Linux tips and tricks

I always welcome suggestions to add new tips, to correct or improve existing ones.

The Shell

When you type command lines, you are interacting with a program call the “shell”. Unless otherwise indicated, the tips assume that you are using the Bash shell.

You can check that with the command-line:

echo $SHELL

For a basic introduction to interacting with the shell, I recommend Learning the shell and The Linux command line by William Shotts[^1].

• Openclassrooms Initiation à Linux (https://openclassrooms.com/en/courses/7170491-initiez-vous-a-linux?archived-source=43538)

Here are some useful shortcuts to edit the command line:

Editing the current command line

• Ctrl-d : delete character under the cursor
• Ctrl-k : delete everything from the cursor till the end of the line
• Alt-d : delete till the end of the current word
• Ctrl-a : move the cursor to the beginning of the line
• Ctrl-e : move the cursor to the end of the line
• Ctrl-B, Alt-B: move backward, by one char or by one word
• Ctrl-F, Alt-F: move forward, by one char or by one word

Navigating the history of command lines:

• Ctrl-r : interactive backward search in the history of command lines
• Ctrl-p : move to previous command line
• Ctrl-n : move to next command line

Job control

• Ctrl-C: Terminate (kill) the running job
• Ctrl-Z: Stop the running job (you can then put in background with bg)

• Ctrl-D: Terminate the Shell and close the Terminal

• Ctrl-L: Clear the terminal

• Ctrl+S: Stop all output to the screen. This is particularly useful when running commands with a lot of long, verbose output, but you don’t want to stop the command itself with Ctrl+C.
• Ctrl+Q: Resume output to the screen after stopping it with Ctrl+S.

Tab completion

To enter the name of a file, you can type the first characters then press the TAB key to autocomplete the end of the name. Thie also work for commands or environment variables. See https://en.wikipedia.org/wiki/Command-line_completion?

Note that bash relies the readline library to interact with the user. Its behavior can be customized by options in $HOME/.inputrc. For example, to enable case-insensitive tab completion:

echo 'set completion-ignore-case On' >> ~/.inputrc

Commands

When you type something on a command line and press enter, you are submitting a command to the shell. The first token is a command, of which there are different types:

• a program, that is an executable file located somewhere on your file system (it can be a binary or a script)
• shell built-in function (e.g. echo)
• a user defined function
• an alias

Try:

type cp
type ll
type echo
type for

The vast majority of commands that you are going to type are programs (scripts or binary). The list of directories containing programs is stored in the environment variable PATH:

printenv PATH

Directories are separated by :.

If you want to add a directory, say /opt/bin to the PATH:

 export PATH="/opt/bin:$PATH"

From now on, the shell will search for programs in /opt/bin`` before scanning the other directories listed inPATH`. The program executed is the first encountered in the list. You can scan the list with:

which -a command

If you "screw up" the PATH, you will no longer have access to programs. In this situation, the best is to close the shell (by pressing Ctrl-D) and open a new one. You can test this situation typing just:

PATH=

If you want a modification of the PATH variable to be permanent, i.e. to be active each time you start a shell, add the export PATH=... line to the file ~/.profile.

Creating scripts

If you happen to often type the same series of commands, it is a good idea to create a script, that is, basically, a text file gathering the sequence of commands to be executed. Then, you will just have to type the filename of this script to execute all the commands.

If it does not exist yet, create a bin directory in your home folder:

mkdir $HOME/bin.

Use a text editor to create a file myscript in this directory, and enter the following on the first line:

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#! /bin/bash

Then type the series of commands (one per line) you want to be executed.

Save the file myscript and enter the commands:

chmod +x ~/bin/myscript
PATH="$HOME/bin:$PATH"

You can now type myscript on the command line to execute the series of commands.

To go further, you should learn how to use arguments to scripts.

Note that you write scripts in other languages than bash, e.g. python.

Startup scripts: .profile, .bashrc, .bash_profile

~/.bash_profile, ~/.profile, .bashrc are scripts that are executed automatically when you start a shell. This allows you to set up your envirement (prompt, PATH), create aliases for common operations, ...

There are two types of shells: login shells (that open after you type your login and password) and nonlogin shells.

Login shells execute ~/.profile and ~/.bash_profile.

Non-login shells only execute ~/.bashrc, not ~/.profile nor ~/.bash_profile

Anything that should be available to graphical applications OR to sh (or bash invoked as sh) MUST be in ~/.profile

• ~/.bash_profileshould just load .profile and .bashrc (in that order)
• ~/.profile has the stuff NOT specifically related to bash, such as environment variables (PATH and friends)
• ~/.bashrc has anything you'd want at an interactive command line. Command prompt, EDITOR variable, bash aliases

A few other notes: * ~/.bashrc must not output anything * Anything that should be available only to login shells should go in ~/.profile * Ensure that ~/.bash_login does not exist.

