Friday, 22 March 2019

Useful Linux commands


 

Conventions


# - requires given linux commands to be executed with root privileges either directly as a root user or by use of sudo command

$ - requires given linux commands to be executed as a regular non-privileged user

How to change the prompt in terminal?
 
user@host:~/path/to/current-dir$ export PS1='$ '
$

Useful Command Line Utility Tools


To print last N commands form Terminal:

$ history N

Using the history command - Media Temple

The output is command history in form:

 2000  sudo apt install ipython ipython-notebook
 2001  sudo apt list --installed | grep python3-pip
 2002  sudo apt list --installed | grep python3-dev
...

To copy only commands (without command number) keep CTRL+ALT pressed and select desired text columns and rows with left mouse click button.

To shorten some long command: How to Create and Use Alias Command in Linux

To list current aliases:

$ alias
alias alert='notify-send --urgency=low -i "$([ $? = 0 ] && echo terminal || echo error)" "$(history|tail -n1|sed -e '\''s/^\s*[0-9]\+\s*//;s/[;&|]\s*alert$//'\'')"'
alias egrep='egrep --color=auto'
alias fgrep='fgrep --color=auto'
alias grep='grep --color=auto'
alias l='ls -CF'
alias la='ls -A'
alias ll='ls -alF'
alias ls='ls --color=auto'

Operating System

How to find what's the version of the installed OS:

$ lsb_release -a
No LSB modules are available.
Distributor ID: Ubuntu
Description: Ubuntu 18.04.3 LTS
Release: 18.04
Codename: bionic

To use this command, we need to install package lsb-release:

$ apt install -y lsb-release


Alternatively, we can use uname --all:

% uname --all   
...
Linux <host_name> 6.1.124-134.200.amzn2023.aarch64 #1 SMP Tue Jan 14 08:15:20 UTC 2025 aarch64 aarch64 aarch64 GNU/Linux

On MacOS:

% uname -a    
Darwin ip-192-168-1-121.us-east-2.compute.internal 23.4.0 Darwin Kernel Version 23.4.0: Wed Feb 21 21:44:54 PST 2024; root:xnu-10063.101.15~2/RELEASE_ARM64_T6030 arm64

Another option is: 

% cat /etc/os-release 
NAME="Ubuntu"
VERSION="20.04.6 LTS (Focal Fossa)"
ID=ubuntu
ID_LIKE=debian
PRETTY_NAME="Ubuntu 20.04.6 LTS"
VERSION_ID="20.04"
HOME_URL="https://www.ubuntu.com/"
SUPPORT_URL="https://help.ubuntu.com/"
BUG_REPORT_URL="https://bugs.launchpad.net/ubuntu/"
PRIVACY_POLICY_URL="https://www.ubuntu.com/legal/terms-and-policies/privacy-policy"
VERSION_CODENAME=focal
UBUNTU_CODENAME=focal

On Mac this file does not exist:

% cat /etc/os-release 
cat: /etc/os-release: No such file or directory

On Mac, use sw_vers:

% sw_vers
ProductName: macOS
ProductVersion: 14.4
BuildVersion: 23E214

Processes



ps = Process Status

a = show processes for all users
u = display the process's user/owner
x = also show processes not attached to a terminal

ps command accepts three different styles of options:
  • UNIX
  • BSD - on Debian Ubuntu
  • GNU
Display only the processes associated with the current terminal session:

$ ps

To view all the processes that the current user owns, for example if there are multiple terminal sessions running for the same user:

$ ps x

The command above returns an additional column called sate information (STAT). The characters in that column have the following meaning:
  • s = indicates that the process is a session leader (i.e., the root process)
  • S = process is in an interruptible sleep state, and is waiting for some event, like user input
  • R = command is actively running
  • T = process has stopped; like when you enter control-Z in the vi text editor
  • + = process is a foreground process


To list all processes in the system, regardless of the owner or controlling terminal:

$ ps -e


To show only command:

% ps -e -o command


To formats the CMD column’s data to display the parent-child relationship between processes:

$ ps -He


To show the prettier hierarchical graph:

$ ps -axjf


Output columns:
  • PPID = parent process ID
  • SID = session ID. The value is usually the same as the PID of the process that started the chain.
  • PGID = parent group process ID
  • TPGID = terminal sessions ID with which the process is associated. If there is no terminal that is associated, then -1 is displayed.
  • UID = user ID associated with the process

