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For a file in sysfs, I tried using 

echo 45>target_size

to set the value to 45. This failed, with an invalid entry. However, if I do:

x=45;echo $x>target_size

it works fine. Further, if I use a very big number, it gives me an error about having an invalid file number. So apparently when you echo a straight number, the terminal is interpreting that as a file descriptor, rather than a literal number. What is the proper way to echo a number?

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  • Welcome to AU! What do you mean by "This failed, with an invalid entry"? What makes you this didn't work?
    – MadMike
    Jan 24, 2017 at 14:14
  • If I write to the sysfs file, it prints a message if it receives something, saying "Attempting to set target size to ___" with whatever you put in, and then if its a valid number it eventually says "Target size now set to ___". But if I just use echo ___, it doesn't print anything. So it's apparently treating $x and the plain number as different, even though they ought to be the same thing.
    – Zephyr
    Jan 24, 2017 at 14:20

2 Answers 2

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It's a matter of how the shell parses your command, I think. When you do 

echo 45>target_size

the shell redirects file descriptor 45 to filetarget_size and then executes echo with no arguments.

If instead you do

echo 45 >target_size

it redirects the standard output descriptor to file target_size and then executes echo 45.

TL;DR add whitespace before the redirection operator.

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  • 2
    Yep, exactly right. Should be worth noting that this seems to be bash-specific. ksh and dash both echo the number into file without issues
    – Sergiy Kolodyazhnyy
    Jan 24, 2017 at 14:21
  • Just checked in with the bash terminal, and this does indeed seem to be it. That's an interesting quirk that the space matters that much. Thanks!
    – Zephyr
    Jan 24, 2017 at 14:28
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    Not so quirky: the shell is very whitespace sensitive. Whitespace is used to separate commands/keywords/arguments. You need whitespace within [[ ]] to separate operators/operands
    – glenn jackman
    Jan 25, 2017 at 1:49
3

@steeldriver's answer is correct - the space between number and redirection operator matters. It seems to be an inconsistent behavior, as in my tests echo 45>output.txt writes 45 to file on ksh93 and dash shells.

As alternative you could always use tee command with here-string operator <<<. 

tee  output_file.txt <<< "45"

Or with those shells that don't have <<< operator, echo 45 | tee output_file.txt

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  • tee is not a builtin thus this solution is slower
    – Tomáš Nesrovnal
    Jan 24, 2017 at 18:04
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    @TomášNesrovnal slower ? Sorry, but that's a slightly bogus comment. Maybe by nano seconds, but definitelly not any slower than a shell built in. In this case speed is irrelevant, too.
    – Sergiy Kolodyazhnyy
    Jan 24, 2017 at 18:07
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    Agreed. Being slower would be important only for a super-critical response time, in which case using a shell would be entirely inappropriate anyway. The days when computers were so slow that they needed maximum optimisation for everyday use are long gone, thank goodness!
    – Paddy Landau
    Jan 31, 2017 at 8:45

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