Shell Programming and Scripting

b=${a/23/BB} # Substitute "BB" for "23".

${parameter/pattern/string}

The pattern is expanded to produce a pattern just as in filename expansion. Parameter
is expanded and the longest match of pattern against its value is replaced with string.
If pattern begins with ‘/', all matches of pattern are replaced with string. Normally only
the first match is replaced. If pattern begins with ‘#', it must match at the beginning of
the expanded value of parameter. If pattern begins with ‘%', it must match at the end of
the expanded value of parameter. If string is null, matches of pattern are deleted and the
/ following pattern may be omitted. If parameter is ‘@' or ‘*', the substitution operation is
applied to each positional parameter in turn, and the expansion is the resultant list. If
parameter is an array variable subscripted with ‘@' or ‘*', the substitution operation is
applied to each member of the array in turn, and the expansion is the resultant list.

Last login: Sat Nov 23 11:02:56 on ttys000
AMIGA:barrywalker~> echo -n 1234567890 > /tmp/string.txt
AMIGA:barrywalker~> hexdump -C < /tmp/string.txt
00000000  31 32 33 34 35 36 37 38  39 30                    |1234567890|
0000000a
AMIGA:barrywalker~> echo -n 1234567890.0987654321 > /tmp/string.txt
AMIGA:barrywalker~> hexdump -C < /tmp/string.txt
00000000  31 32 33 34 35 36 37 38  39 30 2e 30 39 38 37 36  |1234567890.09876|
00000010  35 34 33 32 31                                    |54321|
00000015
AMIGA:barrywalker~> _

b=${a/23/BB}

root@OpenWrt:~# a=1234
root@OpenWrt:~# echo ${a/23/BB}
1BB4

root@OpenWrt:~# a=12341234
root@OpenWrt:~# echo ${a/23/BB}
1BB41234
root@OpenWrt:~# echo ${a//23/BB}
1BB41BB4