2004.12_Command Line-Bash Magic.pdf

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Command Line

Bash Magic

The secrets we’re going to learn cannot be summed up in words. In fact, all of

the magic we’ll conjure up within these pages can be represented with the

characters ! , $ , ? , and - . BY STEVEN GOODWIN

Note that the command is stored in the

history with its components fully substi-

tuted, not the !$ version you type, as can

be seen with,

Without specifying which arguments are

to be used (by appending :$ , for exam-

ple), the entire command will be

repeated. There is a special case to

repeat the entire previous command

( !-1 ), and that is !! . You can also request

the most recent command that starts

with a particular group of letters. Bash

will match as many, or as few, characters

as you give it, looking for the most

recent command that began with all the

specified letters.

to type? The shell has a few

short symbols that will save

you many keystrokes if you know how to

use them.This article examines historical

substitution and other tricks.

$ history | tail -3

604 mkdir new_folder

605 cd new_folder

606 history | tail -3

!$ is a special case of a much larger tech-

nique called history substitution The

symbol consists of two components: ! ,

which indicates which command to use

(in this case, the most recent), and $ ,

which means the last argument. Both

sections can be replaced with a multi-

tude of different variations. Let us first

look at replacing the $ .

Notice that, in general, a colon is

required. This is used to separate the ! ,

indicating the last command, and the

details of which argument within that

command to use. The command in ques-

tion doesn’t have to be last one, either.

You can retrieve the next-to-the-last com-

mand with !-2, for example.

It Was Supposed To Be

So Easy

Was Supposed To Be So Easy. Typing

commands into the shell is all well and

good but can be cumbersome when

we’re presented with long filenames. Tab

completion can help in some cases, but

if there are a lot of files with very similar

names, we usually end up having to type

the whole thing anyway. But in many

circumstances, if tab completion is the

answer, we’re probably asking the wrong

question. We need to ask instead about

history substitution – and !$ .

$!m

mount /cdrom

For those that would prefer an interac-

tive history, press Control+R, start

typing, and watch bash automatically

find the most recent matching com-

mand.

Advanced users may care to supple-

ment the !$ argument with extra

qualifiers. These extra qualifiers will

retrieve a specific portion of the argu-

ment and no more. They are detailed in

Table 2. These qualifiers work with all

the above options, allowing for some

very powerful, and obtuse, shell control.

These qualifiers can be combined if

separated by colons. They are evaluated

in a left-to-right fashion, for example,

The Last Argument

This curious symbol is a place-holder for

the last argument of the previous com-

mand. !$ is useful for situations like this,

$ mount /cdrom

$cp/cdrom/*.pdf /media/library

$ umount !-2:$

umount /cdrom

Table 1: Symbols for History Substitution

$ mkdir new_folder

$cd!$

cd new_folder

Symbol

Description

Typed Example

Resultant command

Notes

!:0 (zero)

Use command

!:0 another

mkdir another

!:1 (one)

Use 1st argument

cd !:1

cd new1

Other arguments can be referenced in the same, numeric, manner

!:$ Use last argument cd !:$ cd new2 You may use !$ instead

!:1-2 Use 1st and 2nd arguments echo !:1-2 echo new1 new2

Omitting the first number uses 0 as default. Omitting the last uses the penultimate argument. !:n* uses the n ‘th argument

to the end (and is the same as !:n-$ )

! All arguments touch ! touch new1 new2

Acts like !:1-$. Useful for handling arguments from scripts, or in creating alias es. You may use !* instead

Here you can see that the argument is

substituted directly before execution,

enabling you to see the full command in

the shell before it is run – although you

don’t have time to cancel it if it’s wrong!

December 2004

www.linux-magazine.com

Special Characters in Bash

W hy type more than you need

Command Line

LINUX USER

$ mplayer U

/media/mp3/rock/file.mp3

$ echo !:1:h:t

rock

be able to delete the files for real, as fol-

lows:

tion to several files. See the following,

for instance:

$!!

rm *.bak

#Create a small hierarchy, U

tom/user, tom/group, tom/all, U

dick/user, dick/group, etc.

$ mkdir -p {tom,dick,harry} U

/{user,group,all}

One popular trick is to use !* as a safety

net before deleting files. First you start

by checking to ensure that the files you

have targeted targeted backup files are

correct.

Brackets

Bash uses several different sets of brack-

ets. The (rounded) brackets are for

subshells, the [square] brackets expand

ranges of letters (remember tr [A-Z] [a-z]

from [1]) and {curly} brackets, or

braces, expand each argument within

them and combine it with the rest of the

argument. Braces provide the benefit of

letting the user perform the same opera-

You cannot include spaces before or after

the commas (although they can be

escaped with \ if necessary), but it is fine

to leave entries blank. Blank entries

enable us to perform tricks like this:

$ls*.bak

one.bak three.bak two.bak

You can delete them with either rm !* ,

rm -i !* (which asks for confirmation

before deleting each file) or rm !*:p

(which prints the potential command to

the window, but doesn’t do anything,

acting as secondary check.)

