Unix substring

${#string} - Length of String

${string:position} - extract substring from $string at $position

-- position argument starts at 0, not 1.

${string/substring/replacement} - replace first match of $substring with $replacement

${string//substring/replacement} - replace all matches of $substring with $replacement
${string/#substring/replacement} - If $substring matches front end of $string, substitute $replacement for $substring
${string/%substring/replacement} - If $substring matches back end of $string, substitute $replacement for $substring

${string#substring} - Strip shortest match of $substring from front of $string

${string##substring} - Strip longest match of $substring from front of $string

${string%substring} - Strip shortest match of $substring from back of $string

${string%%substring} - Strip longest match of $substring from back of string

EXAMPLES
file=jack_of_all_trades
echo ${#file}
18

echo ${file:12}

trades

echo ${file:5:2}

of

echo ${file/of/OF}

jack_OF_all_trades

echo ${file//a/A}

jAck_of_All_trAdes

echo ${file:0:4}
jack

echo ${file/#j/A}

Aack_of_all_trades

echo ${file/%s/A}

jack_of_all_tradeA

file=aaa_bbb_ccc_aaa

echo ${file#aaa}

_bbb_ccc_aaa

echo ${file%aaa}

aaa_bbb_ccc_