Shell Programming and Scripting

Hi, there,

I wrote a script like this:

#!/bin/bash
#put something into a LIST
for item in $LIST
do
cat $item >> /tmp/output
done

My question is that if I have 5 items in that LIST, should it be opened and closed every time when the ">>" works? So that file will be opened and closed 5 times, and it'll be a performance issue when it has a big number of items in that list. Am I right?

If so, anyway to improve it?

Thanks!

you don't usually cat a variable, unless that variable contains the name of the file.
and no, don't worry about the >> .

first of all
not cat $item >> /tmp/output
echo $item >> /tmp/output

you can use this to convert the list to lines and insert into file at once:
echo $LIST | awk '{for (i=1; i<=NF; ++i) {print $i}}' > /tmp/output

we don't know where OP got his $LIST, so it may contain file names. So cat $item may be correct.

I will assume that cat is correct... to open the file once, you could:

Code:

for item in $LIST ; do
      cat $item 
done >> /tmp/output

Oh, yes, it's a list of some process ids. I think you can treat them as file names, right?
Well, the real use case is that I have some users who run multiple firefox process independently on Mac machines and I need to know which profile those process are accessing. In another word, I need to know in this case, which profiles are "in use".

Firefox itself has a flag file called .parentlock in each profile folder and it suppose to be created once the profile is in use and deleted once the profile quits. But there's a bug with Apple's file system so the file won't be removed once it's created. That means I can't use the file as a flag.

So I decided to use some help of unix shell script, bash is installed by default so I'd like to use that. The idea is:
1) use 'ps -U' to get running processes for current user
2) use 'grep', 'sed' and 'awk' to find out all firefox processes and the profile name it attached, dump to a file

The script looks like:

Code:

#!/bin/bash

if [ -z $1 ]; then
        echo "No output file name."
        exit -1
fi

if [ -f $1 ]; then
        echo "Output file exists. Remove it."
#       exit -2
        rm -rf $1
fi

firefox_suspects=`ps -U \`whoami\`|grep firefox-bin|awk '{print $1}'`
for pid in $firefox_suspects;
do
        lsof -p $pid | grep '.parentlock' | awk '{print $10}' | sed 's!^.*/\([^/]*\)/\.parentlock$!\1!' >> $1
done

Well, now I know I can improve it by simply move the '>>' after 'done'. That's nice. But does it mean there *IS* performance issue in my old script and the outfile ($1) will be opened and closed as many times as the number of the items in that list?

Thanks for all the responses.

Wish everybody a Happy New Year!

Yes, you are opening the file in append mode many times and this will cause a minor performance issue.