Math

• bc -l
• expr: there must be spaces between expression parts
• \*: * has a special meaning to Bash so it needs to be escaped
• let
• calculation: $((b++)) includes assignment (assignment first then evaluation), ((b++)) no assignment only evaluation

Commands

• command substitution:

  $(command)
  `command`
  

• (): runs commands in a subshell
• {}: runs commands in the current shell

Variables

• assignments: no space on either side of the equal sign
• global by default
• $@: list of arguments; $1, $2, etc: first, second, etc. argument; $#: number of arguments

User input

• read

Logic and If-else

• $?: last exit status. && continues only after $? == 0, || continues only after $? != 0
• conditional expression: [ ] POSIX compliant, [[ ]] extension to the standard, supporting extra operations
• logical flags:
  • [[ 4 -gt 3 ]] && echo t || echo f : prints t;
  • [[ -e math.sh ]] : checks if a file exists;
  • -d : a dir exists;
  • -z, -n: string length is zero, non-zero
• logical operators:
  • regex: [[ "sean" =~ ^s.+n$ ]] && echo t || echo f : prints t;
  • NOT: [[ ! 7 -gt 2 ]] && echo t || echo f : prints f;
  • =, != : string equal, not equal

Arrays

• indexed and associative
• init: a=(1 2 3), declare -a a=(1 2 3) (indexed), declare -A (associative), declare: local by default
• get: ${a[0]}, ${a[*]} expanding to one single argument, ${a[@]} expanding to distinct arguments, a part: ${a[*]:1:2} : 2 3, length: ${#a[*]} : 3
• set: a[0]=0
• concat: a+=(3 4 5)

Braces

• sequence: { .. }, a{0..4}c, {1..3}{a..c}; with variables: start=4; end=9; eval echo {$start..$end} : 4 5 6 7 8 9
• combine: {{1..3},{a..c}}

Loops

for i in 1 2 3
do
  ...
done

while [[ ]]
do
  ...
done

Functions

#f () {
function f {
  local result=0
  ...
  echo $result
}