See: * https://superuser.com/questions/789448/choosing-between-bashrc-profile-bash-profile-etc * https://stackoverflow.com/questions/902946/about-bash-profile-bashrc-and-where-should-alias-be-written-in * http://mywiki.wooledge.org/DotFiles

Jumping directly to directories

If you are tired of typing intermediate directory names when changing directory, check out the Directory Bookmarks functions for bash at https://github.com/icyfork/dirb/.

Download https://raw.githubusercontent.com/icyfork/dirb/master/dirb.sh in your $HOME folder and add the following line to the file $HOME/.bashrc:

source $HOME/dirb.sh

Once installed, you can save bookmarks for specific directories (command s) and later jump into them directly (command g). Here are all the available operations:

s       Save a directory bookmark
g       go to a bookmark or named directory
p       push a bookmark/directory onto the dir stack
r       remove saved bookmark
d       display bookmarked directory path
sl      print the list of directory bookmarks
sl -l                  long list
sl -p                  path list

Killing a program that is no longer responsive

It may happen that a program monopolizes most of the CPU, but does not longer respond to input. Such a program is crashed and should be "killed".

For applications running in a terminal, first try to press Ctrl-C.

If this does not work, or if the application is running in its own window but refusing to close, open a terminal and type:

pkill program_name

You can also use the command ps -ef to locate the application and note down the "process identification number" in the 'PID' column. Then, type:

kill PID

(in place of PID, use the number associated to the process listed in 'ps' output). Check if the program was destroyed with the ps command; if not:

kill -9 PID

If the whole graphics system no longer responds, you can try to open a text mode terminal with Ctrl-Alt-F1 or Ctrl-Alt-F4, log in and kill the programs that causes problem. Sometimes, the only solution is to kill Xorg, the display server).

It the keyboard does not repond anymore, before switching off the computer, you can try to connect from another computer on the same network using ssh and to kill the applications or do a proper shutdown (typing 'halt' on the command line).

Connecting to a remote computer using ssh

A secure method to connect to a remote computer:

ssh computername

or

ssh login@computername

if the login id on the remote computer is different than the one on the local computer.

If you plan to launch graphical application on the remote computer, you need to add the -X option:

ssh -X login@computername

Note: you may have to run xhost + on the local, client, computer.

If you often connect to a computer, you can create an alias in $HOME/.profile:

alias ssh-myserver="ssh -X login@computername"

Note that:

• the client computer must have the ssh client (sudo apt install openssh-client)
• the remote computer must be running a sshd server (run sudo apt install openssh-server on it).

You can troubleshoot connection issues with

ssh -vv login@computer

Setting up SSH

To avoid having to type your login password each time you use ssh or scp, you can setup SSH to use public and private keys to perform the authentification automagically.

First, you must generate keyfiles, once, on your local computer. To do so:

ssh-keygen

This generates, among other files, a public key stored in a file ~/.ssh/identity.pub). You now need to copy this key in the authorized_keys file inside the ~/.ssh directory of the remote computer you want to connect to.

ssh-copy-id  remotecomputer

If you have let an empty passphrase, you can know use ssh or scp without entering your password. But so can do anyone who access your account on your local computer.

So you may prefer to use a passphrase. To avoid having to type it each time you log to the remote computer, copy the following lines in your ~/.bash_profile:

eval `ssh-agent`
ssh-add < /dev/null

You will be prompted for the passphrase only once: when you login on the local computer (See the explanations about ssh-agent at http://mah.everybody.org/docs/ssh).

Executing commands on a remote computer, without login

ssh login@computername command

Keeping a remote session alive

Once connected on the remote computer, execute:

tmux

When you want to leave, press Ctrl-b d. The terminal is detached but not closed.

Next time you connect to this remote computer, to continue your work, you can access the session:

tmux a

See https://danielmiessler.com/study/tmux/ for a primer on tmux, or read the book Tmux 2: Productive Mouse-Free Development by Brian Hogan.

Copy files to or from a remote computer

scp -r localdir remotelogin@remotecomputer:remotedir

rsync -avh localdir/ remotelogin@remotecomputer:remotedir

tar  -cf - dir | ssh login@remotehost tar -xvf -

Setting up X11 forwarding with ssh

To allow graphical applications running on the server to display their windows on the local computer, when using ssh:

From https://unix.stackexchange.com/questions/12755/how-to-forward-x-over-ssh-to-run-graphics-applications-remotely

X11 forwarding needs to be enabled on both the client side and the server side.