$ ps aux

   a      Lift the BSD-style "only yourself" restriction, which is imposed 
          upon the set of all processes when some BSD-style (without "-") 
          options are used or when the ps personality setting is BSD-like.  
          The set of processes selected in this manner is in addition to the 
          set of processes selected by other means.  An alternate 
          description is that this option causes ps to list all processes 
          with a terminal (tty), or to list all processes when used together 
          with the x option.

   u      Display user-oriented format.

   x      Lift the BSD-style "must have a tty" restriction, which is imposed 
          upon the set of all processes when some BSD-style (without "-") 
          options are used or when the ps personality setting is BSD-like.
          The set of processes selected in this manner is in addition to the 
          set of processes selected by other means.  An alternate 
          description is that this option causes ps to list all processes 
          owned by you (same EUID as ps), or to list all processes when used 
          together with the a option.

To see the entire command (to prevent command truncating):

$ ps auxww

-w Use 132 columns to display information, instead of the default which is your window size. If the -w option is specified more than once, ps will use as many columns as necessary without regard for your window size. Note that this option has no effect if the “command” column is not the last column displayed.



User management


 Linux User Management | My Public Notepad

File and Directory Ownership and Permissions



Working with directories


To list the content of the directory:

ls

To list directories in the current directory, names only, and each name in its own line:

$ ls -1d *

-1     list one file per line
-d     list directories themselves, not their contents
*        list all items in the current directory (they might be filtered by other flags, like -d)

To list all files (including hidden) in the current directory:

ls -a

To list files in some specific directory use:

$ ls target_directory

Example:
$ ls /usr/local/go/

To list directories and files in form of a tree install tree package:

$ sudo apt install tree

...and use it as e.g.:

$ tree -I *node*

-I = ignores directories that match given pattern

To diff two directories use:

$ diff -r dir1 dir2 

This shows which files are only in dir1 and those only in dir2 and also the changes of the files present in both directories if any. If any file does not end with a newline character, this will be reported as well:

$ diff -r dir1 dir2
diff -r dir1/test.txt dir2/test.txt
1c1
< First line in dir1/test.txt
\ No newline at end of file
---
> First line in dir2/test.txt
\ No newline at end of file
Binary files /dir1/test.bin and /dir2/test.bin differ
Only in /dir1/subdir1/: subsubdir1
Only in /dir2/subdir1/: file.zip
---

What does 1c1 in diff tool mean?
1c1 indicates that line 1 in the first file was c hanged somehow to produce line 1 in the second file.
They probably differ in whitespace (perhaps trailing spaces, or Unix versus Windows line endings?).

man diff
Difference between two directories in Linux [closed]

To find the location of some directory starting from the root directory (/) use:

$ find / -type d -name dir_name

What does the suffix .d (in directory name) mean in Linux? - Server Fault

Time and Date


To convert Unix time to human readable, use:

$ date -d @1583944041
Wed 11 Mar 16:27:21 GMT 2020


Working with files in Linux | My Public Notepad

Package management


apt (Advanced Packaging Tool) - It is not a command itself but a package which contains set of tools which manage installation and removal of other packages.

apt-get

apt-get update - downloads the package lists from the repositories and "updates" them to get information on the newest versions of packages and their dependencies. It will do this for all repositories and PPAs.

http://askubuntu.com/questions/222348/what-does-sudo-apt-get-update-do

apt-cche


add-apt-repository - adds a repository to the list of repositories


To apply latest security updates on Ubuntu:

sudo apt-get update

sudo apt-get -y upgrade


Difference Between apt and apt-get Explained

Should I use apt or apt-get?

You might be thinking if you should use apt or apt-get. And as a regular Linux user, my answer is to go with apt.

 apt is the command that is being recommended by the Linux distributions. It provides the necessary option to manage the packages. Most important of all, it is easier to use with its fewer but easy to remember options.

 I see no reason to stick with apt-get unless you are going to do specific operations that utilize more features of apt-get.



How to add repository to Apt sources?


