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AWK Tips

killing processes by name (in this example we kill the process called netscape)

kill `ps auxww | grep netscape | egrep -v grep | awk '{print $2}'`

or with awk alone:

ps auxww | awk '$0~/netscape/&&$0!~/awk/{print $2}' |xargs kill

It has to be adjusted to fit the ps command on whatever unix system you are on. Basically it is: "If the process is called netscape and it is not called 'grep netscape' (or awk) then print the pid"

UNIX Basics Examples with awk A short introduction

Games with ls

1. Renaming within the name:
   ls -1 *old* | awk '{print "mv "$1" "$1}' | sed s/old/new/2 | sh
   (although in some cases it will fail, as in file_old_and_old)
    
2. remove only files:
   ls -l * | grep -v drwx | awk '{print "rm "$9}' | sh
   or with awk alone:
   ls -l|awk '$1!~/^drwx/{print $9}'|xargs rm
   Be careful when trying this out in your home directory. We remove files!
    
3. remove only directories
   ls -l | grep '^d' | awk '{print "rm -r "$9}' | sh
   or
   ls -p | grep /$ | wk '{print "rm -r "$1}'
   or with awk alone:
   ls -l|awk '$1~/^d.*x/{print $9}'|xargs rm -r
   Be careful when trying this out in your home directory. We remove things!

    

An alterative way to pass shell variables into AWK program from the command line using "variable assignment" parameters:

Pseudo-files

AWK knows another way to assign values to AWK variables, like in the following example:

$ awk '{ print "var is", var }' var=TEST file1 

This statement assigns the value "TEST" to the AWK variable "var", and then reads the files "file1" and "file2". The assignment works, because AWK interprets each file name containing an equal sign ("=") as an assignment.

This example is very portable (even oawk understands this syntax), and easy to use. So why don't we use this syntax exclusively?

This syntax has two drawbacks: the variable assignment are interpreted by AWK the moment the file would have been read. At this time the assignment takes place. Since the BEGIN action is performed before the first file is read, the variable is not available in the BEGIN action.

The second problem is, that the order of the variable assignments and of the files are important. In the following example

$ awk '{ print "var is", var }' file1 var=TEST file2

the variable var is not defined during the read of file1, but during the reading of file2. This may cause bugs that are hard to track down.

Comparing Strings

Sometimes you want to compare a string or pattern with another string, for example a particular field or a variable. You can compare two strings in various ways, including whether one contains the other, whether they are identical, or whether one precedes the other in alphabetical order.

You use the tilde (~) sign to test whether two strings match. For example,

$2 ~ /^15/

checks whether field 2 begins with 15. This pattern matches if field 2 begins with 15 regardless of what the rest of the field may contain. It is a test for matching, not identity. If you wish to test whether field 2 contains precisely the string 15 and nothing else, you could use

$2 ~ /^15$/

You can test for nonmatching strings with !~. This is similar to ~, but it matches if the first string is not contained in the second string.

You can use the == operator to check whether two strings are identical, rather than whether one contains the other. For example,

$1==$3

checks to see whether the value of field 1 is equal to the value of field 3.

Do not confuse == with =. The former (==) tests whether two strings are identical. The single equal sign (=) assigns a value to a variable. For example,

$1=15

sets the value of field 1 equal to 15. It would be used as part of an action statement. On the other hand,

$1==15

compares the value of field 1 to the number 15. It could be a pattern statement.

The != operator tests whether the values of two expressions are not equal. For example,

$1 != "pencils"

is a pattern that matches any line where the first field is not "pencils."