On the client side, the -X (capital X) option to ssh enables X11 forwarding, and you can make this the default (for all connections or for a specific conection) with ForwardX11 yes in ~/.ssh/config.

On the server side, X11Forwarding yes must be specified in /etc/ssh/sshd_config. Note that the default is no forwarding (some distributions turn it on in their default /etc/ssh/sshd_config), and that the user cannot override this setting.

The xauth program must be installed on the server side. If there are any X11 programs there, it's very likely that xauth will be there. In the unlikely case xauth was installed in a nonstandard location, it can be called through ~/.ssh/rc (on the server!).

Note that you do not need to set any environment variables on the server. DISPLAY and XAUTHORITY will automatically be set to their proper values. If you run ssh and DISPLAY is not set, it means ssh is not forwarding the X11 connection.

To confirm that ssh is forwarding X11, check for a line containing Requesting X11 forwarding in the ssh -v -X output. Note that the server won't reply either way, a security precaution of hiding details from potential attackers.

Printing

To get a list of available printers:

lpstat -p -d

To check the status of all printers:

lpstat -a

To print file.pdf (or more precisely to put in the printing queue) of the printer printername:

lpr -P printername file.pdf

To print two copies of a file

lpr -# 2 filename.pdf

To print 2 pages per side:

lpr -o number-up=2 -o sides=two-sides-long-edge filename.pdf

To remove a printing job:

lprm job-id

(job-id is the number reported by the lpr or lpstat commands).

If you use the same printer most of the time, you can create a script like the following in your ~/bin directory:

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#! /bin/sh
export PRINTER=my-beautiful-printer
lpr -P "$PRINTER" -o media=A4 "$*"

In case of printing problem, first Check that that the cups service is running:

systemctl  status cups.service

If you need to manage or add printers, open a browser on http://localhost:631

Check out Linux 101: Manage printers and printing for more information.

Getting information about the system

Which computer am I currently working on?

To display the network node name (also called the hostname):

hostname

or

uname -n

What is my public IP address?

sudo apt install curl
curl ifconfig.me

To know your IP address on the local area network:

ip addr

(you must identify the physical interface (ethernet card or wifi card) and check for the inet line)

Check available space on local disks

df -hT -x squashfs -x tmpfs

I actually added the following in my .bashrc:

alias df="df -hT -x squashfs -x tmpfs"

if you need to make space you can search for large folders or files using:

ncdu
du -h | sort -hr | less

If there is a quota system that limits the amount of space you can use on your account, you can check how much is available:

quota -s

Listing available disk partitions

lsblk | grep -v loop   # excludes loop devices
blkid

List the processes currently running on the system

To list the processes currently running:

ps auf 
ps axuf   # also show process no tied to a terminal

The most important columns are TIME and RSS which show the time used by process since it started and the amount of real memory it uses.

If you want to list just some programs, for example matlab, type

pgrep -a matlab

For a real-time display of processes, you can use top or htop but a more comprehensive too is glances:

glances

Not only does it display CPU and memory usage, but also DISK I/O and network I/O. You can sort processes, for example, by CPU usage, etc (Press h in glances to see the help). Glances is extremely useful to identify bottlenecks (see https://livebook.manning.com/book/linux-in-action/chapter-13/74)

You may have to install it with pip install glances or sudo apt install glances.

Finds the process owning a file

Sometimes, it can useful to find the process that owns an open file:

lsof  filename

(See http://www.thegeekstuff.com/2012/08/lsof-command-examples/)

Getting detailed information about your system

sudo inxi -b
nvidia-smi  # if you have nvidia GPUs

To check how many CPU/cores are available on your machine:

lscpu -e
lscpu

To check the total amount of RAM installed on your computer and how much is currently being used by Linux:

free -h

Which Linux distribution is running:

inxi -b
lsb_release -a

Note: you may need to install the packages inxi and lsb-core:

# deb based linuxes: sudo apt install lsb-core
# rpm-based linuxes: yum install redhat-lsb-core
# redhat/fedora: dnf install redhat-lsb-core

Which version of the linux kernel is running:

uname -a

To display detailed hardware information:

lshw -short
hwinfo --short
lspci

Monitoring temperatures

sudo apt install lm-sensors hddtemp
sudo sensors-detect
sensors

You can then install psensor to have a GUI monitoring the temperatures:

sudo apt install psensor
psensor

Check the performance of your computer

You can monitor your system with glances:

glances -t 5

or with htop:

htop -d 50 --sort-key PERCENT_CPU
htop -d 50 --sort-key M_RESIDENT

There are more specialized tools that focus on subsystems. For example, you can monitor the global activity of the CPUs with:

mpstat 5

To monitor the memory usage in real-time:

vmstat -S M 10

If any of the indicators si (swap in) or so (swap out) are high, your computer lacks memory and is using the swap (memory on disk).

