topic Re: Shell variable containing pipe in Operating System - HP-UX

Shell variable containing pipe

Stephen Mudd — Thu, 18 Sep 2003 14:41:28 GMT

Hello,

Can someone explain why a remote shell interprets a pipe, but a local shell doesn't?
e.g.
# COMMAND='echo hello | grep hell'
# remsh other_host $COMMAND
hello
# $COMMAND
hello | grep hell

Why does the local shell execute the echo, but not react to the pipe?
What syntax should I use to get the same result as the remsh?

Re: Shell variable containing pipe

Stefan Farrelly — Thu, 18 Sep 2003 14:48:17 GMT

your command worked. Works fine for me with a pipe in a variable or not.

You did the remote command;

echo hello | grep hell

and hell is part of hello so it echoed back hello. This is normal behaviour.

Re: Shell variable containing pipe

Stanimir — Thu, 18 Sep 2003 14:57:38 GMT

Normal behavior! :)

Use:
#COMMAND=`echo hello|grep hell`
#echo $COMMAND

Regards,STAN

Re: Shell variable containing pipe

Todd McDaniel_1 — Thu, 18 Sep 2003 14:58:22 GMT

/agree

On your local host, the echo command only regurgitates what you give it in quotes, it will not treat it as an executable command... merely as plain text.

You would have to pass the variable to a script or one-liner... or change the quotes to back tics... as below.

You will need to change it to back tics..the one next to the 1 key....

# COMMAND=`echo hello | grep hello`
# echo $COMMAND
hello

Re: Shell variable containing pipe

Todd McDaniel_1 — Thu, 18 Sep 2003 15:03:44 GMT

Its amazing to me 3 of us posted at the exact same time LOL... I didnt copy the guy above me LOL...so please dont penalize me on points!!

of course zero for this post of course!

Re: Shell variable containing pipe

Stanimir — Thu, 18 Sep 2003 15:06:04 GMT

Sorry Tod!
Really your answer is more complete
and usefull.

Regards,Stan

Re: Shell variable containing pipe

Rodney Hills — Thu, 18 Sep 2003 15:37:45 GMT

It has to do when the shell is interpreting the command.

On the local system
$COMMAND
is replaced with the the contents of COMMAND and the echo command is executed. But the "|" has no meaning except on the command line you typed. It is not processed by the shell.

If you wanted $COMMAND to recognize the "|", then use the "eval" shell command (see man sh-posix).
eval $COMMAND

Then the shell will rescan the command line a second time, see the "|" and do the pipe operation.

The reason it works on the remsh is the same situation. When $COMMAND is run on the remote host, the remote host shell rescans the command line and sees the "|" as its special meaning.

HTH

-- Rod Hills

Re: Shell variable containing pipe

Stephen Mudd — Thu, 18 Sep 2003 16:26:35 GMT

Thanks for your replies. I should have realised what was going on without making myself look like a complete novice. Never mind. We all have off days occasionally.

Using "eval" is preferable to using "echo $(command)" or backquotes because it retains the formatting, e.g.

# COMMAND1="/usr/sbin/lanscan |cut -c1-8,24-80"
# COMMAND2=$(/usr/sbin/lanscan |cut -c1-8,24-80)
# echo $COMMAND2
Hardware Crd Hdw Net-Interface NM MAC HP-DLPI DLPI Path In# State NamePPA ID Type Support Mjr# 0/0/0/0 0 UP lan0 snap0 1 ETHER Yes 119 0/4/0/0 1 UP lan1 snap1 2 ETHER Yes 119 0/6/0/0 2 UP lan2 snap2 3 ETHER Yes 119 0/2/0/0 0 UP ixe1 5 X25 No 77 0/2/0/0 0 UP ixe2 6 X25 No 77
# eval $COMMAND1
Hardware Crd Hdw Net-Interface NM MAC HP-DLPI DLPI
Path In# State NamePPA ID Type Support Mjr#
0/0/0/0 0 UP lan0 snap0 1 ETHER Yes 119
0/4/0/0 1 UP lan1 snap1 2 ETHER Yes 119
0/6/0/0 2 UP lan2 snap2 3 ETHER Yes 119
0/2/0/0 0 UP ixe1 5 X25 No 77
0/2/0/0 0 UP ixe2 6 X25 No 77
# remsh other_host $COMMAND1
Hardware Crd Hdw Net-Interface NM MAC HP-DLPI DLPI
Path In# State NamePPA ID Type Support Mjr#
0/0/0/0 0 UP lan0 snap0 1 ETHER Yes 119
0/5/0/0 2 UP lan2 snap2 2 ETHER Yes 119
0/10/0/0 1 UP lan1 snap1 3 ETHER Yes 119
0/8/0/0 0 UP ixe1 5 X25 No 79
0/8/0/0 0 UP ixe2 6 X25 No 7