For application named app:

$ sudo install -m 0755 -d /etc/apt/keyrings

$ sudo curl -fsSL https://download.app.com/linux/debian/gpg -o /etc/apt/keyrings/app.asc

$ sudo chmod a+r /etc/apt/keyrings/app.asc

# Add the repository to Apt sources:
$ echo \
  "deb [arch=$(dpkg --print-architecture) signed-by=/etc/apt/keyrings/app.asc] https://download.app.com/linux/debian \
  $(. /etc/os-release && echo "$VERSION_CODENAME") stable" | \
  sudo tee /etc/apt/sources.list.d/app.list > /dev/null


debian - what is the function of /etc/apt/sources.list.d? - Stack Overflow


-----------------------------------------------------------------------------------------------------------------------


To download a file into some specific directory:

cd /dest_dir
wget https://example.com/archive_file.tar.gz


To install a package/software in Ubuntu, it is usually enough to copy it to /usr/local directory.
To move dir1 to some other location e.g. /usr/local use:

mv new_app /usr/local


Linux Directories

Unix Filesystem Hierarchy | My Public Notepad

Installing Software on Linux | My Public Notepad


Working with Archive files


Tar Format


To unpack the .tar.gz in the current directory use:

$ tar -zxvf archive_file.tar.gz

-x = extract
-f (--file) = use archive file; this flag has to be the last in the list of flags and to be followed by the archive file name
-v (--verbose) = verbose output
-z (--gzip, --gunzip, --ungzip)  = filter the archive through gzip

To unpack only the specific directory from the archive use:

$ tar -zxvf archive_file.tar.gz dir_name

To unpack archive to the specific directory:

$ tar -zxvf archive_file.tar -C path/to/dest_dir 

-C (--directory) stands for "Change to directory"

Example:

$ tar -xzf go1.12.1.linux-amd64.tar -C /usr/local

$ sudo tar -xzvf Postman-linux-x64-7.5.0.tar.gz -C /opt

Why compressed directories cannot be extracted in /opt?

To unpack multiple rar files first install unrar:

$ sudo apt-get install rar unrar

then go to the directory where all x.party.rar files are and execute:

$ unrar x -e file.part1.rar


ZIP format


Zip one subdirectory ( e.g. ./modules/foo) while traversing it recursively (-r), but exclude (-x) some its subdirectories and files (on MacOS we need to put paths with wildcards under quotes):

% zip -r module-foo.zip \
./modules/foo \
-x "./modules/foo/.terraform/*" \
-x ./modules/foo/.terraform.lock.hcl

Unzipping into the specified directory:

$ unzip bookmarks.zip -d bookmarks-dev-413

 $ unzip bazel-1.2.1-dist.zip -d bazel bazel-1.2.1-dist

 Unzip files in particular directory or folder under Linux or UNIX - nixCraft


Networking

Introduction to Linux Networking | My Public Notepad

Hardware management


To verify if you're running a 64-bit system use GNU Core Utility: uname.

Swap Memory Management


To check virtual memory statistics (the latest reading) use:

$ vmstat
procs -----------memory---------- ---swap-- -----io---- -system-- ------cpu-----
 r  b   swpd   free   buff  cache   si   so    bi    bo   in   cs us sy id wa st
 0  0 138816 9157820 1739872 11514740    0    1  1559   366  102   64 26  7 66  0  0

or, to see readings every 1 second:

$ vmstat 1

To disable and then reenable swap:

$ sudo swapoff -a
$ sudo swapon -a


performance - How to empty swap if there is free RAM? - Ask Ubuntu

Displays/Monitors Management


$ xrandr --listmonitors
Monitors: 2
 0: +*HDMI-1 2560/677x1440/381+0+0  HDMI-1
 1: +eDP-1 1920/344x1080/193+112+1440  eDP-1

To see details for the current display:

$ xrandr --current
Screen 0: minimum 320 x 200, current 1920 x 1080, maximum 8192 x 8192
eDP-1 connected primary 1920x1080+0+0 (normal left inverted right x axis y axis) 344mm x 193mm
   1920x1080     60.03*+  60.01    59.97    59.96    59.93    48.02  
   1680x1050     59.95    59.88  
   1600x1024     60.17  
   1400x1050     59.98  
   1600x900      59.99    59.94    59.95    59.82  
   ...
   360x202       59.51    59.13  
   320x180       59.84    59.32  
HDMI-1 disconnected (normal left inverted right x axis y axis)
DP-1 disconnected (normal left inverted right x axis y axis)
HDMI-2 disconnected (normal left inverted right x axis y axis)
DP-2 disconnected (normal left inverted right x axis y axis)
HDMI-3 disconnected (normal left inverted right x axis y axis)

To list all video devices (e.g. webcams):

$ ls /dev/video*

To check the number of processors:

 $ grep -c ^processor /proc/cpuinfo | sed 's/^0$/1/'
4

To see details for all processors:

$ cat /proc/cpuinfo

To list CPU architecture information, use lscpu. E.g.