You can check the file input/ouput volume and speed on the local drives:

iostat -x 2 5
iostat -h -d 10

Check the speed of your ethernet connection. Three tools are available:

mii-tool

ethtool

iperf

Or the general network performance:

netstat -i 10

Large TX-ERR or RX-ERR indicate a problem.

Check disk IO performance:

man fio

fio --name TEST --eta-newline=5s --filename=fio-tempfile.dat --rw=read --size=500m --io_size=10g --blocksize=1024k --ioengine=libaio --fsync=10000 --iodepth=32 --direct=1 --numjobs=1 --runtime=60 --group_reporting

fio --name TEST --eta-newline=5s --filename=fio-tempfile.dat --rw=write --size=500m --io_size=10g --blocksize=1024k --ioengine=libaio --fsync=10000 --iodepth=32 --direct=1 --numjobs=1 --runtime=60 --group_reporting

(from https://askubuntu.com/questions/87035/how-to-check-hard-disk-performance)

Create a RAM disk

sudo mkdir -p /mnt/ramdisk
sudo mount -t tmpfs tmpfs /mnt/ramdisk -o size=1024M
sudo chown `whoami`:`whoami` /mnt/ramdisk
ls -al /mnt/ramdisk

Check power consumption

Two tools can be used to monitor power usage:

sudo powertop
powerstat

If you have a nvidia card:

nvidia-smi

Check open network connections

ss -tr

Users

Who am I?

As far a the computer is concerned, the identity of the current user (its user_id), can be printed with:

whoami

Note that your login name and home directory are stored in the environment variables LOGNAME and HOME.

Each login is associated to a UserID (UID), an integer, and to a list of GroupIDs (GUID). You can list the information associate to the current login:

id

Check who is logged on the computer

To see who is currently logged on the system, use

who

or more simply:

w

If you are superuser, you can see a journal of the logins with the command:

sudo last

Who is that user?

To determine a person behind an user_id, use finger:

finger <user_id>

Change your identity

To temporally become newuser:

su - newuser

Of course, you will be prompted for newuser's password.

If you want to become root:

sudo -i

When you are done, type:

exit

Change your password

To change your password on the local system:

passwd

Change the login shell

To change your login shell, e.g. from /bin/csh to /bin/bash:

chsh -s /bin/bash

Change group

Check which groups you belong to using id, then use

newgrp group

From now, the files and directories you create will belong to group group

To modify the group of already existing files in directory dir:

chgrp -R group dir

Changing you UserID number

Each login is associated to a number called the UID. If for any reason you need to change your UID number, here is how to do it:

usermod -u <NEWUID> <LOGIN>
groupmod -g <NEWGID> <GROUP>
find / -user <OLDUID> -exec chown -h <NEWUID> {} \;
find / -group <OLDGID> -exec chgrp -h <NEWGID> {} \;
usermod -g <NEWGID> <LOGIN>

Grant a user the ability to run commands as root (sudo)

sudo usermod -aG sudo userlogin

Of course, you need to be in the list of sudoers yourself to be able to execute this command.

Using sudo is better than using su, check out why at https://phoenixnap.com/kb/sudo-vs-su-differences

Files and directories

Where am i?

To know the current working directory:

pwd

You can also go back to your home directory by just typing

cd

Listing files and subdirectories

ls
ls -1         # in one column
ls -l         # with detailed format
ls -t | head  # most recent files only
ls -d */      # only directories
tree -d
tree -d -L 2   # limit depth to 2

Copying, renaming, moving or deleting files

To copy a file inside the same directory, giving it name2:

cp file1 file2

To copy a file from the current directory to the existing directory target_dir:

cp file1 target_dir

To copy all the files from the current directory to another directory:

cp * target_dir

To do the same thing but showing a progress bar:

rsync --info=progress2 * target_dir

To rename a file:

mv file1 file2

To move a file to the existing directory dir:

mv file1 dir

To delete a file:

rm file

To avoid being asked for confirmation:

rm -f file

Creating, Copying, moving or deleting directories

To create a new directory:

mkdir -p newdir

To copy the directory dir in the destination directory destdir:

cp -a dir destdir

(Note: the -a option does a recursive copy, that is, includes the subdirectories and preserves the attributes of files)

Or rsync -a --info=progress2 dir/ destdir

To move the whole directory dir inside the existing destdir:

mv dir1 destdir

To rename directory dir as dir2:

mv dir dir2

To delete the directory dir and all its content:

rm -rf dir

Renaming files, replacing their name by their creation date

Here is a script that replaces filenames by creation date (this can be useful for a photo album)

