Bash

BASH variable scope in while loop

while read line; do echo "$line"; done < <(cat file.txt)
while read file; do echo "$file"; done <file.txt

character replacement with tr

• replace a single character with a newline character

cat file|tr "," "\n" # replace every comma with a new line character

• replace several subsequent identical characters with one occurence of the character

cat file|tr -s " " # replace group of spaces with a single space

• replace several subsequent identical characters with one occurence of another character

cat file|tr -s " " "," # replace every single space or group of spaces with a comma

internal variables

$$ pid of current bash process
$! pid of last job run in background
$? exit status of last command
$- flags passed to bash (using set)
$# number of command-line arguments
$* all of the positional parameters, seen as a single word
$@ same as $*, but each parameter is a quoted string

associative arrays

$ typeset -A names
$ names=(["Linus"]="Torvalds" ["Richard"]="Stallman")
$ for lastname in "${names[@]}"; do echo $lastname; done
Stallman
Torvalds
$ for firstname in "${!names[@]}"; do echo $firstname; done
Richard
Linus

$ declare -a letters
$ letters=(Z N E)
$ echo "amount of letters: ${#letters[*]}"
amount of letters: 3
$ for letter in "${letters[@]}"; do echo $letter; done
Z
N
E

indirect expansion

$ myvar="the content of myvar"
$ another_name="myvar"
$ echo ${!another_name}
the content of myvar

some tips

description	code	result
right pattern deletion	str="bash is great" echo ${str%%is*}	bash
left pattern deletion	str="bash is great" echo ${str##*is}	great
set upper case	str="bash is great" echo ${str^^}	BASH IS GREAT
set down case	str="BASH IS GREAT" echo ${str,,}	bash is great
pattern subsitution	str="bash is great, really great" echo ${str/great/fantastic}	bash is fantastic, really great
global pattern subsitution	str="bash is great, really great" echo ${str//great/fantastic}	bash is fantastic, really fantastic

shell parameter expansion

Below word can be one of:

• tilde expansion
• parameter expansion
• command substitution
• arithmetic expansion
• string

• when $parameter is null or unset => substitute with the expansion of word:

${parameter:-word}

• when $parameter is null or unset => assigns the expansion of word to parameter, substitute with $parameter

${parameter:=word}  

• when $parameter is null or unset => print to stderr the expansion of word and exit

${parameter:?word}

• when $parameter is not null or set => substitute with the expansion of word, otherwise substitute with nothing

${parameter:+word}