Stupid question: if [ "$i" == `$(cat ${LOGFILESSHD}

frenchn00b · 05-19-2008, 01:03 PM

Code:

cat /var/log/auth.log  | grep "blabla  blabla blabla"  | while read i ; do 
echo "**$i**" 
allre=`$(cat ${LOGFILESSHD} | grep "${i}" )`
...

the problem is that allre is always egal to nothing

It would work if I had no spaces in "blabla blabla blabla"
and with writing
allre="$(cat ${LOGFILESSHD} | grep ${i} )"
with ".

I have a problem, very stupid , sorry, with " ...
Could somebdy be kind for giving the solution ? I bump bump bump ... on that bash but cannot find it... no idea really

thank you
Best regards

colucix · 05-19-2008, 01:26 PM

Code:

allre=`$(cat ${LOGFILESSHD} | grep "${i}" )`

this syntax is wrong. the backticks and the syntax $(...) are (almost) equivalent for command substitution. If you use them together you can get unpredictable results. Indeed, at first the command inside $(...) is evaluated, then the result is evaluated again as a command, because of the embedding backticks. You stated that it works using double quotes: what prevent you from using them?

Note that if you're trying to access the variable outside the while loop, it will result empty. This is because the while loop is called after a pipe, hence in its own subshell with its own local variables. Once the subshell is closed, the variables are lost.

To prevent this behavior, use process substitution, e.g.

Code:

while read i ; do 
echo "**$i**" 
allre=`$(cat ${LOGFILESSHD} | grep "${i}" )`
done < <(cat /var/log/auth.log  | grep "blabla  blabla blabla")

frenchn00b · 05-19-2008, 03:17 PM

Quote:

Originally Posted by colucix

Code:

allre=`$(cat ${LOGFILESSHD} | grep "${i}" )`

this syntax is wrong. the backticks and the syntax $(...) are (almost) equivalent for command substitution. If you use them together you can get unpredictable results. Indeed, at first the command inside $(...) is evaluated, then the result is evaluated again as a command, because of the embedding backticks. You stated that it works using double quotes: what prevent you from using them?

Note that if you're trying to access the variable outside the while loop, it will result empty. This is because the while loop is called after a pipe, hence in its own subshell with its own local variables. Once the subshell is closed, the variables are lost.

To prevent this behavior, use process substitution, e.g.

Code:

while read i ; do 
echo "**$i**" 
allre=`$(cat ${LOGFILESSHD} | grep "${i}" )`
done < <(cat /var/log/auth.log  | grep "blabla  blabla blabla")

hi, thank you hence this should maybe be withotut the $ to make it as command:

Code:

while read i ; do 
echo "**$i**" 
allre=`cat ${LOGFILESSHD} | grep "${i}" `
done < <(cat /var/log/auth.log  | grep "blabla  blabla blabla")

but what is suprising is that line:

Code:

 done < <(cat /var/log/auth.log  | grep "blabla  blabla blabla")

I didnt know that we could use ( ) for commands to bound them right and left, it's like the : '

"< <()" is wicked command. I knew >> and >

Ah, still in the same subject, I also get this < pipeline problem:

Code:

ls -tr | tail -n 1 | xargs mplayer

cannot work when the filename has some spaces...

frenchn00b · 05-19-2008, 03:33 PM

I have got also that problem with " and ' ... fighthing ...

Code:

if [ "$i" == "`cat ${LOGFILESSHD} | grep "${i}"`" ] ; then

colucix · 05-19-2008, 03:37 PM

Quote:

Originally Posted by frenchn00b

"< <()" is wicked command. I knew >> and >

This syntax is specific to process substitution. For details and some more examples, have a look at chapter 22 of the Advanced Bash Scripting Guide.

Quote:

Ah, still in the same subject, I also get this < pipeline problem:

Code:

ls -tr | tail -n 1 | xargs mplayer

cannot work when the filename has some spaces...

You can try

Code:

ls -tr -Q | tail -n 1 | xargs mplayer

The -Q option embeds file names in double quotes, so that when it is passed as argument to xargs the blank space is correctly interpreted by the shell.

frenchn00b · 05-19-2008, 04:44 PM

Not easy;

this works :

Code:

essai=$(host  `cat ~/logs.log  | awk ' { print $11 }  '` ) ; echo $essai

but if I put the ", then not working anymore

Code:

essai="$(host  `cat ~/logs.log  | awk ' { print $11 }  '` ) ; echo $essai"

colucix · 05-19-2008, 05:16 PM

Code:

essai="$(host  `cat ~/logs.log  | awk ' { print $11 }  '` ) ; echo $essai"

Hmmm... can you explain in real words what you are trying to achieve by this command? What is the expected output and what exactly the problem is? Error messages? Unexpected behavior? Can you provide a real example?

At a first glance, it does not make sense. In the first version - without double quotes - it assigns to the variable essai the output of the command enclosed in $(...) then echoes the value of essai. The second version - with double quotes - assigns the output of the command concatenated with the string " ; echo something", where something is the value of essai. I miss what's your aim and what the problem is, sorry.