${line}┘"
  tput sgr 0
}

function box_success() {
  tput bold
  tput setaf 2
  echo " ┌──────────┐"
  echo " │ Success! │"
  echo " └──────────┘"
  tput sgr0
}

function box_failed() {
  tput bold
  tput setaf 1
  echo " ┌─────────┐"
  echo " │ Failed! │"
  echo " └─────────┘"
  tput sgr0
}

box_out "Hello there!"
box_success
box_failed
  • QUICK SINGLE-LINE FORMATTED PRINTING
red() { echo -e "\033[31m [ $1 ] \033[0m" ; }
green() { echo -e "\033[32m [ $1 ] \033[0m" ; }
blue() { echo -e "\033[34m [ $1 ] \033[0m" ; }
yellow() { echo -e "\033[33m [ $1 ] \033[0m" ; }
purple() { echo -e "\033[35m [ $1 ] \033[0m" ; }
cyan() { echo -e "\033[36m [ $1 ] \033[0m" ; }

red_bold() { echo -e "\033[1;31m [ $1 ] \033[0m" ; }
green_bold() { echo -e "\033[1;32m [ $1 ] \033[0m" ; }
blue_bold() { echo -e "\033[1;34m [ $1 ] \033[0m" ; }
yellow_bold() { echo -e "\033[1;33m [ $1 ] \033[0m" ; }
purple_bold() { echo -e "\033[1;35m [ $1 ] \033[0m" ; }
cyan_bold() { echo -e "\033[1;36m [ $1 ] \033[0m" ; }

red_italic() { echo -e "\033[3;31m [ $1 ] \033[0m" ; }
green_italic() { echo -e "\033[3;32m [ $1 ] \033[0m" ; }
blue_italic() { echo -e "\033[3;34m [ $1 ] \033[0m" ; }
yellow_italic() { echo -e "\033[3;33m [ $1 ] \033[0m" ; }
purple_italic() { echo -e "\033[3;35m [ $1 ] \033[0m" ; }
cyan_italic() { echo -e "\033[3;36m [ $1 ] \033[0m" ; }

red_underline() { echo -e " \033[4;31m[ $1 ]\033[0m " ; }
green_underline() { echo -e " \033[4;32m[ $1 ]\033[0m " ; }
blue_underline() { echo -e " \033[4;34m[ $1 ]\033[0m " ; }
yellow_underline() { echo -e " \033[4;33m[ $1 ]\033[0m " ; }
purple_underline() { echo -e " \033[4;35m[ $1 ]\033[0m " ; }
cyan_underline() { echo -e " \033[4;36m[ $1 ]\033[0m " ; }
red "This is an error message."
green "This is a success message."
blue_bold "This is an informational message."
yellow_bold "This is a warning message."
purple_italic "This is a custom message."
cyan_underline "This is also a custom message."
  • BRACE EXPANSION
echo abc{mnp}xyz
mkdir num_{0..z}
  • CHECK EXIT STATUS OF THE PREVIOUS COMMAND
echo $?

OR

case $? in
  0) echo "OK";;
  1) echo "Err";;
  *) echo "Unhandled error";;
esac
  • RUNNING A SCRIPT IN THE CURRENT CONTEXT OR LOADING ITS VARIABLES AND FUNCTIONS
source ./scriptName.sh argument1 argument2

OR

. ./scriptName.sh argument1 argument2
  • RESTORING SESSION SETTINGS
. .profile
  • HIDE THE BLINKING CURSOR DURING SCRIPT EXECUTION AND RESTORE IT AFTERWARDS
tput civis
tput cnorm