Bash Bash Parameter Expansion Substrings and subarrays
Example
var='0123456789abcdef'
# Define a zero-based offset
$ printf '%s\n' "${var:3}"
3456789abcdef
# Offset and length of substring
$ printf '%s\n' "${var:3:4}"
3456
4.2
# Negative length counts from the end of the string
$ printf '%s\n' "${var:3:-5}"
3456789a
# Negative offset counts from the end
# Needs a space to avoid confusion with ${var:-6}
$ printf '%s\n' "${var: -6}"
abcdef
# Alternative: parentheses
$ printf '%s\n' "${var:(-6)}"
abcdef
# Negative offset and negative length
$ printf '%s\n' "${var: -6:-5}"
a
The same expansions apply if the parameter is a positional parameter or the element of a subscripted array:
# Set positional parameter $1
set -- 0123456789abcdef
# Define offset
$ printf '%s\n' "${1:5}"
56789abcdef
# Assign to array element
myarr[0]='0123456789abcdef'
# Define offset and length
$ printf '%s\n' "${myarr[0]:7:3}"
789
Analogous expansions apply to positional parameters, where offsets are one-based:
# Set positional parameters $1, $2, ...
$ set -- 1 2 3 4 5 6 7 8 9 0 a b c d e f
# Define an offset (beware $0 (not a positional parameter)
# is being considered here as well)
$ printf '%s\n' "${@:10}"
0
a
b
c
d
e
f
# Define an offset and a length
$ printf '%s\n' "${@:10:3}"
0
a
b
# No negative lengths allowed for positional parameters
$ printf '%s\n' "${@:10:-2}"
bash: -2: substring expression < 0
# Negative offset counts from the end
# Needs a space to avoid confusion with ${@:-10:2}
$ printf '%s\n' "${@: -10:2}"
7
8
# ${@:0} is $0 which is not otherwise a positional parameters or part
# of $@
$ printf '%s\n' "${@:0:2}"
/usr/bin/bash
1
Substring expansion can be used with indexed arrays:
# Create array (zero-based indices)
$ myarr=(0 1 2 3 4 5 6 7 8 9 a b c d e f)
# Elements with index 5 and higher
$ printf '%s\n' "${myarr[@]:12}"
c
d
e
f
# 3 elements, starting with index 5
$ printf '%s\n' "${myarr[@]:5:3}"
5
6
7
# The last element of the array
$ printf '%s\n' "${myarr[@]: -1}"
f
