Display information about command type.
type: type [-afptP] name [name ...]
test
help test
[[ hello == world ]]; echo $?
[[ hello =~ el ]]; echo $?
[[ "hello world" =~ "lo wor" ]]; echo $? # if the string contain ,
[[ "hello world" =~ ^h ]]; echo $? # if the string starts with an h,
[[ "hello world" =~ ^h.+d$ ]]; echo $? # if the string starts with an h, has at least one character after it, and ends with a d.
[[ $VAR =~ \?$ ]]; echo $? # if the string end with an ?
Print all environment variables
printenv
View all variables in the shell with
declare -p [-p stands for print]
let: let arg [arg ...] Evaluate arithmetic expressions.
Evaluate each ARG as an arithmetic expression. Evaluation is done in
fixed-width integers with no check for overflow, though division by 0
is trapped and flagged as an error. The following list of operators is
grouped into levels of equal-precedence operators. The levels are listed
in order of decreasing precedence.
id++, id-- variable post-increment, post-decrement
++id, --id variable pre-increment, pre-decrement
-, + unary minus, plus
!, ~ logical and bitwise negation
** exponentiation
*, /, % multiplication, division, remainder
+, - addition, subtraction
<<, >> left and right bitwise shifts
<=, >=, <, > comparison
==, != equality, inequality
& bitwise AND
^ bitwise XOR
| bitwise OR
&& logical AND
|| logical OR
expr ? expr : expr
conditional operator
=, *=, /=, %=,
+=, -=, <<=, >>=,
&=, ^=, |= assignment