Command to append line to a text file without opening an editor

You can append a line of text to a file by using the >> operator:

echo "hello world" >> my_file.txt

or in your case

echo "alias list='ls -cl --group-directories-first'" >> config.fish

Please take note of the different types of quotes.

There's plenty of methods of appending to file without opening text editors, particularly via multiple available text processing utilities in Ubuntu. In general, anything that allows us to perform open() syscall with O_APPEND flag added, can be used to append to a file.

• GNU version of dd utility can append data to file with conv=notrunc oflag=append

  printf "\nalias list='ls -cl --group-directories-first'\n" | dd conv=notrunc oflag=append bs=1 of=config.fish
  

  Portably we could use something like this on the right side of pipeline:

  dd conv=notrunc seek=$(wc -c < testFile.txt) bs=1 of=testFile.txt
  

  Note the use of bs=1 , which is to prevent short reads from pipeline

• The tee command can be used when you need to append to file and send it to stdout or to next command in pipeline

  tee -a config.fish <<< "alias list='ls -cl --group-directories-first'"
  

• awk has append operator >> which is also portable and defined by POSIX specifications

  awk 'BEGIN{ printf "alias list=\x27ls -cl --group-directories-first\x27\n" >> "config.fish"  }'
  

• We can combine sed's flag $ to match the last line with a for appending and -i for in-place editing.

  sed -i '$a alias list='"'"'ls -cl --group-directories-first'"'" config.fish
  

• We could even implement something like dd in Python 3:

    #!/usr/bin/env python3
    # read bytes from stdin, append to specified file
    import sys

    with open(sys.argv[1],'ab') as f:
         f.write(sys.stdin.buffer.read())

See also:

• What's the difference between <<, <<< and < < in bash?
• What is the differences between &> and 2>&1
• How to redirect stderr to a file
• What is the difference between “cat < filename” and “cat filename”?
• What is the difference between “Redirection” and “Pipe”?

Adding to Stefano's answer, you can also use cat:

• Using a heredoc:

  $ cat >> config.fish <<'EOF'
  > alias list='ls -cl --group-directories-first'
  > EOF
  

  <<'EOF' means "take the following as input, until you reach a line that is just EOF". The quotes mean to take the input literally.

• Or inputting the line on stdin:

  $ cat >> config.fish
  

  Then paste or type in the line, press Enter to go to a new line, then press Ctrl+D to mark the end.