More Commands

CS155 Commands3

sed – stream editor

sed is a non-interactive editor that operates on streams.
The most common usage is some form of:
sed 's/pattern/replace/' file > new
sed does not change the contents of the original file. Use > to send changed output to a new file.
A nice introduction: http://www.grymoire.com/Unix/Sed.html

sed syntax

All lines:	`sed '`command`'` filename
One line:	`sed '`line command`'` filename
Range of lines:	`sed '`startline,endline command`'` filename

Commands:

s/old/new/ (with optional g flag)
d
Many others

sed line specification

A line specification in sed can be:

A number
$ - the last line
/pattern/

sed: A Few Examples

Replace all instances of hello with goodbye in new_file:
sed 's/hello/goodbye/g' new_file
Delete all lines that occur between foo and bar in file:
sed '/foo/,/bar/d' file
Delete all lines that contain the word “bob” in data:
sed '/bob/d' data

sed: A Few Examples

Delete lines 2–5 of new_file (but don't change new_file!):
sed '2,5d' new_file
Replace foo with bar in the first 5 lines of file:
sed '1,5s/foo/bar/g' file

sed: More Examples

Remove some punctuation characters:
sed 's/[,.?-]//g' filename
Strip blanks from beginning of lines:
sed 's/^ *//' filename
Strip blanks from end of lines:
sed 's/ *$//' filename

A Little More Regular Expression Syntax

In regular expressions, we escape characters by preceding them with a backslash.
Escaping a character does one of two things:
- removes the special meaning of special characters
- gives special meaning to non-special characters
Examples:
- . matches any character, but \. matches only the dot itself.
- ? matches itself, but \? is a special character

Alternation through Escaping

\| can be used to match either of two patterns:
grep 'one\|two' files

diff: Comparing Files

Usage: diff file1 file2
diff describes the modifications that should be applied to file1 to create file2

% sed '/^four/d' <~/pub/numbers >some-numbers
% echo "booga booga" >>some-numbers
% diff ~/pub/numbers some-numbers
4d3
< four
14d12
< fourteen
100a99
> booga booga

line1start, line1end action line2start, line2end
- a – add
- c – change
- d – delete

file: Getting Information on File Type

The file command attempts to determine the type of a file:

% file ~
/s/bach/a/class/cs155: directory
% file ~/pub/numbers
/s/bach/a/class/cs155/pub/numbers: ASCII text
% file /bin/bash
/bin/bash: ELF 64-bit LSB shared object, x86-64, version 1 (SYSV), dynamically linked, interpreter /lib64/ld-linux-x86-64.so.2, for GNU/Linux 3.2.0, BuildID[sha1]=e4abb3b0b8951a4f746530ae2e9b85559d5fb98f, stripped
% gzip -v <~/monster >monster.gz
 59.6%
% file ~/monster monster.gz
/s/bach/a/class/cs155/monster: UTF-8 Unicode text
monster.gz:                    gzip compressed data, last modified: Fri Jan 26 20:53:31 2018, from Unix, original size 401

It doesn’t just look at the file suffix—it looks at the contents. Let’s lie:

% man -Tps date >date-manual.jpg
% file date-manual.jpg
date-manual.jpg: PostScript document text conforming DSC level 3.0

CS155: Introduction to Unix

Spring 2018

Commands 3

More Commands

sed – stream editor

sed syntax

sed line specification

sed: A Few Examples

sed: A Few Examples

sed: More Examples

A Little More Regular Expression Syntax

Alternation through Escaping

diff: Comparing Files

file: Getting Information on File Type