Linux Format - UK (2019-12)

14 LXF257 December 2019 http://www.linuxformat.com

ANSWERS

text files. The --bytes option tells it to use

binary data instead. Normally tail will

output the last x bytes (or lines) of a file.

Prefixing the number with + means ‘start

with byte x’ instead. Depending on how

you are counting bytes, you may need to

pass ‘x+1’ to tail.

You usually see dd mentioned when

copying image data to disks, but it is far

more capable than that.

$ dd if=BinaryData.bin bs=X skip=1 |

<program>

This should do what you want. The bs

option tells dd to copy data in blocks of

that size – the default is 512 bytes. The

skip tells it to skip the first block, so you

get all but the first x bytes. As there is no

of (output file) option, dd sends its output

to stdout, which is then piped to your

program. If the data file is static, but you

want to use the stripped version more than

once, you could use the split command to

create a new file without the first x bytes:

$ split --bytes=X BinaryData.bin

This will split the file into x-sized

chunks, with numeric suffixes. If x is more

than half the size of the original file,

BinaryData.bin01 is all you need,

otherwise you need to join the other pieces

back together:

$ rm BinaryData.bin

$ cat BinaryData.bin?? >NewBinaryData.

bin

This may be a cumbersome or elegant

approach, depending on your needs, but

you can also output to a file with tail or dd.

Whichever you use is a matter of personal

preference, all will do what you need.

Q

Persistent Peppermint

I installed Peppermint from the

DVD to the hard disk on my laptop, a

Samsung R60 Plus, but when I press

Shut Down it won’t power off. It says

shut down, and then nothing.

Wolfgang

A

Ah, the curse of the pretty splash

screen. These boot and shutdown

screens provide a more friendly-looking

alternative to the traditional screen of

scrolling text messages. However, those

messages tell you what is happening, or

not happening, during boot and shutdown.

So the first step is to get rid of the screen.

You can usually do this by pressing Esc

when it appears. If that doesn’t work, edit

the boot options from the initial boot

menu, usually by pressing E, and remove

and references to splash or quiet – we

want splash-free and noisy. When you

have done this, your computer will still

stop during the shutdown process, but you

will be able to see what it was trying to do

when it stopped. This could be unmounting

a network share, a problem if the network

is brought down before shares are

unmounted, or shutting down a service

that doesn’t want to stop.

Once you have identified the culprit, a

quick trip to your preferred search engine

with the relevant message will usually

provide the solution. One situation that

sometimes arises is that the computer

goes through the boot process and even

displays “Shutting down” or “Powering

off”, but doesn’t switch off. In this case

rebooting works – only shutting-down fails.

One possible solution to this is to edit the

boot options as above and add acpi=force

or noacpi. If one of those works, you can

edit /etc/default/grub and add the

option to the GRUB_CMDLINE_LINUX_

DEFAULT setting and run:

$ sudo update-grub

to make the change permanent (see page

72 for more about using GRUB).

If rebooting works, the more kludgy

solution is to reboot, then hit the power

switch when the BIOS screen appears.

At this point there is no operating system

loaded and no disks mounted, so a hard

shutdown can do no harm.

QUICK REFERENCE TO... MULTI COMMANDS

When using the command line, there

are often occasions when you need to

run several commands in sequence –

this can also happen when using a GUI,

but the solution is nowhere near as

simple. The classic example of this is

the standard method of compiling

software from source, this requires you

to run ./configure (possibly with some

arguments) followed by make then

make install.

Each of these steps can take

anywhere from a few seconds to many

hours, depending on the complexity of

the code and speed of your machine.

Waiting for one to complete before

running the next is inefficient, so

instead you could do:

./configure; make; make install

The semicolons cause the commands

to be executed in sequence, as if you

had run each one individually. Some of

you may have already noticed a

potential problem here: what happens

if ./configure or make fails? Will the

subsequent commands try to execute

anyway? The answer is yes, which

would hide any error messages in the

following output, and you may not even

realise there was an error until you try

to use the program. A safer way of

running these commands is:

./configure && make && make install

&& is a logical operator. This

command line actually means ‘if

./configure is true and make is true

and make install is true’. Fortunately

for us, the shell determines if a

command is true by running it to see if

it gives any errors. If a command fails,

there is no point in running the next one

because the test has already failed, so

joining commands with && runs them

in sequence, but stops as soon as one

of them returns an error, meaning we

no longer need to babysit the shell.

The companion command to &&

is || , which means ‘or’. So in

command1 || command

command2 only runs if command

fails. This is less useful in the interactive

shell, but is used often in shell scripts:

somecommand1 || echo “Something

broke!”