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#! /bin/bash

for fullfile in "$@";
do
  filename=$(basename "$fullfile")
  extension="${filename##*.}"
  filename="${filename%.*}"
mv -n "$fullfile" "$(date -r "$fullfile" +"%Y%m%d_%H%M%S").${extension}";
done

Check or modify the rights of access to a file or a directory

When you use ls -l to list the files in a directory, the first string of characters, made of x, r, w, -... specifies the access rights (Consult Understanding file permissions on Unix: a brief tutorial)

To allow everybody to read a file aga in the current directory:

chmod a+r aga

To allow everyone to enter a directory mydir and read its content:

chmod a+rx mydir

To make all subdirs and files readable by everyobody:

find -type d -exec chmod a+rx '{}' '+'
find -type f -exec chmod a+r  '{}' '+'

If, when using ls -l, there is a + sign is trailing the rights, it means that ACL (Access Control List), is set on the files or directories. The chmod command will not work: you must then use the getfacl and setfacl commands to list or modify the access/write rigths

Linking files

To avoid copying a file in several places on the same disk, it is a better idea to use a hard link:

ln existingname newname

Thus the same file can have several names (and be in several directories at the same time). Importantly, this only works if the directories are on the same filesystem.

To create a symbolic link (somewhat similar to a 'shortcut' in Windows):

ln -s filename newname

If you delete or move the file, the symbolic links will be 'dangling'.

To find and remove dangling links in a directory:

symlinks -rd directory

Locating files or directories

Finding files or directories

To locate the file named 'filename' in all the subdirectories of the current directory:

find -iname 'filename'

You can use a pattern in place of 'filename', e.g. to return the list of all .doc files in the current directory and its subdirectories:

find -iname '*.doc'

The depth of subdirectories visited can be limited:

find -maxdepth 2 -name '*.doc'

With -o your can do an 'or'. For example, to search for for files with extension nii or img:

find -name '*.nii' -o -name '*.img'

With !, you can negate a search:

find ! -name '*.nii'

You can specify a time-range:

find -mtime 0  # find the files created or modified in the last 24hours
find -mtime +30 -mtime -60  # find files modified in the last 30-60 days
find -newermt 20171101 ! -newermt 20171201 -name '*.pdf' -ls  # find pdf files modified between two dates

You can specify that you only search for, e.g., directories, using the -type argument:

find -type d # list all subdirectorectries
find -type d -mtime -10  # find the directories created or modified in the last 10 days:

You can find and delete all empty directories:

find . -type d -empty -print
find . -type d -empty -delete

You can filter on permissions

find -perm -o+x -ls -type f  # list all file with the execute flag set on 'others'

You can also execute a command on each file:

find -name '*~' -exec rm '{}' '+'  # delete all files '*~'
find -name '*.py' -exec mv -t path '{}' '+'  # move all py files to path
find -name '*.txt' -print0 | xargs -0 grep -l Alice   # show files

Note that xargs can be parallelized with the -P option:

find -name '*.nii' -o '*.img' -print0 | xargs -0 -P 10 gzip  # gzip all image files

Consult info find and info xargs for more information.

To accelerate file search, you can generate a database of all filenames on your filesystem:

updatedb

And then use the command

locate PATTERN

Note that the locate will return all files where PATTERN matches any substring in the full pathname (including directories).

Read the manual:

man locate

Search files by content

grep PATTERN file

where PATTERN is a regular expression (See man grep).

To search files recursively in subdirectories, you can combine find and grep:

find -type f -name "*.tex" -print0 | xargs -0 grep -n PATTERN

But this is complex! An interesting alternative is to use ack (https://beyondgrep.com/). By default, it does a recursive search and it can focus on certain file types.

ack --python -w TOKEN  # search only python file matching on word 'TOKEN'

To install ack under ubundu:

sudo apt install ack-grep

Another search tool, which I have not used but is said to be faster than ack is ag http://conqueringthecommandline.com/book/ack_ag:

sudo apt install silversearcher-ag

Tools like grep and ack are useful to search within text files but pretty useless for binary files. If you want to search within .pdf or .doc files, you first need to extract the textual content and then index it. Then, you will be able to search files by their content. To this end, you can install and use a tool like recoll (see http://www.lesbonscomptes.com/recoll/). One issue though it that the index can quickly grow very large.

