{string:position:length}

所以在这种情况下

$str="abcdefghijklm"
$echo "${str:0:5}"
abcde

查看其他用法：

$echo "${str:0}"      # default: start from the 0th position
abcdefghijklm
$echo "${str:1:5}"    # start from the 1th and get 5 characters
bcdef
$echo "${str:10:1}"   # start from 10th just one character
k
$echo "${str:5}"      # start from 5th until the end
fghijklm

取自：
– wooledge.org – How can I use parameter expansion? How can I get substrings? How can I get a file without its extension,or get just a file’s extension?
– Shell Command Language – 2.6.2 Parameter Expansion