Understanding sed command

Question

Please excuse if the question is too naive. I am new to shell scripting and am not able to find any good resource to understand the specifics. I am trying to make sense of a legacy script. Please can someone tell me what the following command does:

sed "s#s3AtlasExtractName#$i#g" load_xyz.sql >> load_abc.sql;

Answer 1

This command will replace all occurrences of s3AtlasExtractName with whatever $i is.

• s - Substitute
• # - Delimiter
• s3AtlasExtractName - Word that needs substituting
• # - Delimiter
• $i - i variable that will be used to replace s3AtlasExtractName
• # - Delimiter
• g - Global Replace all instance of s3AtlasExtractName in a single line and not just the first occurrence of it

So this will parse through load_xyz.sql and change all occurrences of s3AtlasExtractName to the value of $i and append the whole of the contents of load_xyz.sql to a file called load_abc.sql with the sed substitutions.

Answer 2

sed is a command line stream editor. You can find information about it here: http://www.computerhope.com/unix/used.htm

An easy example is shown below where sed is used to replace the word "test" with the word "example" in myfile.txt but output is sent to newfile.txt

sed 's/test/example/g' myfile.txt > newfile.txt

It seems that your script is performing a similar function by replacing the content of the load_xyz.sql file and storing it in a new file load_abc.sql Without more code I am just guessing but it seems that the parameter $i could be used as counter to insert similar but new values into the load_abc.sql file.

Answer 3

In short, this reads load_xyz.sql and replaces every occurrence of "s3AtlasExtractName" by whatever has been stored in the shell variable "i".

The long version is that sed accepts many subcommands with different formattings. Any "simple" sed command will look like 'sed '. The first letter of the subcommand tells you which operation sed is going to do with your files.

The "s" operation stands for "substitution" and is the most commonly used. It is followed by a Perl-like regexp: separator, regexp to look for, separator, value to substitute, separator, PREG flags. In your case, the separator is '#' which is pretty unusual but not forbidden, so the command substitues '$i' to every instance of 's3AtlasExtractName'. The 'g' PREG flag tells sed to replace every occurrence of the pattern (the default is to only replace its first occurrence on every line in the input).

Finally, the use of "$i" inside a double-quote-delimited string tells the shell to actually expand the shell variable 'i' so you'll want to look for a shell statement setting that (possibly a 'for' statement).

Hope this helps.

edit: I focused on the 'sed' part and kinda missed the redirection part. The '>>' token tells the shell to take the output of the sed command (ie the contents of load_xyz.sql with all occurrences of s3AtlasExtractName replaced by the contents of $i) and append it to the file 'load_abc.sql'.