darker.sh

        
            
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            #!/usr/bin/env bash
        
            
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            set -ueo pipefail
        
            
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            #set -x
        
            
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            darker_channel() {
        
            
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              value="$1"
        
            
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              light_delta="$2"
        
            
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              result="$(bc <<< "ibase=16; $value - $light_delta")"
        
            
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              if [[ "$result" -lt 0 ]]; then
        
            
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                result=0
        
            
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              fi
        
            
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              if [[ "$result" -gt 255 ]]; then
        
            
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                result=255
        
            
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              fi
        
            
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              echo "$result"
        
            
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            }
        
            
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            darker() {
        
            
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              hexinput="$(tr '[:lower:]' '[:upper:]' <<< "$1")"
        
            
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              light_delta="${2-10}"
        
            
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              a="$(cut -c-2 <<< "$hexinput")"
        
            
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              b="$(cut -c3-4 <<< "$hexinput")"
        
            
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              c="$(cut -c5-6 <<< "$hexinput")"
        
            
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              r="$(darker_channel "$a" "$light_delta")"
        
            
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              g="$(darker_channel "$b" "$light_delta")"
        
            
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              b="$(darker_channel "$c" "$light_delta")"
        
            
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              printf '%02x%02x%02x\n' "$r" "$g" "$b"
        
            
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            }
        
            
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            darker "$@"
        
            
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