$cpfilename{,.bak}

Table 2: Extra Qualifiers

Qualifier

Description

Typed Example

Result

Head

cd !$:h

cd /media/mp3/rock

Tail

echo !$:t

echo new

$rm!*:p

rm *.bak

Print - don’t execute

rm -rf !$:p

rm -rf /media/mp3/rock/new

Quote words

cd !$:q

cd ‘/media/mp3/rock/new’

x also does this, but breaks at newlines, spaces and tabs

You can then press the up arrow, or type

!! , to retrieve the last command you just

typed. After you press return, you’ll then

Remove extension

echo !:1:r

echo extra/ripped

Any preceding directory is left intact

Keep only the extension

echo !:1:e

echo .mp3

It keeps the dot!

www.linux-magazine.com December 2004

LINUX USER

Command Line

which expands to the arguments file-

name and filename.bak .

per se , but it is a convention amongst

many tools, such as cat . The same effect

can also be achieved by using /dev/stdin

as the input file.

This example uses - to concatenate

standard input with an existing file.

zero (0) if the last command was suc-

cessful and non-zero if the last command

wasn’t successful. The precise value of

the exit status is determined by the

severity of the error, so a minor infrac-

tion would be one (1), whilst the

inability to execute the command in the

first place would be 127. As well as

chaining commands together, this sym-

bol can be used as part of a self-aware

installer,

$! and the Process ID

The $! symbol holds the process ID of

the last command you ran as a back-

ground process. This is the same ID that

appears on screen:

$ cat - original > new

This text appears as the first U

line in 'new'. Followed by the U

rest of the "original" file

$ finger steev &

[1] 18959

$ echo $!

18959

01 gawk >/dev/null 2>&1

02 if [$?-eq 0];then

03 USE_AWK="gawk"

04 else

05 nawk >/dev/null 2>&1

06 if [$?-eq 0];then

This feature can serve as a poor man’s

timeout , whereby commands are run in

the background, and, if they haven’t

completed after a specific time, they are

killed!

And so from one minus sign, to two! The

double minus means “end of parame-

ters.” This symbol provides an elegant

way to tell the shell that whatever fol-

lows is a command argument (like a

filename), even if it looks like an option.

This becomes an essential feature when

you realize that -a is a legitimate file-

name in Linux, as well as a command

line option. Create a file called -a in your

home directory and try to delete it,

USE_AWK="nawk"

01 TEMPFILE=`mktemp`

02 DURATION=$1

03 shift

05 $* >$TEMPFILE &

06 PID=$!

08 sleep $DURATION

10 kill -9 $PID 2>/dev/null

12 cat $TEMPFILE

13 rm $TEMPFILE

08 else

echo "I need an awk

- any awk - to run!"

exit 1;

11 fi

12 fi

$rm-a

rm: invalid option -- a

Try `rm --help' for more info.

The preceding script picks an available

version of AWK from those installed on

the system. If gawk is reachable, the exit

status return code is 0 and gawk is used.

If gawk is not reachable on the system,

the script continues with nawk . The

order that the versions are referenced in

the script determines the priority of the

versions, while the redirection keeps our

screen tidy. Not every command can

work without arguments, so sometimes

you will have to invoke the help or ver-

sion number to get a zero return code.

Experiment with fitting these Bash

techniques into your own routine.

They’ll save you time and keystrokes.

The same error would occur if you typed

rm * in a directory where such a file

occurred. However, being surround by so

many other files, the problem would be

less obvious. By using -- to indicate the

end of the parameter list, our intention

can be realized quite simply.

It would be called thus:

./timeout.sh 2 finger U

steev@somehost.com

Powers of the Minus Sign

In shell programming, the minus sign

doesn’t have negative connotations, as it

is used in two of its most powerful, and

under-used, features. When used on its

own, a single minus is often used to

denote standard input in place of a file-

name. This is not a feature of the shell

$rm---a

You could also delete this file from a GUI

file manager or with the command rm

./-a , but neither are suitable for a batch

file that has to handle indeterminable

files. In these cases -- should always be

used, since a malicious user could name

his files ‘-a’, ‘-b’, ‘-c’, and so on, hoping

that a file scanner would either misun-

derstand the files or terminate

prematurely, leaving other suspect con-

tent undisturbed.

■

INFO

[1] Steven Goodwin:“Hidden in plain sight”,

Linux Magazine, Issue 43 / June 2004,

p48–49

Alias

When builders go down

the pub they talk about

football. Presumably

therefore, when foot-

ballers go down the

pub they talk about

builders! When Steven Goodwin goes

down the pub he doesn’t talk about

football. Or builders. He talks about

computers. Constantly…

Some people set up aliases in their .bashrc or

.alias files to ease the burden of typing.

Some standard inclusions are alias ll='ls -l'

and alias la='ls -A' .However, there are

numerous others referenced by Google and

employed by the web-surfing public. After

all these years, my favourite is still alias

cd..='cd ..' for all those times my fingers miss

the space bar!

$? and Exit Status

$? is another useful variable. This exple-

tive holds the exit status . The exit status

describes the result of the last command.

By convention this exit status will be

December 2004

www.linux-magazine.com

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