Comparing files or directories

Comparing two files

To list all the lines that differ between file1 and file2:

diff file1 file2

meld provides a nicer, graphical way to show the differences between two files or two directories.

meld file1 file2

When comparing text file, you may want to ignore changes in whitespaces (e.g. wrapping of paragraphs), then use wdiff.

wdiff file1.txt file.txt

To compare two latexfiles:

latexdiff file1.tex file2.tex

To create a patch listing the changes from version1 to version2:

diff -aur version1 version2 >dir2.diff

To apply the patch to version1 and generate version2:

patch -p1 <dir2.diff

Compare two directories

To compare two directories:

diff -r --brief dir1 dir2

diff compares the contents of the files. For large directory, this may be too slow. To run a faster comparison based on file sizes, you can use:

  rsync --dry-run --recursive --size-only -i  source/ target/

Synchronizing two directories bidirectionaly

unison

Backups

To back up my laptop, I use rsnapshot. I use an external harddrive with a large ext4 partition (~4 times the sizae of my laptop harddrive).

sudo apt install rsnaphost

Configuring rsnapshot essentially consists of editing /etc/rsnapshot.conf to specify where to save snapshots. In my case:

snapshot_root   /media/cp983411/WD_BLACK/rsnapshot/

Another nice backup utility, with a graphical interface, is:

backintime

It can be set up to automatically start so that you just have to plug your backup hardrive to performe a backup. Check out http://backintime.readthedocs.io.

Web

Aspiring web pages:

wget  address
curl  address

Nice ftp programs

If you need to use ftp, you can use the following clients

ncftp
lftp

Git

Use git to keep an history of your projects and collaborate

Another approach to synchronise dirs is to use git repositories.

Learn about git by reading https://git-scm.com/book/en/v2

See also git-annex

Create a copy of a local git repository on github.com

git push --mirror git@github.com:username/project.git

Disable the Touchpad while typing

killall syndaemon
syndaemon -i 1 -KRd

Unfreeze the mouse

sudo rmmod psmouse
sudo modprobe psmouse

The system is not responding

Try Ctrl-Alt-F1 to open a terminal. From there, you might be able to do:

sudo shutdown now

Alternatively, press Alt+PrintScr, and, keeping this key pressed, type, slowly, reisub. This mysterious sequence is explained at https://linuxconfig.org/how-to-enable-all-sysrq-functions-on-linux#h6-the-sysrq-magic-key or https://en.wikipedia.org/wiki/Magic_SysRq_key

change the brightness of the display

sudo brightlight -r     # read
sudo brightlight -i 10  # increase
sudo brightlight -d 10  # decrease

or

xbacklight -set 50

or

xrandr --output eDP1 --brightness 0.5

Lock the screen under X11

Assuming that xscreensaver is running in the background.

xscreensaver-command -lock

or:

i3lock -d 30 # if you use i3wm

Suspend to RAM

systemctl suspend

Suspend to disk

systemctl hibernate

Note: To hibernate on disk, the size of the swap partition must be larger than the RAM size.

Reboot

systemctl reboot

Shutdown

systemctl poweroff

Generating passwords

    pwgen

Graphics

Manipulating Images

Make sure to have ImageMagick installed (e.g. sudo apt install imagemagick on a Debian-based system)

To get information about an image:

identify image.png

To display an image (gif, .jpg, .png, .tiff, eps, ...) use:

display file.gif
eog image.png

To convert from one format to another:

convert file.jpg file.png

To resize an image:

convert img.png -resize 66%  img_small.png
convert img.png -resize 400x400 img_400.png

To juxtapose several images:

montage -tile 4x4  *.png -geometry 1024x768 output.png

To superimpose images:

composite img1.png img2.png result.png

For more complex manipulations of bitmap image, I mostly use The Gimp

gimp file.jpg

Photography

To manipulate photographs, checkout:

• darktable
• Lightzone
• RawTherapee

Drawing

To draw on canvas (with pencils, brush, ...)

• mypaint
• krita

Creating graphics

To edit vector graphics files, e.g. .svg:

inkscape

To create graphs:

dot

To plot data, I use R or `Python``:

import matplotlib.pyplot as plt
import numpy as np

Take a screenshot

To take a snapshot, that is, copy a portion of the screen into an image file, you can use ImageMagick's command import:

import file.png

You will then be able to select a rectangle on the screen with the mouse, which will be copied in file.png.

Other screenshot programs include gnome-screenshot, ksnapshot, scrot, maim... See https://wiki.archlinux.org/index.php/Screen_capture for a list.

Make a screencast

Voir http://www.linuxlinks.com/article/20090720142023520/Screencasting.html

Under i3, see https://github.com/synaptiko/.files/blob/4a6a549dfe0c22d19f38e32129b5c05de2bb6d34/i3/record-screen.sh

Sound

Assuming that your Linux distribution is running the pulseaudio sound server --- which can be checked with pactl list ---, install pavucontrol to control the sound levels and which sound card each software is using.

