How to replace text in a column

Your syntax is almost right. To test for string equality in awk, use a double equals sign: ==. To assign a value, use a single equals sign.

So use if ($4 == "E:") and you should get the result you want.

The full command would look like this. Note there is only one character difference from the command you used; that was your only error:

awk -F , '{ if ($4 == "E:") $4="";}1' OFS=, data.final

To show some different syntax and different approaches, the following versions are exactly equivalent:

awk -F, -v OFS=, '$4 == "E:" { $4 = "" }; 1' data.final

awk 'BEGIN { FS=OFS="," }; $4 == "E:" { $4 = "" }; {print}' data.final

awk -F, -v OFS=, '{sub( /^E:$/, "", $4); print}' data.final

Notes regarding the above:

1. If your whole code block is just an if/then, you can just use the condition as a filter for the code block. Hence $4 == "E:" {$4 = ""} is exactly equivalent to {if ($4 == "E:") {$4 = ""}}
2. It is good practice to enclose "then" statements in a code block even when there is only one of them, i.e. if ($4 == "E:") {$4 = ""} rather than if ($4 == "E:") $4 = "";
3. -F sets the value of FS and -v can be used to set the value of any variable before awk considers the first line of the first file. (You likely knew that.) You can also use a BEGIN block to do the same thing; worth knowing for when you want to make the awk script stand alone.
4. The reason why 1 prints lines in awk is that it is a condition (a filter) which always evaluates to true, and the default action in awk when no code block is attached to the filter is to print $0. So 1 by itself is equivalent to 1 {print} or 1 {print $0} or just {print}.
5. In my last variation, I used a sub function to substitute the regex /^E:$/ (start of string, E:, end of string) with "" in $4.

Since the sub function returns the number of substitutions that were made (either 1 or 0; use gsub to make more than one replacement), you can code golf this problem by adding one to the sub function result to ensure you have a pattern that is always true so that the resulting line is printed whether a substitution is made or not. Here is the code golf version, not recommended for beginner use if you are putting this in a script you will then maintain:

awk -F, -v OFS=, 'sub(/^E:$/,"",$4)+1' data.final

:)

With sed:

sed -r 's/^([^,]+,[^,]+,[^ ]+,)E:(,)/\1\2/' file.txt

The 4th comma separated field will be made blank if it contains only E:.

Example:

% cat file.txt
12,324,32342,E:fsdsf,23432,34534,45345,324
13,3224,342,E:werwefsdsf,23432,34534,45345,324
121,3244,33442,E:,23432,34534,45345,324

% sed -r 's/^([^,]+,[^,]+,[^ ]+,)E:(,)/\1\2/' file.txt 
12,324,32342,E:fsdsf,23432,34534,45345,324
13,3224,342,E:werwefsdsf,23432,34534,45345,324
121,3244,33442,,23432,34534,45345,324

Assuming your file name is file, you can try the following:

while read -r line; 
do 
var="$(echo "$line" | cut -d ',' -f 4)";

  if [[ "$var" = "E:" ]]; then echo "$line" | sed s/"$var"/''/g ; 
  else echo "$line";
  fi; 

done < file

or:

while read -r line; do var="$(echo "$line" | cut -d ',' -f 4)"; if [[ "$var" = "E:" ]]; then echo "$line" | sed s/"$var"/''/g ; else echo "$line";fi; done < file

Explanation:

1. while read -r line; reads file line by line
2. var="$(echo "$line" | cut -d ',' -f 4)"; sores the string at 4th place separated by , into variable var
3. if [[ && "$var" = "E:" ]]; then echo "$line" | sed s/"$var"/' '/g ; if $var has string exactly E: then sed s/"$var"/''/g ; replaces it with blank ""
4. else echo "$line"; other wise it prints line as it is

Example output (as expected from question):

• file:

  $ cat file
  12,324,32342,E:fsdsf,23432,34534,45345,324
  
  13,3224,342,E:werwefsdsf,23432,34534,45345,324
  
  121,3244,33442,E:,23432,34534,45345,324
  

• Running command:

  $ while read -r line; do var="$(echo "$line" | cut -d ',' -f 4)"; if [[ "$var" = "E:" ]]; then echo "$line" |sed s/"$var"/' '/g ; else echo "$line";fi; done < file
  12,324,32342,E:fsdsf,23432,34534,45345,324
  
  13,3224,342,E:werwefsdsf,23432,34534,45345,324
  
  121,3244,33442,,23432,34534,45345,324
  

You can also redirect your output to file by using >> file2 or |tee file2 at the last of command:

while read -r line; do var="$(echo "$line" | cut -d ',' -f 4)"; if [[ "$var" = "E:" ]]; then echo "$line" |sed s/"$var"/' '/g ; else echo "$line";fi; done < file | tee file2