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grep in a variable

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# 1  
Old 05-29-2012
grep in a variable
Hello,

I usually search extensively and have to date found what I've needed. However, this one's got me stumped. I need to create a variable as follow. The issue however is that upon execution, it freezes. $var1 isn't always present in usage.log and this is fine but I'd like it to continue with the script if no match is found. Any ideas? Thanks.
Code:
var2=`grep $var1 /conf/usage.log | tail -1 | cut -d " " -f 2`

Last edited by joeyg; 05-29-2012 at 03:26 PM.. Reason: wrap commands and data with CodeTags
# 2  
Old 05-29-2012
try this -
Code:
var2=`cat /conf/usage.log|grep $var1| tail -1 | cut -d" " -f2`

# 3  
Old 05-29-2012
Thanks, that did the trick but error control leaves much to be desired Smilie
# 4  
Old 05-29-2012
Have you researched the
Code:
>2

option for error handling?
redirect errors somewhere. Two examples:
Code:
cat myfile 2>/dev/null
cat myfile 2>>/logfile.txt

The first send errors no-where
The second, appends to the end of a file called logfile.txt
# 5  
Old 05-29-2012
No, good point. I'll read into this. Thanks.
# 6  
Old 05-29-2012
The problem with the freezing is not freezing, but grep waiting for input. This is the case when var1 is empty and grep then thinks that "/conf/usage.log" is the search string. This can be avoided by double quoting var1 (which is good practice anyway):
Code:
var2=`grep "$var1" /conf/usage.log | tail -1 | cut -d " " -f 2`

In any case, you should still test for var1 being empty, because otherwise the last line of /conf/usage.log ends up in var2, no matter what it consists of...
Last edited by Scrutinizer; 05-29-2012 at 04:23 PM..
# 7  
Old 05-29-2012
grep not finding the pattern is not really an error, but grep succeeding in finding nothing. Just test for null before using it. Break it into separate steps depending on how granular you need to get to handle various conditions. It depends on your requirements. For instance, do you halt if $var1 is not set? Halt if $var1 is set but not found in the log? i.e. only grep if $var1 is not null:
Code:
#  Test to see if $var1 is null
if [[ -v "$var1" ]]; then  # Make sure to quote it so the test "sees" the null.
  echo '$var1 is null'
  # Code for var1 being null goes here.
else
  search=`grep $var1 /conf/usage.log`
  if [[ -v "$search" ]]; then  
    echo "[$var1] is not found"
    # Code for var1 not found goes here
  else  
    var2=`echo $search | tail -1 | cut -d " " -f 2` 
  fi
fi

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