使用awk将符合特定规则的第一列单词小写化的命令修正问题

Let's break down what's going wrong with your command and how to fix it.

Your Scenario #

You want to convert the first column to lowercase only if it matches the regex ^[A-Z][a-z]+$, with fields separated by tabs.

• Input:

  Agne	XXX
  Agn_Ball	YYY
  

• Desired Output:

  agne	XXX
  Agn_Ball	YYY
  

• Original Command:

  awk -F '\t' 'BEGIN {OFS='\t'} $1 ~ /^[A-Z][a-z]+$/ {print tolower($1),$2}1' < test.tmp > test.tmp2
  

• Actual (Incorrect) Output:

  agne	XXX
  Agne	XXX
  Agn_Ball	YYY
  

What's Causing the Duplicate Line? #

The issue is the trailing 1 at the end of your awk script. In awk, any non-zero value (like 1) is treated as a true condition, which triggers the default action: print $0 (print the entire original line).

For lines where the first column matches your regex:

1. Your script first runs print tolower($1),$2 (outputs the modified line)
2. Then the 1 triggers print $0 (outputs the original unmodified line)
   That's why you get duplicate lines for matching entries.

Fixed Commands #

Here are two clean ways to fix this:

Option 1: Modify the Field and Print Once

This approach updates the first column in-place when it matches, then prints every line (modified or original) once:

awk -F '\t' 'BEGIN {OFS="\t"} $1 ~ /^[A-Z][a-z]+$/ { $1 = tolower($1) } 1' test.tmp > test.tmp2

• When the regex matches, we overwrite $1 with its lowercase version
• The trailing 1 then prints the entire line (with the modified $1 if applicable)

Option 2: Explicit Conditional Printing

If you prefer separate logic for matching vs non-matching lines, use an if/else block (no trailing 1 needed):

awk -F '\t' 'BEGIN {OFS="\t"} {
  if ($1 ~ /^[A-Z][a-z]+$/) {
    print tolower($1), $2
  } else {
    print $0
  }
}' test.tmp > test.tmp2

This explicitly handles each case, ensuring every line is printed exactly once.

Either of these will give you the desired output without duplicate lines.

内容的提问来源于stack exchange，提问作者Luca