What is the purpose of -e in sed command?

Question

I can't find any documentation about the sed -e switch, for simple replace, do I need it?

e.g.

sed 's/foo/bar/'

VS

sed -e 's/foo/bar/'

Answer 1

From the man page:

-e script, --expression=script

    add the script to the commands to be executed

So you can use multiple -e options to build up a script out of many parts.

$ sed -e "s/foo/bar/" -e "/FOO/d"

Would first replace foo with bar and then delete every line containing FOO.

Answer 2

This might work for you:

sed -e '/foo/i\' -e 'bar' -e '/fred/a\' -e 'barny' -e '/harry/c\' -e 'potter' file

In each case the i(insert),a(append) and c(change) commands need to be terminated by a newline.

Normally commands can be separated by a ; e.g. /foo/d;/bar/d and grouped by {...} e.g. /foo/{h;d} but for the i,a,c commands the -e provides a way of separating the commands.

The alternative is to use the shell(bash) to insert a newline:

sed '/foo/i\bar'$'\n''/fred/a\barney'$'\n''/harry/c\potter' file

Answer 3

In the help for sed you will find that -e flag stands for:

 -e script, --expression=script
              add the script to the commands to be executed

You are not the first to be confused after encountering -e. In your example sed 's/foo/bar/' and sed -e 's/foo/bar/' are equivalent. In both cases s/foo/bar/ is the script that is executed by sed. The second option is more explicit, but that is probably not the reason that you often see -e used. The reason for that is that -e makes it possible to use more than one script with the same invocation of sed. That still leaves the question why use it if you are using only one script? The following example might explain that:

echo "foo" | sed -e 's/foo/bar/' -e 's/bar/foo/'

Will output "foo".

However,

echo "foo" | sed 's/foo/bar/' -e 's/bar/foo/'

will fail because now sed interprets the first s/foo/bar/ as a filename. Hence, to make life easier for future you, you can already start using the -e flag today. That way future you can just append the command with an extra script by using -e <script>.

N.B. In other answers using ; as an alternative to use multiple scripts has been suggested. While that is technically possible in some cases, it is not encouraged, because it quickly leads to very hard to read code. See an example here: https://www.grymoire.com/Unix/Sed.html#uh-61

Answer 4

-e, equivalent to --expression, is optional, unless you are stringing several expressions together (not common) as shown in another Answer.

Aside: ; can be used instead in the same expression to execute several expressions one after the other.