Replace one substring for another string in shell script
To replace the first occurrence of a pattern with a given string, use ${parameter/pattern/string}
:
#!/bin/bash
firstString="I love Suzi and Marry"
secondString="Sara"
echo "${firstString/Suzi/"$secondString"}"
# prints 'I love Sara and Marry'
To replace all occurrences, use ${parameter//pattern/string}
:
message='The secret code is 12345'
echo "${message//[0-9]/X}"
# prints 'The secret code is XXXXX'
(This is documented in the Bash Reference Manual, §3.5.3 "Shell Parameter Expansion".)
Note that this feature is not specified by POSIX — it's a Bash extension — so not all Unix shells implement it. For the relevant POSIX documentation, see The Open Group Technical Standard Base Specifications, Issue 7, the Shell & Utilities volume, §2.6.2 "Parameter Expansion".
This can be done entirely with bash string manipulation:
first="I love Suzy and Mary"
second="Sara"
first=${first/Suzy/$second}
That will replace only the first occurrence; to replace them all, double the first slash:
first="Suzy, Suzy, Suzy"
second="Sara"
first=${first//Suzy/$second}
# first is now "Sara, Sara, Sara"