sed

• insert a line at the start of a file:

sed -i '1s/^/your text\n/' file

• replace newline char:

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

• remove two adjacent lines (N; joins each line to the following line):

sed 'N;s/line 1\nline 2/replacement/g' filename

• cut file between lines:

sed -n '3598627,3607135p' /var/log/syslog

• insert a hash at the start of every line:

sed 's/^/#/' file.txt

• and then remove the hash:

sed 's/^#//g' file.txt

• insert a line after a match:

sed -i '/string_to_match.*/a string_to_insert' /file

• match pattern and print including trailing three lines:

sed -n '/somestring/,+3 p' filename

• match pattern and delete including trailing three lines:

sed '/somestring/,+3 d' filename