mjhjmtu

echo "abc" >>file.txt puts a newline after abc, not before. If you end up with abc on its own line, that means that the newline before abc was already present in file.txt.

Note that it is perfectly normal for a text file to end in a newline. On unix, a line consists of a sequence of characters other than null⁰ or newline followed by a newline.¹ Therefore any non-empty text file ends with a newline character.

If you want to add text to the last line of a file, then you can't do it with >>, because this always appends to the file, so it always writes after the last newline. Instead you need a tool that is capable of modifying an existing file. For example, you can use sed:

sed '$ s/$/abc/' file.txt >file.txt.new && mv file.txt.new file.txt

In the sed command, the first $ means “do the following command only on the last line”, the command s/REGEX/REPLACEMENT/ replaces REGEX by REPLACEMENT, and the regular expression $ matches at the end of the line.

Linux's sed command has a built-in feature to automate this create-new-file-and-replace sequence, so you can shorten that to

sed -i '$ s/$/abc/' file.txt

⁰ _{That's a null byte, which ASCII calls NUL and Unicode calls U+0000. Text processing programs may or may not cope with this character.}
¹ _{See the definitions of Text File, Line, and Newline Character in the "Definitions" section of the Base Definitions chapter of IEEE 1003.1-2008:2016.}

I don't think it's possible with echo command, use the following sed approach instead:

sed -i '$ s/$/abc/' file.txt

• 
  -i - modify the file inplace
  


• 
  $ - indicate the last record/line


• 
  s/$/abc/ - substitute the end of the line $ with substring abc (for the last record)

If you have the truncate command and your text file has NL as its last character you can remove it and then append your text like this:

truncate --size -1 file.txt
echo "abc" >>file.txt

(Note that truncate cares nothing about file content, and in this example simply reduces the file size by one byte. If your last character is not a single byte, ie it's a multi-byte "wide" character then you will introduce corruption.)

Assuming that the file does not already end in a newline and you simply want to append some more text without adding one, you can use the -n argument, e.g.

echo -n "some text here" >> file.txt

However, some UNIX systems do not provide this option; if that is the case you can use printf, e.g.

printf %s "some text here" >> file.txt

(the initial %s argument being to guard against the additional text having % formatting characters in it)

From man echo (on macOS High Sierra):

-n

Do not print the trailing newline character. This may also be achieved by appending 'c' to the end of the string, as is done by iBCS2 compatible systems. Note that this option as well as the effect of 'c' are implementation-defined in IEEE Std 1003.1-2001 ("POSIX.1") as amended by Cor. 1-2002. Applications aiming for maximum portability are strongly encouraged to use printf(1) to suppress the newline character.

What you want is to add it at the end of the last line, so just before the delimiter of that last line, so just before the last character of the file.

With ksh93, you can do:

echo abc 1<> file >#((EOF - 1))

Where 1<> is the standard operator to open a file in read+write mode (and more importantly without truncation) on stdout and >#((...)) is a ksh93-specific seeking operator (here to seek to before the last character). Note that echo writes abc<newline> where the a overwrites the newline that was there and echo adds its own newline.

The zsh equivalent:

zmodload zsh/system
sysseek -w end -u 1 -1; echo abc 1<> file

Probably a UUOC but you could also do:

echo "$(cat file.txt)abc" >file.txt

As Gilles points out this command is limited:

This mostly works, but not if there is sometimes a blank line at the end of the file and a blank line immediately before. E.g. a file with a fixed number of lines, where the last line is initially empty and is extended over time, and the next-to-last-line may sometimes be empty.

Additionally be wary of using cat on files you are not familiar with:

• CVE-2003-0063


• CVE-2008-2383


• CVE-2010-2713


• CVE-2003-0020


• CVE-2012-3515


• More

Conclusion:

Use sed