varname=value # defines a variable
varname=value command # defines a variable to be in the environment of a particular subprocess
echo $varname # checks a variable's value
echo $$ # prints process ID of the current shell
echo $! # prints process ID of the most recently invoked background job
echo $? # displays the exit status of the last command
read <varname> # reads a string from the input and assigns it to a variable
let <varname> = <equation> # performs mathematical calculation using operators like +, -, *, /, %
export VARNAME=value # defines an environment variable (will be available in subprocesses)
array[0]=valA # how to define an array
array[1]=valB
array[2]=valC
array=([2]=valC [0]=valA [1]=valB) # another way
array=(valA valB valC) # and another
${array[i]} # displays array's value for this index. If no index is supplied, array element 0 is assumed
${#array[i]} # to find out the length of any element in the array
${#array[@]} # to find out how many values there are in the array
declare -a # the variables are treated as arrays
declare -f # uses function names only
declare -F # displays function names without definitions
declare -i # the variables are treated as integers
declare -r # makes the variables read-only
declare -x # marks the variables for export via the environment
${varname:-word} # if varname exists and isn't null, return its value; otherwise return word
${varname:=word} # if varname exists and isn't null, return its value; otherwise set it word and then return its value
${varname:?message} # if varname exists and isn't null, return its value; otherwise print varname, followed by message and abort the current command or script
${varname:+word} # if varname exists and isn't null, return word; otherwise return null
${varname:offset:length} # performs substring expansion. It returns the substring of $varname starting at offset and up to length characters
${variable#pattern} # if the pattern matches the beginning of the variable's value, delete the shortest part that matches and return the rest
${variable##pattern} # if the pattern matches the beginning of the variable's value, delete the longest part that matches and return the rest
${variable%pattern} # if the pattern matches the end of the variable's value, delete the shortest part that matches and return the rest
${variable%%pattern} # if the pattern matches the end of the variable's value, delete the longest part that matches and return the rest
${variable/pattern/string} # the longest match to pattern in variable is replaced by string. Only the first match is replaced
${variable//pattern/string} # the longest match to pattern in variable is replaced by string. All matches are replaced
${#varname} # returns the length of the value of the variable as a character string
*(patternlist) # matches zero or more occurrences of the given patterns
+(patternlist) # matches one or more occurrences of the given patterns
?(patternlist) # matches zero or one occurrence of the given patterns
@(patternlist) # matches exactly one of the given patterns
!(patternlist) # matches anything except one of the given patterns
$(UNIX command) # command substitution: runs the command and returns standard output