Five Shell Programming Tips

 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.

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

   Prev - Next >>