Bash

kopert · 09-17-2012, 05:15 AM

Hi Everyone,
I am new to bash scripting and I just recently encountered a problem:
I want to receive all terminal names(tty1, tty2 etc.), which are currently in use (using who command), and filter myself of the result.

So I do this in command line:
echo "$(who | cut -b10-20 | grep -v {$(who am i | cut -b10-20)})"

It seems that problem is in passing parameter -v {$(who am i | cut -b10-20)}

I know that there are different ways to solve this problem, using functions etc. But for learning reasons I would like to know what am I doing wrong.

Thanks
kopert

pixellany · 09-17-2012, 05:55 AM

The first thing I see is that this does not work:

Code:

grep {string} filename

it needs to be

Code:

grep "string" filename

The best way to learn these things is to try to commands individually to be sure how they work.

For your specific problem, I suggest searching on your username, not the terminal #. (eg--using a terminal emulator, I show up twice in the output of "who".

Finally, stringing too much stuff together can lead to problems.

David the H. · 09-17-2012, 06:02 AM

First of all, please use ***[code][/code] tags*** around your code and data, to preserve formatting and to improve readability. Please do not use quote tags, bolding, colors, or other fancy formatting.

Next, what output do you get? What are the error messages, if any?

Also, what exactly does the who command output for you? I don't have a multi-user system, so I can't run any tests without it. The same goes for whoami, (not "who am i" !). But in any case you can probably substitute the built-in $USER variable instead.

In short, it usually helps when we can see the same things you do.

Don't forget to quote your substitutions, unless you want the output to be word-split afterwards, just like when using variables.

What do you think the "{..}" brackets are supposed to be doing here? In this configuration they would be treated as part of the grep expression. If they were moved inside the "$(..)" then they could be considered a command grouping; but only if the final "}" keyword is separated from the last command with a semi-colon. i.e.

Code:

{ command1 ; command2 ;}

This syntax is necessary when the grouping is all on a single line.

But since the command substitution already acts as a grouping pattern, they wouldn't be necessary anyway.

Finally, the initial echo and substitution brackets are completely pointless. The chain itself will print to stdout.

Assuming the commands themselves are correct, try this:

Code:

who | cut -b10-20 | grep -v "$( whoami | cut -b10-20 )"
who | cut -b10-20 | grep -v "$USER"

There are probably better ways to do this, however.

pixellany · 09-17-2012, 06:12 AM

Try this:

Code:

me=$(who am i|awk '{print $1}'
who | grep -v $me | awk '{print $2}'

But, note this:

Code:

[mherring@herring_lap play]$ whoami
mherring
[mherring@herring_lap play]$ who am i
mherring pts/0        2012-09-17 06:36 (:0.0)

which leads to this:

Code:

who | grep -v $(whoami) | awk '{print $2}'

Overlap with previous post acknowledged......

David the H. · 09-17-2012, 06:43 AM

So wait, there really is a "who am i" command string? Well, whaddayaknow?

Edit: And how about removing the unnecessary grep too?

Code:

who | awk -v me="$(whoami)" '$0 !~ me {print $2}'

kopert · 09-17-2012, 07:05 AM

Thank you for your answers!

Quote:

Originally Posted by pixellany

For your specific problem, I suggest searching on your username, not the terminal #. (eg--using a terminal emulator, I show up twice in the output of "who".

I would like to get all terminal names except one, on wich command was executed.
In your solution user will get all terminal names except those on wich he is logged in.

Quote:

Originally Posted by pixellany

Code:

me=$(who am i|awk '{print $1}'
who | grep -v $me | awk '{print $2}'

This solution works very well, when changing $1 to $2.
However, without using variable "me", this is not working:

Code:

who | grep -v $(who am i | awk '{print $2}') 
who | grep -v $(who -m | awk '{print $2}')

Quote:

Originally Posted by David the H.

First of all, please use *** tags*** around your code and data, to preserve formatting and to improve readability. Please do not use quote tags, bolding, colors, or other fancy formatting.

Next, what output do you get? What are the error messages, if any?

Also, what exactly does the who command output for you? I don't have a multi-user system, so I can't run any tests without it. The same goes for whoami, (not "who am i" !). But in any case you can probably substitute the built-in $USER variable instead.

I am sorry, my bad.

The output of "who" command is:

Code:

testusr1 tty2         2012-09-17 13:12
testusr1 tty1         2012-09-17 13:12
testusr2 tty3         2012-09-17 13:17
testusr3 tty4         2012-09-17 13:04
root     tty5         2012-09-17 13:29

The output of "whoami" command is:

Code:

testusr1

The output of "who am i" or "who -m" is:

Code:

testusr1 tty1         2012-09-17 13:12

The output of command:

Code:

who | cut -b10-13 | grep -v "$( who -m | cut -b10-13 )"

+ who
+ cut -b10-13
++ who -m
+ grep -v ''

Snark1994 · 09-17-2012, 08:32 AM

Quote:

Originally Posted by David the H.

So wait, there really is a "who am i" command string? Well, whaddayaknow?

It seems to vary:

Code:

[joshua:~]$ who
joshua   :0.0         2012-09-17 09:42
[joshua:~]$ whoami
joshua
[joshua:~]$ who am i
[joshua:~]

My version of 'who' (GNU coreutils 8.17) certainly doesn't like you adding 'am i' to it... Though I note that I have two manpages for it, and the second one says

Quote:

who [OPTION]... [ FILE | ARG1 ARG2 ]
...
-m only hostname and user associated with stdin
...
If FILE is not specified, use /var/run/utmp. /var/log/wtmp as FILE is common. If ARG1 ARG2 given, -m presumed: 'am i' or 'mom likes' are usual

gnashley · 09-17-2012, 11:38 AM

'who' returns the names of all logged-in users, 'whoami' returns the equivalent of the bash environmental variable $USER

kopert · 09-18-2012, 04:13 AM

Finally I found a way to get all terminal names (except one, on which command was issued), without using variables.

Code:

who | awk '{print $2}' | grep -v "$(ps --no-headers | awk '{print $2}' | uniq)"

Still don't know why it is not working when changing:

Code:

"ps --no-headers" to "who am i" or "who -m" or even "tty".

Best,
kopert