SED Command – Why Sed Replace is Not Working

sed

sed -i 's/#DirectoryIndex index.html/#DirectoryIndex index.php /etc/httpd/conf/httpd.conf'

which gives out the error

sed: -e expression #1, char 57: unknown option to `s'

I am trying to replace index.html with index.php

You need to close the string before the filename. And you missed the closing /.

sed -i 's/#DirectoryIndex index.html/#DirectoryIndex index.php/' /etc/httpd/conf/httpd.conf

Jay's answer will work almost unchanged with sed:

sed -i 's/^Defaults[ \t]+requiretty/# Defaults requiretty/g' /etc/ssh/sshd_config

No need to pipe cat to sed:

$ sed 's/<a href="\/tag\//<a href="http:\/\/www.mydomain.com\/tag\//g' wordpress_posts.sql