echo does not print command output as expected

Question

This following command prints nothing on my machine

echo `python3.8 -c 'print ("*"*10)'`

whereas

python3.8 -c 'print ("*"*10)'

does. Why?

What OS/shell? I get `**********` in zsh (macOS default) and a list of the things in my current directory in bash, for example. — jonrsharpe, Oct 25 '21 at 13:21
Unless you are trying to embed the output of Python in a larger string, capturing the output only to write it to standard output again with `echo` isn't necessary. — chepner, Oct 25 '21 at 13:32
Similar: [When should I wrap quotes around a shell variable?](https://stackoverflow.com/questions/10067266/when-to-wrap-quotes-around-a-variable); relevant: [Security implications of forgetting to quote a variable in bash/POSIX shells](https://unix.stackexchange.com/questions/171346/security-implications-of-forgetting-to-quote-a-variable-in-bash-posix-shells) — glenn jackman, Oct 25 '21 at 14:17

mkrieger1 · Accepted Answer · 2021-10-25T13:33:51.673

3

The first example does command substitution for the argument to echo and is equivalent to the command echo **********. This happens to output a list of directory contents, since ********** seems to be equivalent to *¹ and is expanded by the shell.

If you want to prevent the shell to expand **********, you need to quote it:

echo "`python3.8 -c 'print ("*"*10)'`"

which is equivalent to echo "**********".

¹_{don't quote me on that (pun intended)}

edited Oct 25 '21 at 13:33

answered Oct 25 '21 at 13:28

mkrieger1

19,194
5
54
65

2

But of course, that's a [useless use of `echo`](https://www.iki.fi/era/unix/award.html#echo) – tripleee Oct 25 '21 at 13:32
Interestingly, in an empty directory, `echo *` does output `*`. – mkrieger1 Oct 25 '21 at 13:44
2

That depends on your glob settings. Bourne shell or Bash out of the box yes; Bash with `nullglob` no. – tripleee Oct 25 '21 at 13:45

score 1 · Answer 2 · answered Oct 25 '21 at 13:30

1

`expr` means to evaluate the contents between ' as a command and replace it with the result.

`print(*)` is equal to ` * ` which is evaluated as all the files in the current directory

The output really depends on the machine used

answered Oct 25 '21 at 13:30

Mar

167
1
8

expr? that does not appear in the question. – glenn jackman Oct 25 '21 at 14:18
expr as in expression, _any expression_ – Mar Oct 25 '21 at 14:23
Oh, I see you mean any expression between backticks. That could be more clearly explained, particularly as you then show a single quote. – glenn jackman Oct 25 '21 at 15:52

echo does not print command output as expected

2 Answers2