$  lscpu | grep "Byte Order"
Byte Order:          Little Endian


SSH


How to generate SSH key pair on Ubuntu
How to test SSH key password on Ubuntu

---

To get base64 encoding of a string:

$ echo -n my_string | base64

or

$ printf my_string | base64

---
TBC...


Getting Information about Hardware


How to get the GPU info?

lshw = list hardware
-C = class

$ sudo lshw -C display
  *-display UNCLAIMED       
       description: 3D controller
       product: GM107GLM [Quadro M620 Mobile]
       vendor: NVIDIA Corporation
       physical id: 0
       bus info: pci@0000:01:00.0
       version: a2
       width: 64 bits
       clock: 33MHz
       capabilities: pm msi pciexpress bus_master cap_list
       configuration: latency=0
       resources: memory:ee000000-eeffffff memory:d0000000-dfffffff memory:e0000000-e1ffffff ioport:e000(size=128) memory:ef000000-ef07ffff
  *-display
       description: VGA compatible controller
       product: Intel Corporation
       vendor: Intel Corporation
       physical id: 2
       bus info: pci@0000:00:02.0
       version: 04
       width: 64 bits
       clock: 33MHz
       capabilities: pciexpress msi pm vga_controller bus_master cap_list rom
       configuration: driver=i915 latency=0

       resources: irq:130 memory:ed000000-edffffff memory:c0000000-cfffffff ioport:f000(size=64) memory:c0000-dffff


NVidia and Intel in same Laptop: which card is used?

$ lspci -k | grep -EA2 'VGA|3D'  
00:02.0 VGA compatible controller: Intel Corporation Device 591b (rev 04)
Subsystem: Dell Device 07a9
Kernel driver in use: i915
--
01:00.0 3D controller: NVIDIA Corporation GM107GLM [Quadro M620 Mobile] (rev a2)
Subsystem: Dell GM107GLM [Quadro M620 Mobile]
Kernel modules: nvidiafb, nouveau


GNU Bash Shell Commands

 
To determine the default shell for the current user:

$ echo "$SHELL"
/bin/bash

or print the name of the current terminal process:

$ echo $0
bash


Robert Muth: Better Bash Scripting in 15 Minutes

Bash script should start with shebang e.g.

#!/bin/bash


---
Escaping and preserving special characters in bash

Bash wildcards: ? (question mark) and * (asterisk) | LinuxG.net
What is the meaning of a question mark in bash variable parameter expansion as in ${var?}? - Stack Overflow
shell - Expansion of variable inside single quotes in a command in Bash - Stack Overflow
linux command line find escape question mark - Server Fault

---

$_


What does 'cd $_' mean?

$_ expands to the last argument to the previous simple command* or to previous command if it had no arguments. Typical use:

mkdir dirA && cd $_


---

To see all Bash commands, execute:

$ help

---

set


$ help set 
set: set [-abefhkmnptuvxBCHP] [-o option-name] [--] [arg ...]
    Set or unset values of shell options and positional parameters.
    
    Change the value of shell attributes and positional parameters, or
    display the names and values of shell variables.
    
