[SOLVED] How to 'watch' a command including both of single and double quote?

quanta · 08-12-2010, 11:35 AM

Hi guys,

I want to watch a number of processes in "D" status repeatedly with following command:

Code:

# watch -n 1 'top -b -n 1 | awk '{if ($8 == "D") {print; count++} } END {print count}''

but it didn't work. I also tried with double quote.

Can I use 'watch' command is this case?

PS: I know 'while' and 'sleep' can do the same but it is dirty workaround.

AlucardZero · 08-12-2010, 11:45 AM

1. You can't do quotes like that. Either escape them or stagger them. ' then " then ', then close them in reverse order. You have ' then ' then " and it sees 'top -b -n 1 | awk ' as one unit.
2. $8 needs to be escaped (\$8) so that the shell doesn't parse it.

Code:

watch -n 1 "top -b -n 1 | awk '{if (\$8 == "D") {print; count++} } END {print count}'"

works without error, though it prints just "2" for me and I don't know why.

David the H. · 08-12-2010, 02:29 PM

You also have to escape the double-quotes inside the awk command.

Code:

watch -n 1 "top -b -n 1 | awk '{if (\$8 == \"D\") {print; count++} } END {print count}'"

Basically, single-quotes escape everything, while double-quotes escape everything inside them except for $, `, and \, and of course another ", which ends the sequence. This means that inside double-quotes you can use \ to escape anything that could be a problem, including other double-quotes. But inside single-quotes you can't use anything, not even the escape character.

Note also that double-quotes lose their special meaning inside single quotes, and vice-versa.

So in this case, you have to use double-quotes around the phrase as a whole, which allows you to use escapes, then you need to escape any [$`\"] characters that are intended to be fed to the commands inside. In this case, you need to protect the $ and " inside the awk expression, because the single-quotes surrounding them have lost their special bash meaning.