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

  • assigments: 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
}