text file command Help me!!! please

[slackman]

i have text file is ("test.txt") on linux fedora core system.

####### inside text file ########
12345
56789
101112


condition
1. i want to add line into text.txt in the line tail that but without to open text editor such as nono,pico
2. i try to user "echo 555555 > text.txt" it's not working . because old data that missing.
3. and command line to add line it that need standard command line on linux.

####### inside text file ########
12345
56789
101112
555555 <<-- new line (i want to add this line without open text editor)

*** if you can use command fix and condition to adding line include line number that very very thanks.

[devils casper]

echo 555555 >> test.txt