- #!/bin/bash 
- s="coffe 
- #milk 
- water"; 
- checkval="#milk"; 
- if [[ $checkval != \#* ]]; then 
-   sed "s/$checkval/#&/" <<< "$s" 
- else 
-   sed "s/$checkval/${checkval:1}/" <<< "$s" 
- fi; 
-   
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