sed is a special editor modifying files automatically. If you want to write a program to make changes in a file, sed is the tool to use.
Now, I introduce the most basic command that called substitution(s). The other commands will be introduced in other articles.
1.sed s/day/night/ day_file
There are four parts of this command.
s: substitute command
/../../: delimiter
day: regular expressiojn pattern for searching
night: replace string
This command will replace the first 'day' with 'night' line by line and show the results to the std. Attention: this command will not change the day_file.
2.sed s/day/night/g day_file
This command will replace every 'day' with 'night' line by line and show the results to the std. Attention: this command will not change the day_file. And g means global.
3.sed s/day/night/g < day_file > night_file
This command will replace every 'day' with 'night' line by line and generate the night_file to store the result. Attention: this command will not change the day_file.
4.sed s/day/night/g day_file > day_file_bak; mv day_file_bak day_file
This command will replace every 'day' with 'night' line by line and make changes to the day_file.(In fact, the command generate a new file named 'day_file_bak' to store the results and rename the 'day_file_bak' to 'day_file'.
