2004.06_Power Tool Secrets-Hidden Gems that Make Linux Life Easy.pdf

KNOW HOW

Power Tools: Secrets

Hidden in plain sight

Linux has a wealth of command line tools. Everybody has their favorite. For

used for speed purposes. The magic file

indicates which bytes need to appear in

which position to indicate a particular

file type. The file description can be as

simple or complex as needed. MP3s, for

example can indicate their bit rate and

their recording frequency.

every cat or more , there’s probably another ten commands that rarely see the

light of day. This month, Steven Goodwin uncovers some hidden gems that

don’t deserve to remain hidden for much longer. BY STEVEN GOODWIN

F inding information is not difficult.

The real problem is knowing what

to look for in the first place.

$whatis apropos

apropos (1) - search the manual U

page names and descriptions

$ file Vexations.mp3

Vexations.mp3: MPEG 1.0 layer U

3 audio stream data, 48 kBit/s U

, 44.1 kHz, jstereo

Anyone with superuser privileges can

recompile a magic.mgc file with:

apropos

This utility will search through each man

page looking for a particular word or

phrase. This can be a keyword or regular

expression, and so can be quite complex.

It will then output each appropriate com-

mand, along with its description. So, if

you’re looking for a command that’s

related to passwords try as in Listing 1:

The -e switch will only match exact

words (so in the above example, the

phrase password s would not get

matched). As you are probably aware,

the number in brackets indicates the sec-

tion of the man pages where a particular

command exists. See Table 1 for details.

You can invoke a particular section by

using:

This is the description that appears on

the first line of the man page, and the

same text that appears with the apropos

command. whatis can also take regular

expressions, and is often used to quickly

check a command without having to for-

mat and display the whole man page.

$ file -C

which

Linux commands are spread across dif-

ferent paths. My system has 2242

commands in seven different directories!

(Press the tab key twice and let tab com-

pletion show you how many are

available, followed by echo $PATH ).

Determining which command is exe-

cuted from which directory is the job of a

command called, er, which !

file

Before reading a text file, with either cat

or more , it is sensible to check that it is,

in fact, text. Failure to do so may cause

your screen to fill

with junk, and re-

quire a reset

command be-

fore your shell

becomes legi-

ble again. This

command lets

us know in

advance what

type of file this is. It

is an improvement

over the use of file exten-

sions (which may be wrong, or

non-existent), as it reads a portion of

the file and uses its contents to deter-

mine the type. For example, a text file

will be one that has carriage returns in

it, and a gzip file will begin with a spe-

cific header with the bytes \037 and

\213.

file determines the type by using a file

of magic numbers stored at /usr

/share/misc /magic and /usr/share/misc

/magic.mgc , the latter being a pre-com-

piled version of the original magic file,

$ which file

/usr/bin/file

man chage 1

If an install, or compile, is causing prob-

lems, or you can not see why a particular

command is not getting called, then

which will indicate the command that is

getting called in its place. This is helpful

in finding out when older (or newer) ver-

sions of programs are being used

erroneously. It can also be used with file

to determine if the command about to be

run is a program, symlink, or script.

whatis

whatis is a very simple command

that produces a useful

description of the

command.

$ file `which which`

/usr/bin/which: Bourne-Again U

shell script text executable

A Saucerful of Secrets

With everything in Linux being treated

as a file, it’s no surprise that a large

number of commands exist to handle

them.

June 2004

www.linux-magazine.com

Linux – Worst Kept Secrets

Power Tools: Secrets

KNOW HOW

Table 1: Man page sections

Listing 1: apropos

Section

Description

01 $ apropos password

02 afppasswd (1)

Executable programs or shell commands

- netatalk password maintenance utility

System calls (functions provided by the

kernel)

03 chage (1)

- change user password expiry information

04 chpasswd (8)

- update password file in batch

Library calls (functions within system

libraries)

05 crypt (3)

- password and data encryption

06 ... and so on ...

Special files (usually found in /dev)

File formats and conventions eg

/etc/passwd

mentary set, we can only replace with a

single character.

