by: Linux/Unix Raspberry Pi

This is a quick reference guide for creating automation scripts and clever averaging of bash features for issuing efficient and reliable commands.

Some might have an example while the most straight forward one won’t.

BASIC USAGE OF VARIABLES, STRINGS, AND NUMBERS

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

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

Operators available:

  • +
    • Addition
  • –
    • Subtraction
  • *
    • Multiplicaton
  • /
    • 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 BACKGROUND
[command] &
  • LISTING BACKGROUND PROCESSED
jobs
  • BRINGING THE BACKGROUND PROCESS TO FOREGROUND
fg [job number]
  • RUN A SECOND COMMAND IF THE FIRST IS SUCCESSFUL
[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

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

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

Note: for any of the looping above the consider using the following commands as applicable:

  1. “break” to exit a loop prematurely.
  2. “continue” to skip the current iteration of a loop and move to the next one.
  • 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
getopts [options] [variable]

E.g.

  • SHIFT
shift

E.g.

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]
  • OUTPUT STDERR INTO STDOUT
[command] > [somewhere] 2>&1
  • DUMPING BOTH OUTPUTS
[command] &> /dev/null
  • PIPING
[command] 2>&1 | tee [file]
  • WRITING TO A FILE
[command] > [file]

SCRIPTING AND AUTOMATION

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

Available options (they are not required):

  • -p
    • Prompts a message.
  • -s
    • Hides the entered characters as if they were secret.
  • -t
    • Times out after a number of seconds.
  • -d
    • Define the delimiter instead of the default new line.
  • -r
    • Allow input 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 PROMPT
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."
  • BRACES EXPANSION
echo abc{mnp}xyz
mkdir num_{0..z}
  • RUNNING SCRIPT IN CURRENT CONTEXT OR LOADING VARIABLES PLUS FUNCTIONS
source ./scriptName.sh argument1 argument2