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}:

firstString="I love Suzi and Marry"
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”.

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