图解sed的hold space和pattern space的交

代码grep3line.sh

以下脚本的功能是查找文本的匹配字符串，然后打印匹配字符串所在行的前一行，匹配字符串所在行，匹配字符串所在行后一行。然后以___（三个下划线）结尾

#!/bin/sh
# grep3 - prints out three lines around pattern
# if there is only one argument, exit

case $# in 
    1);;
    *) echo "Usage: $0 pattern";exit;;
esac;
# I hope the argument doesn't contain a /
# if it does, sed will complain

# use sed -n to disable printing 
# unless we ask for it
sed -n '
'/$1/' !{
    #no match - put the current line in the hold buffer
    x
    # delete the old one, which is 
    # now in the pattern buffer
    d
}
'/$1/' {
    # a match - get last line
    x
    # print it
    p
    # get the original line back
    x
    # print it
    p
    # get the next line 
    n
    # print it
    p
    # now add three dashes as a marker
    a\
---
    # now put this line into the hold buffer
    x
}'

执行脚本

echo -e '1\n2\n3\n4' | ./grep3line.sh 2
# 输出
1
2
3
___

图解过程