    Options:
      -a  Mark variables which are modified or created for export.
      -b  Notify of job termination immediately.
      -e  Exit immediately if a command exits with a non-zero status.
      -f  Disable file name generation (globbing).
      -h  Remember the location of commands as they are looked up.
      -k  All assignment arguments are placed in the environment for a
          command, not just those that precede the command name.
      -m  Job control is enabled.
      -n  Read commands but do not execute them.
      -o option-name
          Set the variable corresponding to option-name:
              allexport    same as -a
              braceexpand  same as -B
              emacs        use an emacs-style line editing interface
              errexit      same as -e
              errtrace     same as -E
              functrace    same as -T
              hashall      same as -h
              histexpand   same as -H
              history      enable command history
              ignoreeof    the shell will not exit upon reading EOF
              interactive-comments
                           allow comments to appear in interactive commands
              keyword      same as -k
              monitor      same as -m
              noclobber    same as -C
              noexec       same as -n
              noglob       same as -f
              nolog        currently accepted but ignored
              notify       same as -b
              nounset      same as -u
              onecmd       same as -t
              physical     same as -P
              pipefail     the return value of a pipeline is the status of
                           the last command to exit with a non-zero status,
                           or zero if no command exited with a non-zero status
              posix        change the behaviour of bash where the default
                           operation differs from the Posix standard to
                           match the standard
              privileged   same as -p
              verbose      same as -v
              vi           use a vi-style line editing interface
              xtrace       same as -x
      -p  Turned on whenever the real and effective user ids do not match.
          Disables processing of the $ENV file and importing of shell
          functions.  Turning this option off causes the effective uid and
          gid to be set to the real uid and gid.
      -t  Exit after reading and executing one command.
      -u  Treat unset variables as an error when substituting.
      -v  Print shell input lines as they are read.
      -x  Print commands and their arguments as they are executed.
      -B  the shell will perform brace expansion
      -C  If set, disallow existing regular files to be overwritten
          by redirection of output.
      -E  If set, the ERR trap is inherited by shell functions.
      -H  Enable ! style history substitution.  This flag is on
          by default when the shell is interactive.
      -P  If set, do not resolve symbolic links when executing commands
          such as cd which change the current directory.
      -T  If set, the DEBUG and RETURN traps are inherited by shell functions.
      --  Assign any remaining arguments to the positional parameters.
          If there are no remaining arguments, the positional parameters
          are unset.
      -   Assign any remaining arguments to the positional parameters.
          The -x and -v options are turned off.
    
    Using + rather than - causes these flags to be turned off.  The
    flags can also be used upon invocation of the shell.  The current
    set of flags may be found in $-.  The remaining n ARGs are positional
    parameters and are assigned, in order, to $1, $2, .. $n.  If no
    ARGs are given, all shell variables are printed.
    
    Exit Status:
    Returns success unless an invalid option is given.



set -x 
  • enables a mode of the shell where all executed commands are printed to the terminal.
  • typically used for debugging
---

What is the difference between [] and [[]]?

Bash Brackets Quick Reference

Test if a command outputs an empty string

if [[ $(ls -A) ]]; then
    echo "there are files"
else
    echo "no files found"
fi

---
Create a directory if this does not exist:

DATA_DIR=data-vol
if [ -d ${DATA_DIR} ]; then
   echo Directory ${DATA_DIR} exists.
else 
   echo Directory ${DATA_DIR} does not exist.
   echo Creating directory ${DATA_DIR} ...
   mkdir ${DATA_DIR}
fi

or, as one-liner:

[ -d path/to/mydir ] || mkdir -p path/to/mydir

(-p option instructs mkdir to create all intermediate parent directories to mydir)

---

Semicolon in conditional structures

The semicolon is needed only when the end of line is missing:

if [ "a" == "a" ] ; then echo "true" ; fi

Without semicolons, you get Syntax error.

---

How to capture exit code of the application most recently executed in Terminal?

$ echo $?

It can also be used in a bash script, e.g.:

ginkgo -r

if [[ $? != 0 ]]; then
   echo "Unit tests failed. Terminating build process..."
   exit 1
fi

---

Variables


$ dependencies=(build-essential cmake pkg-config libavcodec-dev libavformat-dev libswscale-dev libv4l-dev libxvidcore-dev libavresample-dev python3-dev libtbb2 libtbb-dev libtiff-dev libjpeg-dev libpng-dev libtiff-dev libdc1394-22-dev libgtk-3-dev libcanberra-gtk3-module libatlas-base-dev gfortran wget unzip)

We can print the array:

$ echo ${dependencies[@]}

 build-essential cmake pkg-config libavcodec-dev libavformat-dev libswscale-dev libv4l-dev libxvidcore-dev libavresample-dev python3-dev libtbb2 libtbb-dev libtiff-dev libjpeg-dev libpng-dev libtiff-dev libdc1394-22-dev libgtk-3-dev libcanberra-gtk3-module libatlas-base-dev gfortran wget unzip

We can print elements with specific indexes in the array:

$ echo ${dependencies[0]}
$ echo ${dependencies[1]}


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