About sed

The sed means as “stream editor”, it is used to perform text things from an input stream to an _output stream.

Glimpse of man page

DESCRIPTION

The sed utility reads the specified files, or the standard input if no files are specified, modifying the input as specified by a list of commands. The input is then written to the standard output.

A single command may be specified as the first argument to sed. Multiple commands may be specified by using the -e or -f options. All commands are applied to the input the order they are specified regardless of their origin.

Suppress the output, and p command to print specific lines.

➜  /tmp echo 'aabbcc' | sed -n '1p'
aabbcc
➜  /tmp echo 'aabbcc' | sed -n '2p'
➜  /tmp 

$p command means to ouput the last line of the stream. By the way, sed treats multiple input files as one long stream.

# Print the first line of one.txt and the last line of two.txt 
➜  /tmp sed -n '1p;$p' one.txt two.txt
one
end two

Helpful Documentations

It’s a good way to use man sed to know the options which sed supports quickly(but on BSD or GNU released OS, the content maybe exist some difference, like linux and OSX). And also, you can find the official documentations at here.

Know the construct of sed command

Syntax

sed [-Ealn] [-e command] [-f command_file] [-i extension] [file ...]

sed commands follow this syntax:

	[addr]X[options]

• X: is a single-letter sed command.
• [addr]: is an optional line address. If specified, the command X will only executed on the matched lines. [addr] canbe a single line number, a regular expression, or a range of lines.

The following example deletes lines 30 to 35 in the input, 30,35 is an address rage. d is the delete command.

sed ’30,35d’ input.txt > output.txt

The following example prints all input until a line starting with the word “import”, then quit with exit code 42. If such line was not found, sed return with 0. /^foo/ is a regular expression address. q is the quit command.

$sed '/^#define/q42' git.c ; echo $?
#include "builtin.h"
#include "config.h"
#include "exec-cmd.h"
#include "help.h"
#include "run-command.h"
#include "alias.h"
#include "shallow.h"

#define RUN_SETUP               (1<<0)
42

Commands within a script or script-file can be separated by semicolons (;) or newlines (ASCII 10). Multiple scripts can be specified with -e or -f options.

The following examples are all equivalent:

sed ’/^foo/d ; s/hello/world/’ input.txt > output.txt

sed -e ’/^foo/d’ -e ’s/hello/world/’ input.txt > output.txt

echo ’/^foo/d’ > script.sed
echo ’s/hello/world/’ >> script.sed
sed -f script.sed input.txt > output.txt

echo ’s/hello/world/’ > script2.sed
sed -e ’/^foo/d’ -f script2.sed input.txt > output.txt