用sed替换匹配的第一次出现

Question

我正在用sed进行查找和替换,用BASH变量替换BASH变量$a(当在新行的开头时)$b:

sed -i "s#^$a#$b#" ./file.txt

这取代了所有匹配^$a.如何只替换^a整个文件中的第一个匹配项？

Answer 1

一种方式使用sed:

sed "s/^$var1/$var2/ ; ta ; b ; :a ; N ; ba" infile

说明:

s/^$var1/$var2/             # Do substitution.
ta                          # If substitution succeed, go to label `:a`
b                           # Substitution failed. I still haven't found first line to 
                            # change, so read next line and try again.
:a                          # Label 'a'
N                           # At this position, the substitution has been made, so begin loop
                            # where I will read every line and print until end of file.
ba                          # Go to label 'a' and repeat the loop until end of file.

由Jaypal提供的相同示例的测试:

内容infile:

ab aa
ab ff
baba aa
ab fff

运行命令:

sed "s/^$var1/$var2/ ; ta ; b ; :a ; N ; ba" infile

结果:

bb aa
ab ff
baba aa
ab fff