Connect a MIDI instrument

Follow the instructions at http://tedfelix.com/linux/linux-midi.html. In a nutshell:

sudo apt install jackd2 jack-tools fluidsynth aconnectgui vmpk qjackctl qsynth  fluid-soundfont-gm

1. To avoid potential latencies, you may want to install a kernel with the PREEMPT option:

   sudo apt-get install linux-lowlatency-hwe-20.04

2. Launch qjackctl, in the setup tab, set Frame/period to 128 to reduce latency, ans press 'start'

3. Use aconnectgui to connect your MIDI keyboard

4. Launch qsynth, add the soundfounds in setup and restart it.

5. In qjackctl, use connect and the patchbay.

Miscellaneous

Access files on a data CD or on a floppy

With some Linux systems, you just insert the CD or the floppy and the content become available in the directory /mnt/cdrom or /mnt/floppy:

ls /mnt/cdrom
ls /mnt/floppy

If the floppy is not write-protected, you can create or copy files in /mnt/floppy just like in any ordinary folder.

Note that if you have several cdrom or floppy drives, they may have names cdrom1, cdrom2, floppy1,...

In some Linux systems, it is necessary to manually mount the cdrom or the floppy before accessing the files, and umount it before ejecting it. For the cdrom:

mount /mnt/cdrom
ls /mnt/cdrom
...
umount /mnt/cdrom
eject

For the floppy:

mount /mnt/floppy
ls /mnt/floppy
umount /mnt/floppy

If you get an error message like mount: only root can do that, ask the system administrator to grant you right to mount floppies by adding the user option the configuration file /etc/fstab. More information in the manual pages of mount and fstab:

man mount
man fstab

Concerning floppies, some systems have mtools installed (see man mtools') which provide themdirandmcopycommands that emulate the old DOS commandsdirandcopy`. It is not necessary to mount the floppy to use them.

Format a floppy

To format the floppy with an ext2 filesystem, and mount it:

fdformat /dev/fd0
mkfs -t ext2 /dev/fd0
mount -t ext2 /dev/fd0 /mnt/floppy

This floppy can be read only on other linux systems. To be able to read it under Windows/DOS, you should use a DOS filesystem with mkdosfs in place of mkfs -t ext2:

mkdosfs /dev/fd0

Split a large file on several floppies

First compress the file, with gzip or bzip2 (see section41). If it still does not fit on a single floppy (1.4Mb), you can use the command split:

split -b1m file

This create a series of x?? files which you can copy on separate floppies.

To reassemble the files:

cat x* >file

Rip an audio CD

to extract all tracks from an audio CD:

cdparanoia -B

To just extract one track:

cdparanoia -w track_number file.wav

If you prefer GUI, you can open konqueror, and type 'audiocd:/' in the address bar. This will show you the content of the CD, which you can copy somewhere else. Copying from the mp3 or ogg folders will do the automatic translations for you.

There are various programs with graphical interface which allow you to rip audio CD: grip and kaudiocreator, rhythmbox.

Convert from wav to mp3

I use lame:

lame file.wav file.mp3

Convert from wav to ogg vorbis

I use oggenc:

oggenc file.wav -o file.ogg

Rip an Audio cd into mp3 or oggenc

You could write a script calling cdparanoia then lame but there is a nifti command line tool, abcde, which queries music databases to find the tracks' song titles.

abcde -o mp3  # rip an audio cd track and converts into mp3

If you prefer a GUI, use asunder

Rip a DVD

Use (handbrake)[https://handbrake.fr/]

Create a data CD

1. Gather all the files you want to save in a given directory, e.g. /tmp/mycd

2. Create an iso image:

   mkisofs -o cd.iso -J -R /tmp/mycd
   ls -l cd.iso
   

   Check that the resulting file cd.iso file is not too large to fit on the CD; if it less than 650Mb, this should be ok.

3. Record on the cd (you must be root).

   You must know which is the device is associated to the CD writer drive.

   cdrecord -scanbus
   

   To determine the x,y,z scsi coordinates of your cd writer. If it does not appear listed, it may be because the ide-scsi parameter was no passed to the Linux kernel (See the HOWTO about CD Writing).

   To record, do:

   cdrecord dev=x,y,z -multi speed=0 -data cd.iso
   

Create an audio CD

To record on an audio CD all the *.wav files which are in the current directory:

cdrecord dev=x,y,z -pad speed=0 -audio *.wav

(x,y,z must be replaced by the numbers returned by cdrecord -scanbus)

Make backups

You can write backup scripts using rsync but it has already been done many time. I have used backintime, but borgbackup looks interesting.

