Linux command: sed

The sed command can be used for varius things see man page:
sed command man page
An example of use of sed to replace characters:
echo <string to replace> | sed <regular expression>
Regular expresion:

s -> replace.
g -> all the 'e' are replaced with 'R'. without g only one 'e' is replaced.
# -> delimitation character can be: {|, /, #}.

Example:

echo "esto es un ejemplo" | sed 's#e#R#g'

Replace New Line Characters

sed ':a;N;$!ba;s/\n/ /g'

This will read the whole file in a loop, then replaces the newline(s) with a space.
Explanation:
Create a label via :a.
Append the current and next line to the pattern space via N.
If we are before the last line, branch to the created label $!ba ($! means not to do it on the last line as there should be one final newline).
Finally the substitution replaces every newline with a space on the pattern space (which is the whole file).

Here is cross-platform compatible syntax which works with BSD sed (as per @Benjie comment):

sed -e ':a' -e 'N' -e '$!ba' -e 's/\n/ /g'

Replace New Line (\n) with ", "

sed -e ':a' -e 'N' -e '$!ba' -e 's/\n/", "/g'

tr -d '\n'

Uppercase to lowercase

sed 's/.*/\L\1/g' < input

Mongo operators to Python List

cat mongodb_operators.txt | grep -E ^\\\$ | awk '{ print "\x27" $1 "\x27" }' | sed s/\\\$//g | sed ':a;N;$!ba;s/\n/, /g'

Print some line of a file

To print line 45 from file rigodon-22011715-3.jl.ok

sed -n 45p rigodon-22011715-3.jl.ok

To print multiple lines:

sed -n -e 5p -e 8p file

To print a range of lines:

sed -n 5,8p file

To print a range and some specific file:

sed -n -e 5,8p -e 10p file

Remove empty lines

sed -i '/^$/d' file.txt