$$ variable as GREP pattern: "weird" behavior

courteous · 12-12-2010, 07:22 AM

First, 35th field of "ls -l /proc/" is column with folder names.

So, the following snippet returns $PID (if found):

Code:

ls -l /proc/ | cut -d' ' -f35 | grep ^${PID}$

This doesn't work:

Code:

ls -l | cut -d' ' -f35 | grep ^$$$

grail · 12-12-2010, 07:55 AM

Well I am going to start with .. What are you talking about?

Quote:

First, 35th field of "ls -l /proc/" is column with folder names.

The 35th field? This is based on what as a delimiter? Here is a snippet of 'ls -l /proc' for me:

Code:

dr-xr-xr-x  4 root       root                     0 2010-12-12 23:21 tty
-r--r--r--  1 root       root                     0 2010-12-12 23:21 uptime
-r--r--r--  1 root       root                     0 2010-12-12 23:21 version
-r--r--r--  1 root       root                     0 2010-12-12 23:21 version_signature
-r--------  1 root       root                     0 2010-12-12 23:21 vmallocinfo
-r--r--r--  1 root       root                     0 2010-12-12 23:21 vmstat
-r--r--r--  1 root       root                     0 2010-12-12 23:21 zoneinfo

I think you would agree that it will be difficult to find 35 fields in this??

Quote:

So, the following snippet returns $PID (if found):

How would this ever return a PID?? Last time I looked, ls has no idea about PIDS. And why would it need to if it is simply listing objects?

Quote:

This doesn't work:

As far as I can tell, none of what you have shown works.

colucix · 12-12-2010, 09:15 AM

Hmmm... I agree with grail: it's not clear at all what you're trying to achieve and what you're talking about. Anyway, the grep command with $$ works for me, if the PID of the current shell is expected. Eventually you may try:

Code:

... | grep ^${$}$

courteous · 12-12-2010, 09:21 AM

Thank you both. 35th field is based on space (' ') delimiter: there must be some better way to extract folder's name (the last "column)? (I've tried with default TAB delimiter, but it doesn't work.)

colucix · 12-12-2010, 09:29 AM

Why you don't use simply ls (without -l) to list the content of /proc? In this way you can avoid extraction.

To by-pass the multiple spaces problem you can use awk which treats multiple spaces as a single delimiter by default. For example:

Code:

ls -l | awk '{print $NF}'

This works if directories or file names don't have spaces in their name (otherwise they are treated as delimiter as well). A workaround in awk is:

Code:

ls -l | awk '{print substr($0,index($0,$9))}'

if the column containing directories names is the 9th (on some systems is the 8th, depending on the timestamp format).