# 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`