DFT Wiki

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 straightforward 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 THE BACKGROUND

[command] &

• LISTING BACKGROUND PROCESSED

jobs

• BRINGING THE BACKGROUND PROCESS TO THE 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

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 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.

While the looping above will run one at a time, the example below can parallelize the execution of the commands, keeping a defined number of simultaneous executions.

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 used to parse short options (like -a, -b, -c) and their optional arguments from command-line arguments.

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 – this command is used to shift the positional parameters to the left, it removes N arguments and shifts all the 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]

• 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}

• CHECK EXIT STATUS OF PREVIOUS COMMAND

echo $?

OR

case $? in
  0) echo "OK";;
  1) echo "Err";;
  *) echo "Unhandled error";;
esac

• RUNNING SCRIPT IN CURRENT CONTEXT OR LOADING VARIABLES PLUS FUNCTIONS

source ./scriptName.sh argument1 argument2

OR

. ./scriptName.sh argument1 argument2

• RESTORING SESSION SETTINGS

. .profile

• HIDE BLINKING CURSOR DURING SCRIPT EXECUTION AND RESTORE AFTERWARDS

tput civis

tput cnorm