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I am trying to store Available and Total Memory into variables in a script file as follows,

read -r Available Total <<EOT
$(free -m | awk '/^Mem/{print $7; print $2;}')
EOT

$ echo $Total

$ echo $Available
1437

But I am unable to store the variable $Total.

But when I do:

$ read Available Total <<EOT
$(echo $(free -m | awk '/^Mem/{print $7; print $2;}'))
EOT

$ echo $Available
1309
$ echo $Total
7865

It works. But shellcheck gives me following suggestion:

Useless echo? Instead of 'echo $(cmd)', just use 'cmd'. [SC2005]

Why the forst example didn't work? and Why the second one works?

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1 Answer 1

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That awk command prints the two values on two separate lines, while read reads one line, expecting to find two fields on it.

Change the print command to print $7, $2; to print the numbers on the same line. Since you tagged this with Bash, you can use a here-string instead of a here-doc to make the full command a bit neater:

$ read -r Available Total <<< $( free -m | awk '/^Mem/{print $7, $2;}' )

Or use a process substitution:

$ read -r Available Total <   <( free -m | awk '/^Mem/{print $7, $2;}' )

If you use echo $(...) without quotes around the command substitution, the output from the command gets word-split, and echo sees the separate lines as separate arguments. It prints all of its arguments joined with spaces, so on a single line.

echo $(foo) is usually rather a useless thing to do, but isn't the same same just foo, for this very reason. It also puts a final newline at the end of the output of foo, if it wasn't there already.

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    Process Substitution would work here as well. Commented Jan 13, 2019 at 13:15
  • @glennjackman This doesn't work read -r Available Total <(free -m | awk '/^Mem/{print $7, $2;}') Could you please correct this? Commented Jan 13, 2019 at 16:47
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    @Nikhil, process substitution needs the redirection operator < here. In itself it just expands to a (pseudo)filename, which read would take as the name of a variable to write to. (It's probably something like /dev/fd/63, which isn't a valid variable name so you get an error.) Commented Jan 13, 2019 at 17:10
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    Yes, read reads stdin, so you need a redirection. Commented Jan 13, 2019 at 18:02

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