sed & awk, 2nd Edition

A Global Perspective on Addressing

One of the first things you’ll notice about sed commands is that sed will apply them to every input line. Sed is implicitly global, unlike ed, ex, or vi. The following substitute command will change every “CA” into “California.”

s/CA/California/g

If the same command were entered from the ex command prompt in vi, it would make the replacement for all occurrences on the current line only. In sed, it is as though each line has a turn at becoming the current line and so the command is applied to every line. Line addresses are used to supply context for, or restrict, an operation. (In short: Nothing gets done in vi unless you tell it which lines to work on, while sed will work on every line unless you tell it not to.) For instance, by supplying the address “Sebastopol” to the previous substitute command, we can limit the replacement of “CA” by “California” to just lines containing “Sebastopol.”

/Sebastopol/s/CA/California/g

An input line consisting of “Sebastopol, CA” would match the address and the substitute command would be applied, changing it to “Sebastopol, California.” A line consisting of “San Francisco, CA” would not be matched and the substitution would not be applied.

A sed command can specify zero, one, or two addresses. An address can be a regular expression describing a pattern, a line number, or a line addressing symbol.

• If no address is specified, then the command is applied to each line.

• If there is only one address, the command is applied ...