Mfcttrf

In the terminal of Ubuntu 18.04 LTS, when I write this command:

echo -e "Hello\n"

Output:

Hellon

But it should print, as the man page says:

Hello

That is, first print Hello, then , then a newline character. Where is the problem?

See these few echo -e commands and their output:

man420@man420-X542UQ:~$ echo -e "HElloaaaaaa\n"
HEllon
man420@man420-X542UQ:~$ echo -e "HEllo\n"
HEllon
man420@man420-X542UQ:~$ echo -3 "HElloa\n"
HEllon

Backslashes are treated specially by echo -e, but first they are sometimes treated specially by the shell (which in this case is bash), according to the shell's quoting rules.

The argument echo actually sees is Hello\n. Because you have passed -e to echo, it treats backslash escapes specially, and \ is collapsed to a single . The final n is not escaped, so it appears literally.

The reason for this is that, prior to and separately from the operation of echo itself, the shell treats specially in some contexts but not others. Unquoted characters are always treated specially, single-quoted are never treated specially, but the treatment of double-quoted characters, as in the command you ran, is more subtle and complicated.

When the shell encounters double-quoted text in a command, as with "Hello\n", it treats as an escape character when it precedes a character that can itself have special meaning inside double quotes and not otherwise.

1. Because sometimes has a special meaning inside "", a that immediately precedes another has the effect of quoting that second . So inside double quotes, the first \ are collapsed into .


2. After those first two characters, there is a third character which is unaffected by those first two and which precedes an n. But n is not a character that is ever treated specially inside double quotes, so the before it is not treated specially in this situation. Thus n stays n.

The effect is that, in double quotes, \n is interpreted as \n.

When echo -e sees \n, the first removes special meaning from the second, so echo prints n literally for that text.

One solution is to remove a . Running echo -e "Hello\n" outputs Hello with an extra newline at the end. A better solution is to use single quotes and write echo -e 'Hellon'. Single quotes are the strongest form of quoting. An even better solution is usually to use printf instead of echo, which in this case would be printf 'Hellonn'.