bash.md

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shells languages

Bash

Language

Variables

https://tldp.org/LDP/abs/html/parameter-substitution.html

a=1 # not a = 1
b=one

environment variables

export VAR_1=val_1
unset VAR_1
# to list
env
env VAR_1=123 command # run a "command" with VAR_1 set to 123
env -u VAR_1 command # run a "command" with VAR_1 unset

if/else/case

# man test
if [[ "a" = "a" ]]; then
  echo "a=a"
fi

if ! grep -q 'smth' file.txt; then
  echo -e "smth" >> file.txt
fi

var=123
case $var in
  123)
    echo 123
    ;;
  PATTERN_N)
    STATEMENTS
    ;;
  *)
    echo whatever
    ;;
esac

functions

func_name () {
  echo params: $*
  echo param1: $1
  echo from function
}

loops

for OUTPUT in $(Linux-Or-Unix-Command-Here)
do
    command1 on $OUTPUT
    command2 on $OUTPUT
    commandN
done

for VARIABLE in file1 file2 file3
do
    command1 on $VARIABLE
    command2
    commandN
done

IO

strings

https://www.gnu.org/software/bash/manual/html_node/Shell-Parameter-Expansion.html

s1="my_"
s1+="\nstring"
# -e honors newlines
echo -e $s1

file=sraka.scss
echo ${file/.scss/.css.scss}

for file in $(git ls-files  -- *.scss G -v css.scss); do; git mv $file ${file/.scss/.css.scss}; done

arrays/dictionaries

https://www.cyberciti.biz/faq/bash-for-loop-array/

array=( one two three )
from_cmd=( $(ls -1 *.lst) )
# or
declare -a ARRAY_NAME_HERE=(value1 value2 valueN)
echo "length: ${#ARRAY_NAME_HERE[@]}"

declare -A ARRAY_NAME_HERE
declare -A fruits
fruits[south]="Banana"
fruits[north]="Orange"
fruits[west]="Passion Fruit"
fruits[east]="Pineapple"

printf "%s\n" "${array[@]}"
printf "%s\n" "${files[@]}"
printf "%s\n" "${limits[@]}"
printf "%s\n" "${fruits[@]}"

for key in "${!fruits[@]}"; do
  echo "Key for fruits array is: $key"
done

for value in "${fruits[@]}"; do
  echo "Value for fruits array is: $value"
done

docstrings

a=ZZZ
cat << XML > test.file
  HERE-DOCUMENT $a
XML

cat << 'XML' > test.file
  HERE-DOCUMENT $a - no interpolation!
XML

program loadScriptFromString "$(cat << JS
console.log("hello")
JS
)"

if true; then
	cat <<- EOF
	++Line with a leading TAB --> TAB TAB TAB.
	EOF
fi

comments

regexp

redirects

redirect stderr to stdout

command 2>&1

redirect stderr to file

command 2>file.txt

CLI

CLI shortcuts

https://cheatography.com/clyde-stiller/cheat-sheets/bash-command-line-shortcuts/

Interpreter settings

option	description
set -e	abort if something went wrong
set -x (set -o xtrace)	print execution statements using PS4
set -u	raise an error about undefined variable

Misc

split line by delimiter: cut merge lines: paste calculator: bc

Examples

calculate the sum of numbers from file (line with needed numbers looks like this: # Offense count: 13”):

grep 'Offense count' .rubocop_todo.yml | cut -d' ' -f 4 | paste -s -d+ - | bc

or

grep 'Offense count' .rubocop_todo.yml | awk -F' ' '{s+=$4} END {print s}'