BASH Cures Cancer

I recently read this article Good Shell Coding Practices on handling command line arguments in scripts. I am going to cover some tips for general shell programming I have learned over the years.

1. grep -q: Do you use the exit status of grep? If so, you the -q switch instead of sending the output to /dev/null
   
#!/bin/bash
if grep 'Synergy' messages >/dev/null
then
echo 'Synergy has logged to messages'
else
echo 'Synergy has NOT logged to messages'
fiif grep -q 'Synergy' messages
then
echo 'Synergy has logged to messages'
else
echo 'Synergy has NOT logged to messages'
fi

   Why use -q instead of >/dev/null? Well its ALOT faster. I have not looked at the source but I believe -q implies -m 1 as they are the same speed when used together. (See below) The option -m 1 tells grep to stop after the first match. When using -q you only care if the file contains your search term and do not care how many times or what those lines look like. Thus it would make sense that -q implies -m 1.
   
[root@mojito ~]# time grep 'Synergy' messages >/dev/nullreal 0m0.360s
user 0m0.266s
sys 0m0.072s
[root@mojito ~]# time grep -q 'Synergy' messages
real 0m0.002s
user 0m0.001s
sys 0m0.001s
[root@mojito ~]# time grep -q -m 1 'Synergy' messages
real 0m0.002s
user 0m0.000s
sys 0m0.002s
2. grep -c: Instead of using grep to match lines in a file or input and then piping that to wc -l, just use grep -c. Its faster and simpler:
   
[root@mojito ~]# time grep 'Synergy' messages | wc -l
495948real 0m0.519s
user 0m0.363s
sys 0m0.118s
[root@mojito ~]# time grep -c 'Synergy' messages
495948
real 0m0.245s
user 0m0.167s
sys 0m0.052s
3. VAL=$( funcName ): Most people new to UNIX/Linux don’t use functions in their scripts. More expirence brings more functions. However, too many people have those functions set environment variables instead of returning the values directly. You cannot use the return command to return values (except exit status) but you can use echo. For example, here is two simple functions using the environment variable method and the echo method:
   
   [root@mojito ~]# cat returnVals.sh
   #!/bin/bash
   function badFunc()
   {
   export badFuncVar=VALUE
   }
   function goodFunc()
   {
   echo VALUE
   }
   badFunc
   var1=$badFuncVar
   var2=$( goodFunc )
   var3=` goodFunc `
   echo "var1=$var1 , var2=$var2 , var3=$var3"
   [root@mojito ~]# ./returnVals.sh

   var1=VALUE , var2=VALUE , var3=VALUE
   As you can see setting the environment variables takes two steps and requires you to remember the name of the variables! Too much work for me. See my discussion on the number of days in a month function.

   Note: “ and $() are the same thing. I personally use $() to call functions and “ to call executables.

4. xargs: Newbies often us loops or variables when xargs would suffice. xargs reads standard input, splits the input by whitepsace (spaces and newlines) and then executes the command you specify with the processed input. The three examples below all delete files in the current working directory matching ‘mess*’ which contain the word ‘Synergy’. However, the third is the simplest and most concise. Its the UNIX solution.Using a variable:
   
[root@mojito ~]# export FILES=`find . -type f -name 'mess*' -exec grep -q 'Synergy' {} \; -print`
[root@mojito ~]# rm -f $FILES
   Using a loop:
   
[root@mojito ~]# for FILE in `find . -type f -name 'mess*' -exec grep -q 'Synergy' {} \; -print`
do rm -f $FILE; done
   Using xargs:
   
[root@mojito ~]# find . -type f -name 'mess*' -exec grep -q 'Synergy' {} \; -print | xargs rm -f
5. read: read is command built into the shell. You can use it to obtain data interactively from a user:
   
[root@mojito ~]# read VAR
value
[root@mojito ~]# echo $VAR
value
   Or you can use read in scripts like the following. If the following script seems complex, don’t get hung up on the specifics. Just notice how the condition in the while loop is ‘read FILE’ which causes the while loop will continue to loop until the variable FILE is not assigned a new value.
   
   [root@mojito ~]# cat logRotate.sh
   #!/bin/bash
   find . -type f -name 'mess*.[0-9]' | sort -r | \
   while read FILE
   do
   NUM=`echo $FILE | awk -F. '{print $3}'` # filename Ex: ./file.$NUM
   BASE=`echo $FILE | awk -F. '{print $2}'`
   ((NUM++))
   mv -f $FILE .$BASE.$NUM
   done
   cp -f messages messages.0
   cat /dev/null > messages

   [root@mojito ~]# ls -l messages*
-rw------- 1 root root 107207426 Feb 10 21:00 messages
-rw------- 1 root root 107207426 Feb 10 20:58 messages.0
-rw------- 1 root root 107207426 Feb 10 20:57 messages.1
-rw------- 1 root root 107207426 Feb 10 20:57 messages.2
-rw------- 1 root root 107207426 Feb 10 20:55 messages.3
-rw------- 1 root root 107207426 Feb 10 20:55 messages.4
-rw------- 1 root root 107207426 Feb 10 20:55 messages.5
-rw------- 1 root root 107207426 Feb 10 20:51 messages.6
[root@mojito ~]# ./logRotate.sh
[root@mojito ~]# ls -l messages*
-rw------- 1 root root 0 Feb 10 21:00 messages
-rw------- 1 root root 107207426 Feb 10 21:00 messages.0
-rw------- 1 root root 107207426 Feb 10 20:58 messages.1
-rw------- 1 root root 107207426 Feb 10 20:57 messages.2
-rw------- 1 root root 107207426 Feb 10 20:57 messages.3
-rw------- 1 root root 107207426 Feb 10 20:55 messages.4
-rw------- 1 root root 107207426 Feb 10 20:55 messages.5
-rw------- 1 root root 107207426 Feb 10 20:55 messages.6
-rw------- 1 root root 107207426 Feb 10 20:51 messages.7