Replace One Substring for Another String in Shell Script

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"

For Dash all previous posts aren't working

The POSIX sh compatible solution is:

result=$(echo "$firstString" | sed "s/Suzi/$secondString/")

This will replace the first occurrence on each line of input. Add a /g flag to replace all occurrences:

result=$(echo "$firstString" | sed "s/Suzi/$secondString/g")

Try this:

 sed "s/Suzi/$secondString/g" <<<"$firstString"

It's better to use Bash than sed if strings have regular expression characters.

echo ${first_string/Suzi/$second_string}

It's portable to Windows and works with at least as old as Bash 3.1.

To show you don't need to worry much about escaping, let's turn this:

/home/name/foo/bar

Into this:

~/foo/bar

But only if /home/name is in the beginning. We don't need sed!

Given that Bash gives us magic variables $PWD and $HOME, we can:

echo "${PWD/#$HOME/\~}"

Thanks for Mark Haferkamp in the comments for the note on quoting/escaping ~.*

Note how the variable $HOME contains slashes, but this didn't break anything.

Further reading: Advanced Bash-Scripting Guide.
If using sed is a must, be sure to escape every character.

If tomorrow you decide you don't love Marry either she can be replaced as well:

today=$(</tmp/lovers.txt)
tomorrow="${today//Suzi/Sara}"
echo "${tomorrow//Marry/Jesica}" > /tmp/lovers.txt

There must be 50 ways to leave your lover.

echo [string] | sed "s/[original]/[target]/g"

• "s" means "substitute"
• "g" means "global, all matching occurrences"

Since I can't add a comment. @ruaka To make the example more readable write it like this

full_string="I love Suzy and Mary"
search_string="Suzy"
replace_string="Sara"
my_string=${full_string/$search_string/$replace_string}
or
my_string=${full_string/Suzy/Sarah}

Using AWK:

firstString="I love Suzi and Marry"
echo "$firstString" | awk '{gsub("Suzi","Sara"); print}'

Pure POSIX shell method, which unlike Roman Kazanovskyi's sed-based answer needs no external tools, just the shell's own native parameter expansions. Note that long file names are minimized so the code fits better on one line:

f="I love Suzi and Marry"
s=Sara
t=Suzi
[ "${f%$t*}" != "$f" ] && f="${f%$t*}$s${f#*$t}"
echo "$f"

Output:

I love Sara and Marry

How it works:

• Remove Smallest Suffix Pattern. "${f%$t*}" returns "I love" if the suffix $t "Suzi*" is in $f "I love Suzi and Marry".

• But if t=Zelda, then "${f%$t*}" deletes nothing, and returns the whole string "I love Suzi and Marry".

• This is used to test if $t is in $f with [ "${f%$t*}" != "$f" ] which will evaluate to true if the $f string contains "Suzi*" and false if not.

• If the test returns true, construct the desired string using Remove Smallest Suffix Pattern ${f%$t*} "I love" and Remove Smallest Prefix Pattern ${f#*$t} "and Marry", with the 2nd string $s "Sara" in between.