Failure to shut down properly can often be caused by the hardware’s ACPI implementation.

14 LXF257December 2019 1112Decmbr 09Go2t 9

ANSWERS

text files. The --bytes option tells it to use
binary data instead. Normally tail will
output the last x bytes (or lines) of a file.
Prefixing the number with + means ‘start
with byte x’ instead. Depending on how
you are counting bytes, you may need to
pass ‘x+1’ to tail.
You usually see dd mentioned when
copying image data to disks, but it is far
more capable than that.
$ dd if=BinaryData.bin bs=X skip=1 |

that size – the default is 512 bytes. The

skip tells it to skip the first block, so you

get all but the first x bytes. As there is no

of (output file) option, dd sends its output

to stdout, which is then piped to your

program. If the data file is static, but you

want to use the stripped version more than

once, you could use the split command to

create a new file without the first x bytes:

$ split --bytes=X BinaryData.bin

This will split the file into x-sized

chunks, with numeric suffixes. If x is more

than half the size of the original file,

BinaryData.bin01 is all you need,

otherwise you need to join the other pieces

back together:

$ rm BinaryData.bin

$ cat BinaryData.bin?? >NewBinaryData.

bin

This may be a cumbersome or elegant

approach, depending on your needs, but

you can also output to a file with tail or dd.

Whichever you use is a matter of personal

preference,allwilldowhatyouneed.

Persistent Peppermint

I installed Peppermint from the

DVD to the hard disk on my laptop, a

Samsung R60 Plus, but when I press

Shut Down it won’t power off. It says

shut down, and then nothing.

Wolfgang

Ah, the curse of the pretty splash

screen. These boot and shutdown

screens provide a more friendly-looking

alternative to the traditional screen of

scrolling text messages. However, those

messages tell you what is happening, or

not happening, during boot and shutdown.

So the first step is to get rid of the screen.

You can usually do this by pressing Esc

when it appears. If that doesn’t work, edit

the boot options from the initial boot

menu, usually by pressing E, and remove

and references to splash or quiet – we

want splash-free and noisy. When you

have done this, your computer will still

stop during the shutdown process, but you

will be able to see what it was trying to do

when it stopped. This could be unmounting

a network share, a problem if the network

is brought down before shares are

unmounted, or shutting down a service

that doesn’t want to stop.

Once you have identified the culprit, a

quick trip to your preferred search engine

with the relevant message will usually

provide the solution. One situation that

sometimes arises is that the computer

goes through the boot process and even

displays “Shutting down” or “Powering

off”, but doesn’t switch off. In this case

rebooting works – only shutting-down fails.

One possible solution to this is to edit the

boot options as above and add acpi=force

or noacpi. If one of those works, you can

edit /etc/default/grub and add the

option to the GRUB_CMDLINE_LINUX_

DEFAULT setting and run:

$ sudo update-grub

to make the change permanent (see page

72 for more about using GRUB).

If rebooting works, the more kludgy

solution is to reboot, then hit the power

switch when the BIOS screen appears.

At this point there is no operating system

loaded and no disks mounted, so a hard

shutdown can do no harm.

When using the command line, there

are often occasions when you need to

run several commands in sequence –

this can also happen when using a GUI,

but the solution is nowhere near as

simple. The classic example of this is

the standard method of compiling

software from source, this requires you

to run ./configure (possibly with some

arguments) followed by make then

make install.

Each of these steps can take

anywhere from a few seconds to many

hours, depending on the complexity of

the code and speed of your machine.

Waiting for one to complete before

running the next is inefficient, so

instead you could do:

./configure; make; make install

The semicolons cause the commands

to be executed in sequence, as if you

had run each one individually. Some of

you may have already noticed a

potential problem here: what happens

if ./configure or make fails? Will the

subsequent commands try to execute

anyway? The answer is yes, which

would hide any error messages in the

following output, and you may not even

realise there was an error until you try

to use the program. A safer way of

running these commands is:

./configure && make && make install

&& is a logical operator. This

command line actually means ‘if

./configure is true and make is true

and make install is true’. Fortunately

for us, the shell determines if a

command is true by running it to see if

it gives any errors. If a command fails,

there is no point in running the next one

because the test has already failed, so

joining commands with && runs them

in sequence, but stops as soon as one

of them returns an error, meaning we

no longer need to babysit the shell.

The companion command to &&

is || , which means ‘or’. So in

command1 || command

command2 only runs if command

fails. This is less useful in the interactive

shell, but is used often in shell scripts:

somecommand1 || echo “Something

broke!”

Failure to shut down properly can often be caused by the hardware’s ACPI implementation.