Connect to a bluetooth device

sudo service bluetooth start
sudo service bluetooth status

rfkill list
rfkill unlock 0:


bluetoothctl
  power on
  devices
  scan on
  pair XXXXXXX
  connect XXXXXX

Convert doc or odt documents to pdf

libreoffice --headless --convert-to pdf *.odt

List the hosts in a NIS domain

If you are connected on a local network administrated by NIS (yellow pages), you can display the list of other computers on the network:

ypcat hosts

Mounting a Samba Share

Assuming you have a SAMBA server with IP 192.168.0.50

smbclient -L 192.168.0.50
sudo mount -t cifs //192.168.0.50/BACKUPS /mnt -o username=chrplr,file_mode=0777,dir_mode=0777

Which shell is running?

When you enter commands on the command line in a terminal, the text you type is interpreted by a program called the 'shell'. There are different shells that speak different dialects. To determine the shell you are communicating with, type:

echo $SHELL

Note: this does not work well for subshells:

bash
echo $SHELL
csh
echo $SHELL
exit
exit

Get help. Find manuals

Many commands have associated man pages. To read the man page associated, for example, to the command cp:

man cp

Some commands also have manuals in the form of info files:

info gawk

On many linux systems, there is additional documentation in the /usr/share/doc folder. The HOWTOs can be especially helpful.

To browse them, install dwww:

sudo apt install dwww
sudo a2enmod cgi
sudo systemctl restart apache2
sudo  dwww-index++

Then:

dwww

Cut'n paste

Cutting & pasting under linux is not always straigtfoward. This is due to the fact that there are various systems of cut'n paste cohabitating.

To copy text, the following works with most applications:

• Click the left button and drag the cursor over the text to be copied.
• Click on the middle button to paste.

Note that this is very convenient: there no need to explicitly 'copy' the text.

If you use the window manager 'kde', there is a useful applet called 'klipper' located on the panel. Klipper keeps copies of the most recent clipboard contents. If a cut'n paste operation does not work, you may open klipper, select the relevant line, and retry to paste. It usually works.

If it does not work, then you can try the Cut/Copy/Paste functions from the applications' menus. Sometimes, it is necessary to save the region as a file in the first application, and insert this file in the second application.

Mount a partition of a usb drive

Insert the USB drive, use lsblk or dmesg to find partitions, then use pmount or udisksctl:

lsblk
pmount /dev/sdb1
udisksctl mount -b /dev/sdb1

Check an SD card

sudo apt install f3
lsblk  # to find out which DEVICE the card is associated to
f3probe sudo ./f3probe --destructive --time-ops DEVICE

Setup an ethernet card to access the internet

You need to know IP, MASK, GATEWAY, DNS, HOSTNAME and DOMAIN:

ifconfig eth0 IP netmask MASK up
route add -net default gw GATEWAY netmask 0.0.0.0 eth0
hostname HOSTNAME
echo "domain DOMAIN" >/etc/resolv.conf
echo "nameserver DNS" >>/etc/resolv.conf

Changing/Editing network connection

nmtui  # text mode
nmcli  # text mode
unity-control-center

Install new software

If it come as a .tar.gz and contain a configure script

tar xzf package.tar.gz
cd package
./configure --prefix=$HOME & make & make install

This install the software in your home directory. To install it for every user, you need to omit the prefix option and be root when calling make install.

If you are on a apt-based system (Debian, Ubuntu):

sudo apt install packagename

If you have the .deb file:

sudo dpkg -i file.deb

If you are on a rpm-based linux system, to install an rpm file:

rpm -i package.rpm

To check if the package is correctly installed:

rpm -V package

To remove it:

rpm -e package

Check if a software package is installed

To check if, say, ghostscript is installed:

rpm -q ghostscript

You can get the list of all installed packages:

rpm -qa

Dynamic libraries

To run, some programs need to access functions in dynamic libraries. Dynamic libraries have the extension .so. They are located in /lib, /usr/lib, /usr/local/lib...

To list the libraries needed by a program:

ldd program

After adding new a new dynamic library, e.g. in /usr/local/lib, you must run, as superuser:

ldconfig -n /usr/local/lib

It is possible, as a user, to tell linux to search libraries in a particular place, using the LD_LIBRARY_PATH variable. For more information about how dynamic libraries are accessed, consult the manual of ld.so:

man ld.so

Configure Multiple Displays

Use the programs xranrd and arandr

  arandr
  xrandr --output eDP1 --rotate left

Command-line fun

  sudo apt install cmatrix
  cmatrix

Get back your sanity with a productive environment

The following works for me.

• Use a window manager that allows you that launch applications pinned on some workspace and to have the workspaces accessible by a fixed keystroke. The tiling window manager i3wm fits the bill.
• use Emacs/Spacemacs or vim as an editor
• Use Linux rather than Windows
• use anaconda3 for Python
• use git for projects

Common file types

Similar resources:

• Linux Commands Cheat Sheet