如何在内联 ssh 命令中使用 awk {print}

Question

I am trying to run an inline ssh command which looks like this:

ssh user@127.0.0.1 "df / -h | awk 'FNR == 2 {print $3}'"

I would expect the output to be 3.8G (as this is the second line, third column) but instead, I am getting /dev/sda1 6.9G 3.8G 2.8G 58% / (the entire second line).

This means that the FNR == 2 is working but the {print $3} is not.

If I run that command directly on the machine that I am ssh'ing into then I get the expected result, just not when calling it through an inline ssh command as above.

This code will eventually ran within a bash script. Is it not possible to use print in this way? If not, is there another method that you can suggest? I am relatively new to terminal life so please bear with me.

Answer 1

The joys of shell quoting. The line:

ssh user@127.0.0.1 "df / -h | awk 'FNR == 2 {print $3}'"

Is parsed by the shell, which invokes ssh with two arguments:

user@127.0.01 and df / -h | awk 'FNR == 2 {print }' df / -h | awk 'FNR == 2 {print }' The $3 was interpolated, and (I'm assuming) was empty. To prevent that, you have many options. One of which is:

ssh user@127.0.0.1 << \EOF
df / -h | awk 'FNR == 2 {print $3}'
EOF

another is:

ssh user@127.0.0.1 sh -c '"df / -h | awk '"'"'FNR == 2 {print $3}'"'"'"'

Answer 2

The problem resides in the way you pass you ssh arguments.

By calling:

ssh user@127.0.0.1 "df / -h | awk 'FNR == 2 {print $3}'"

You are passing two arguments:

• user@127.0.0.1
• "df / -h | awk 'FNR == 2 {print $3}'"

Since your second argument is given inside double quotes, the $3 variable will be expanded. You can prevent this variable expansion by escaping the dollar sign:

ssh user@127.0.0.1 "df / -h | awk 'FNR == 2 {print \$3}'"