Note that when we redirect both input

and output, the names must differ. This

is because the redirection truncates the

destination file to zero before the com-

mand executes. To replace the original

file, you could include this command in

a small script that uses mktemp to gener-

ate a temporary, and unique, filename.

Games

Macro packages and conventions eg

man(7), groff(7)

$ uname -v | tr -c [a-zA-Z0-9] _

_44_SMP_Sun_Dec_28_19_07_54_ U

GMT_2003_

System administration commands

Kernel routines [Non standard]

tr is short for translate, and substitutes

one set of characters with another. It can

also remove characters or squeeze multi-

ple occurrences of them together to be

replaced by a single instance. In all

cases, input is taken from stdin, and fed

to stdout. Translations are often used to

demonstrate rot13 processing, or con-

verting text into lowercase. In these

cases, both sets of characters must be of

equal size.

There is also a snappy shortcut for

alphanumeric characters called :alnum: ,

which can be used in place of the rather

verbose [a-zA-Z0-9] .

Another feature of tr lets us delete

individual characters. This gives an easy

and quick way of removing spurious car-

riage return codes from text files that

have originated from within the Win-

dows universe.

cut

cut is a program that is used to extract

columns of data from the input. A popu-

lar use of this command is to extract

information from commands like ls and

ps which are naturally tabulated. By

default, the columns are considered sep-

arate by the presence of a tab. However,

this can be changed to any arbitrary

character or symbol with the -d switch.

Each column is termed a field , and you

can output individual fields, or groups

therefore, using -f .

tr -d "\r" < windows.txt > U

linux.txt

#wecan use the A-Z shorthand U

rather than the alphabet in full

$tr[A-Z] [a-z] < afile U

> a_lower_case_file

$tr[a-zA-Z] [n-za-mN-ZA-M] U

< afile > a_rot13_file

Referring again to our filename fixer

above, we could also have decided to

delete the problematic characters,

instead of changing them. Giving us,

$ls-l|tail +2 | tr -s ' ' | U

cut -d ' ' -f 3 | sort | uniq

root

steev

$ uname -v | tr -c -d [:alnum:]

44SMPSunDec28190754GMT2003

However, tr has many more uses than

that. It can be used to validate potential

filenames by changing non-alphanu-

meric characters into a filesystem

friendly underscore. This requires the -c

(complement) option, meaning ‘any-

thing except the following’. As we have

no way of knowing how many letters

and numbers might exist in the comple-

Squeezing is a technique to compress

several repeated characters into one. We

could use this to replace multiple blank

lines with a single carriage return.

The above example uses a little extra

massaging, firstly to remove the initial

line from ls ( tail +2 , see Table 2) and

then to squeeze the spaces between each

field into one ( tr -s ). From here cut simply

outputs field three (the user name) of

each file in the directory. Since there are

likely to be a lot of duplicates, we sort the

names into alphabetical order and then

use uniq , which acts like tr ‘s squash, but

acts upon successively identical lines.

cut can output multiple fields by

applying the -f option with a comma

(which gives only the columns

requested), or a hyphen (which shows a

range of columns).

$tr-s'\n' <many_blank_lines U

>few_blank_lines

Table 2: Secret Arguments

Arguement

Description

unzip -a

Converts all text files to use Linux-style carriage returns when decompressing

echo -n

Used on its own will produce zero length files (normal echo adds a carriage return)

cat -n

Adds numbers before each line in the file. Very useful when printing program listings. To number

non-blank lines use -b

tail +2

Outputs the whole file, starting from the second line. Useful for removing the headings on

programs like ls and ps

grep -v

Instead of returning every line that matches, -v causes grep to match every line that doesn’t.

grep -v grep will therefore ignore the grep process(es) itself which can be useful when searching

the process list

$#Give the time & date U

of every file

$ls-l|tail +2 | tr -s ' ' | U

cut -d ' ' -f 6-8

ls -1

Lists each file name on its own line, without the ‘total’ header

■

www.linux-magazine.com

June 2004

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