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Bash: How do I get the command history in a screen session?

05/07/2017 Comments off

If I start a screen session with screen -dmS name, how would I access the command history of that screen session with a script?

Using the , the last executed command appears, even in screen.

ANSWER:

I use the default bash shell on my system and so might not work with other shells.

this is what I have in my ~/.screenrc file so that each new screen window gets its own command history:

Default Screen Windows With Own Command History

To open a set of default screen windows, each with their own command history file, you could add the following to the ~/.screenrc file:

screen -t "window 0" 0 bash -ic 'HISTFILE=~/.bash_history.${WINDOW} bash'
screen -t "window 1" 1 bash -ic 'HISTFILE=~/.bash_history.${WINDOW} bash'
screen -t "window 2" bash -ic 'HISTFILE=~/.bash_history.${WINDOW} bash'

Ensure New Windows Get Their Own Command History

The default screen settings mean that you create a new window using Ctrl+a c or Ctrl+a Ctrl+c. However, with just the above in your ~/.screenrc file, these will use the default ~/.bash_history file. To fix this, we will overwrite the key bindings for creating new windows. Add this to your ~/.screenrc file:

bind c screen bash -ic 'HISTFILE=~/.bash_history.${WINDOW} bash'
bind ^C screen bash -ic 'HISTFILE=~/.bash_history.${WINDOW} bash'

Now whenever you create a new screen window, it’s actually launching a bash shell, setting the HISTFILE environmental variable to something that includes the current screen window’s number ($WINDOW).

Command history files will be shared between screen sessions with the same window numbers.

Write Commands to $HISTFILE on Execution

As is normal bash behavior, the history is only written to the $HISTFILE file by upon exiting the shell/screen window. However, if you want commands to be written to the history files after the command is executed, and thus available immediately to other screen sessions with the same window number, you could add something like this to your ~/.bashrc file:

export PROMPT_COMMAND="history -a; history -c; history -r; ${PROMPT_COMMAND}"
Categories: Système Tags: , , ,

Ubuntu Check RAM Memory Chip Speed and Specification From Within a Linux System

22/06/2016 Comments off

I want to add more RAM to my server running Ubuntu Linux. How do I find out my current RAM chip information such as its speed, type and manufacturer name within a Linux system without opening the case?

You need to use the dmidecode command which is a tool for dumping a computer’s DMI (some say SMBIOS) table contents in a human-readable format. This table contains a description of the system’s hardware components (such as RAM), as well as other useful pieces of information such as serial numbers and BIOS revision. Thanks to this table, you can retrieve hardware information without having to probe for the actual hardware. Open a command-line terminal (select Applications > Accessories > Terminal), and then type:

$ sudo dmidecode --type memory

OR

# dmidecode --type memory | less

OR

$ sudo dmidecode --type 17

Sample outputs:

# dmidecode 2.10
SMBIOS version fixup (2.51 -> 2.6).
SMBIOS 2.6 present.
Handle 0x0011, DMI type 16, 15 bytes
Physical Memory Array
	Location: System Board Or Motherboard
	Use: System Memory
	Error Correction Type: None
	Maximum Capacity: 4 GB
	Error Information Handle: Not Provided
	Number Of Devices: 4
Handle 0x0012, DMI type 17, 27 bytes
Memory Device
	Array Handle: 0x0011
	Error Information Handle: No Error
	Total Width: 72 bits
	Data Width: 64 bits
	Size: 2048 MB
	Form Factor: DIMM
	Set: 1
	Locator: DIMM#1A
	Bank Locator: Bank 1
	Type: DDR2
	Type Detail: Synchronous
	Speed: 667 MHz
	Manufacturer: Not Specified
	Serial Number: Not Specified
	Asset Tag: Not Specified
	Part Number: Not Specified
Handle 0x0013, DMI type 17, 27 bytes
Memory Device
	Array Handle: 0x0011
	Error Information Handle: No Error
	Total Width: 72 bits
	Data Width: 64 bits
	Size: 2048 MB
	Form Factor: DIMM
	Set: 1
	Locator: DIMM#2A
	Bank Locator: Bank 2
	Type: DDR2
	Type Detail: Synchronous
	Speed: 667 MHz
	Manufacturer: Not Specified
	Serial Number: Not Specified
	Asset Tag: Not Specified
	Part Number: Not Specified
Handle 0x0014, DMI type 17, 27 bytes
Memory Device
	Array Handle: 0x0011
	Error Information Handle: No Error
	Total Width: 72 bits
	Data Width: 64 bits
	Size: 2048 MB
	Form Factor: DIMM
	Set: 1
	Locator: DIMM#1B
	Bank Locator: Bank 1
	Type: DDR2
	Type Detail: Synchronous
	Speed: 667 MHz
	Manufacturer: Not Specified
	Serial Number: Not Specified
	Asset Tag: Not Specified
	Part Number: Not Specified
Handle 0x0015, DMI type 17, 27 bytes
Memory Device
	Array Handle: 0x0011
	Error Information Handle: No Error
	Total Width: 72 bits
	Data Width: 64 bits
	Size: 2048 MB
	Form Factor: DIMM
	Set: 1
	Locator: DIMM#2B
	Bank Locator: Bank 2
	Type: DDR2
	Type Detail: Synchronous
	Speed: 667 MHz
	Manufacturer: Not Specified
	Serial Number: Not Specified
	Asset Tag: Not Specified
	Part Number: Not Specified

10 Amazing and Mysterious Uses of (!) Symbol or Operator in Linux Commands

16/06/2016 Comments off

Linux-logo-without-version-number-banner-sized-348x196The '!' symbol or operator in Linux can be used as Logical Negation operator as well as to fetch commands from history with tweaks or to run previously run command with modification. All the commands below have been checked explicitly in bash Shell. Though I have not checked but a major of these won’t run in other shell. Here we go into the amazing and mysterious uses of '!' symbol or operator in Linux commands.

1. Run a command from history by command number.

You might not be aware of the fact that you can run a command from your history command (already/earlier executed commands). To get started first find the command number by running ‘history‘ command.

$ history

History shell command

Now run a command from history just by the number at which it appears, in the output of history. Say run a command that appears at number 1551 in the output of ‘history‘ command.

$ !1551

History by number

And, it runs the command (top command in the above case), that was listed at number 1551. This way to retrieving already executed command is very helpful specially in case of those commands which are long. You just need to call it using ![Number at which it appears in the output of history command].

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