Bash¶

nano filename.sh
  #!/bin/bash
  myscriptcontent
chmod u+x filename.sh

Associative Arrays¶

https://clubmate.fi/associative-arrays-in-bash/

declare -A names=(
[caleb]=sargeant
[network]=guru
)

for i in "${!names[@]}"
do
  first_name=$i
  last_name=${names[$i]}
  echo "$first_name : $last_name"
done

Getopt¶

# Execute getopt for arguments to be passed to script, identified by "$@"
PARSED_OPTIONS=$(getopt -n "$0" -o hfi: --long "help,file,input:" -- "$@")

# Bad arguments or something wrong with getopt
if [ $? -ne 0 ]; then
  exit 1
fi

# getopt juju
eval set -- "$PARSED_OPTIONS"

# go through each option and use shift to analyse 1 argument at a time, discarding $1, making $2 $1, etc.
while true;
do
  case "$1" in
    -h|--help)
      echo "usage $0 -h -f -i or $0 --help --file --input"
      shift;;

    -f|--file)
      echo "file"
      shift;;

    -i|--input)
    echo "input"

      if [ -n "$2" ]; then
        echo "Arguement: $2"
      fi
      shift 2;;

    --)
    shift
    break;;
  esac
done

Usage¶

usage() {
printf "Usage: "$0" <region> \n
        Regions available:
        SA\t(South Africa)
        UK\t(United Kingdom)
        US\t(United States)\n\n" 1>&2;
        }

case $1 in
  (SA|UK|US) ;;
  #unhash the next line - it's just messing with RST
  #(*) usage
esac

if [ "$1" == "SA" ]; then
  export MYVAR="varSA"
elif [[ "$1" == "UK" ]]; then
  export MYVAR="varUK"
elif [[ "$1" == "US" ]]; then
  export MYVAR="varUS"
fi

Functions¶

function() {
  echo $@
}

# function accepts positional parameters via $@

sed¶

# Removes all comments including those with tabs and spaces
sed -e '/^[ \t\n]*#/d'

# Replacing Text (in this case remove ")
sed 's/"//g'

# Delete first line
sed -i -e 1d

awk¶

# Print the first column using "," as separator
awk -F "," '{print $1}'

date¶

https://www.tutorialkart.com/bash-shell-scripting/bash-date-format-options-examples/

# yyyymmdd
date +%Y%m%d

cut¶

# Remove the filename extension (eg. ".exe")
name=$(echo "$filename" | cut -f 1 -d '.')

Remove First Character¶

https://stackoverflow.com/questions/6594085/remove-first-character-of-a-string-in-bash

cut -c2-

Prompt for Input¶

Using read

Simple & common

while true; do
  read -p "Do you wish to install this program?" yn
    case $yn in
      [Yy]* ) make install; break;;
      [Nn]* ) exit;;
    * ) echo "Please answer yes or no.";;
  esac
done

Using select

No need to sanitize input
Prompts you with choice you want
Automatically loops (no need for while true loop to retry)

echo "Do you wish to install this program?"
select yn in "Yes" "No"; do
  case $yn in
    Yes ) make install; break;;
    No ) exit;;
  esac
done

Output Formatting¶

Source for more formatting options: http://misc.flogisoft.com/bash/tip_colors_and_formatting

echo -e "\e[1mbold\e[0m"
echo -e "\e[3mitalic\e[0m"
echo -e "\e[4munderline\e[0m"
echo -e "\e[9mstrikethrough\e[0m"
echo -e "\e[31mHello World\e[0m"

While True Loop¶

https://unix.stackexchange.com/questions/193352/is-using-while-true-to-keep-a-script-alive-a-good-idea

while true
do
  echo my commands
  sleep 1
done

For Loops¶

Loop through directory names https://unix.stackexchange.com/questions/86722/how-do-i-loop-through-only-directories-in-bash

for d in */ ; do
  echo "$d"
done

Loop through file names https://stackoverflow.com/questions/10523415/execute-command-on-all-files-in-a-directory

for file in /dir/* ; do
    cmd [option] "$file" >> results.out
done

Arithmetic¶

Bash is limited to integer math

Sum¶

Adding all numbers from output

https://stackoverflow.com/questions/450799/shell-command-to-sum-integers-one-per-line

awk '{sum+=$0} END{print sum}'

Divide¶

https://stackoverflow.com/questions/1088098/how-do-i-divide-in-the-linux-console

x=10
y=5

# Don't enclose variables in quotes this time
$ echo $(( $x / $y ))
2

Scientific Notation & Rounding¶

https://stackoverflow.com/questions/8356698/how-to-remove-decimal-from-a-variable https://unix.stackexchange.com/questions/104332/remove-scientific-notation-bash-script

# Round & remove scientific notation (0f is the number of decimals)
$ echo 2.123456 | awk '{ print sprintf("%.0f", $1); }'
2

# Round down
$ printf %.0f 1.89
2

Incrementing¶

$ echo $((n=n+1))
1
$ echo $((n=n+1))
2

wc¶

Number of Lines¶

wc -l myfile.txt

Number of Words¶

wc -w myfile.txt

Number of Characters¶

wc -m myfile.txt

Count Number of Lines of Output¶

https://unix.stackexchange.com/questions/72819/count-number-of-lines-of-output-from-previous-program

`command | tee >(wc -l)`

jq¶

https://stackoverflow.com/questions/52732473/how-to-pass-bash-variable-as-a-key-to-jq

Querying using a bash variable: jq ".$bash_var"

ls¶

https://stackoverflow.com/questions/14352290/listing-only-directories-using-ls-in-bash

https://stackoverflow.com/questions/5168071/list-sub-directories-with-ls

List Directories: ls -d */

https://stackoverflow.com/questions/7992689/how-to-loop-all-files-in-sorted-order-in-bash

Loop through sorted output: ls *.png | sort -V

tr¶

Remove whitespace

https://stackoverflow.com/questions/369758/how-to-trim-whitespace-from-a-bash-variable

| tr -d '[:space:]')

pwd¶

https://stackoverflow.com/questions/1371261/get-current-directory-name-without-full-path-in-a-bash-script

# Get current working directory as variable in bash
$ result=${PWD##*/}
$ echo $result
caleb.sargeant

# Using basename
$ pwd
/Users/caleb.sargeant
$ basename $(pwd)
caleb.sargeant

https://stackoverflow.com/questions/8426058/getting-the-parent-of-a-directory-in-bash

dir=/home/caleb.sargeant/Desktop/Test
parentdir="$(dirname "$dir")"

cat¶

Be careful with quotation with cat: https://stackoverflow.com/questions/12636170/bash-script-error-with-cat-and-if

tr¶

$ echo __ | tr _ -
--

if¶

Check if a file type exists in directory https://stackoverflow.com/questions/3856747/check-whether-a-certain-file-type-extension-exists-in-directory

count=$(ls -1 *.json 2>/dev/null | wc -l)
if [ $count != 0 ]; then
  echo true
fi

Check if variable not set

if [[ -z "$REGION" ]]; then
  echo "$REGION not set"
  exit 1
fi

Remove First Line¶

https://superuser.com/questions/284258/remove-first-line-in-bash

Search for Text in Files¶

grep -rnw '/etc/' -e 'NULL'

Extract Text Before Character¶

https://stackoverflow.com/questions/20348097/bash-extract-string-before-a-colon

# Replace colon with whatever:
cut -d: -f1
# or
awk -F: '{print $1}'
# or
sed 's/:.*//'

History¶

https://unix.stackexchange.com/questions/212872/how-to-get-last-n-commands-from-history

# Where n is the number of lines you want to "tail":
history n