CODEWITHSUNDEEP
pythonbash
Bryce Thomas
Bryce Thomas

Reputation: 10799

Bash/Python: does `echo` command insert a newline when acting as input to another command?

I have a simple python script line_printer.py:

import fileinput
i = 1
for line in fileinput.input():
    print 'Line', str(i), ':', line.strip()
    i+=1

I'm trying to understand how piping data in from the echo command to this script affects the result versus reading the data in from file.

Consider the following call and output:

$ echo -e "chicken\ncow\npig"
chicken
cow
pig
$

This to me looks like echo has appended an invisble \n after the "g" in pig. So, how come when I call:

echo -e "chicken\ncow\npig" | python line_printer.py

I get:

Line 1 : chicken
Line 2 : dog
Line 3 : cow

as the output and not:

Line 1 : chicken
Line 2 : dog
Line 3 : cow
Line 4 :

At first I thought the behaviour of Python's fileinput module might be to discard the final line in a file if it is blank. But when I try using the contents of a file some_lines.txt:

chicken
dog
cow
<blank line>

as the input:

python line_printer.py some_lines.txt

The output I get is:

Line 1 : chicken
Line 2 : dog
Line 3 : cow
Line 4 :

So why does line_printer.py give different results on the input depending on whether it originated from stdin versus originated from a file? Best I can tell, both stdin (from echo) and the file (from some_lines.txt) finish with a \n, so I would either expect the output of both to include the Line 4 : or the output of neither to include it.

Upvotes: 2

Views: 11810

Answers (2)

Jonathan Ben-Avraham
Jonathan Ben-Avraham

Reputation: 4841

If you look in the bash source, bash-4.2/builtins/echo.def you can see that the builtin echo command always (line 113) outputs a final \n (line 194) unless the -n was specified (line 198) or output of echo is used as a string (line 166). You can test this by doing

echo `echo "Ho ho ho"` | od -c

You will see only one \n because the output of echo "Ho ho ho" is evaluated as a string in the expression echo `echo "Ho ho ho"`.

It doesn't seem to have any relation to the terminal setup.

Upvotes: 1

jim mcnamara
jim mcnamara

Reputation: 16389

This command will answer your question:

echo 'hi' | od -c

The reason for the trailing \n character is that stdout on a terminal by default uses line buffering - meaning it will only display output data that ends with the newline character.

Play around with the printf command:

printf "%s"  foo
printf "%s\n" anotherfoo

Upvotes: 1

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