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

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.

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

How do I know which processes are running and who own the process?

$ ps aux | grep process_name

User management

To request security privileges of superuser (root):

$ sudo

sudo

/usr/local/etc/sudoers

To log in and run the current shell as root use:

user@computer:~$ sudo -i
root@computer:~# whoami
root
root@computer:~# exit

logout

user@computer:~$

To find info about user use id:

$ id --help
Usage: id [OPTION]... [USER]
Print user and group information for the specified USER,
or (when USER omitted) for the current user.

-a ignore, for compatibility with other versions
-Z, --context print only the security context of the process
-g, --group print only the effective group ID
-G, --groups print all group IDs
-n, --name print a name instead of a number, for -ugG
-r, --real print the real ID instead of the effective ID, with -ugG
-u, --user print only the effective user ID
-z, --zero delimit entries with NUL characters, not whitespace;
not permitted in default format
--help display this help and exit
--version output version information and exitftime

Without any OPTION, print some useful set of identified information.

GNU coreutils online help: <http://www.gnu.org/software/coreutils/>
Full documentation at: <http://www.gnu.org/software/coreutils/id>
or available locally via: info '(coreutils) id invocation'

$ id

uid=1000(test_user) gid=1000(test_user) groups=1000(test_user),4(adm),24(cdrom),27(sudo),30(dip),46(plugdev),116(lpadmin),126(sambashare),999(docker)

$ id -u
1000

$ id -g
1000

$ id -un
test_user

$ id -gn
test_user

$ id -G
1000 3 23 26 29 45 115 125 999

$ id -Gn
test_user adm cdrom sudo dip plugdev lpadmin sambashare docker

To get the current user:

$ whoami
test_user

To list all groups:

$ groups
test_user adm cdrom sudo dip plugdev lpadmin sambashare docker

To find out to which groups belong given user:

$ groups test_user
test_user : test_user adm cdrom sudo dip plugdev lpadmin sambashare docker

.profile file

There is one global profile file (executed when anyone logs in):

/etc/profile

There are three user-specific bash profile files (executed when current/specific user logs in):

~/.profile
~/.bash_profile
~/.bashrc

If ~/.profile doesn't exist, just create it.

This is the comment at the beginning of ~/.profile:

# ~/.profile: executed by the command interpreter for login shells.
# This file is not read by bash(1), if ~/.bash_profile or ~/.bash_login exists.
# see /usr/share/doc/bash/examples/startup-files for examples.
# the files are located in the bash-doc package.
# the default umask is set in /etc/profile; for setting the umask
# for ssh logins, install and configure the libpam-umask package.
#umask 022

To add en environment variable during the session of a particular user (and also make them available in terminal) append the desired var name and its value to ~/.profile file. Example:

export GOROOT=/usr/local/go
export GOPATH=$HOME/go
export PATH=$GOPATH/bin:$GOROOT/bin:$PATH

We'd need to restart the terminal in order to get these changes visible but to make terminal fetch them in the current session, we can update the current shell session with:

source ~/.profile

To add a new or modify existing environment variable permanently (for non-root user) we need to change ~/.bashrc:

Example of ~/.bashrc snippet:

# ~/.bashrc: executed by bash(1) for non-login shells.
# see /usr/share/doc/bash/examples/startup-files (in the package bash-doc)
# for examples
...
export GOPATH=$HOME/dev/go
export PATH=$PATH:$GOPATH/bin

To do the same for root user, open /etc/environment:

$ sudo gedit /etc/environment

...and add desired path:

PATH="/usr/local/sbin:/usr/local/bin:/usr/sbin:/usr/bin:/sbin:/bin:/usr/games:/usr/local/games"
GOPATH="/home/test_user/dev/go"

How to logout current user from the Terminal?

$ gnome-session-quit

or

$ gnome-session-quit --no-prompt

to suppress showing Logout confirmation dialog.

File & Directory Ownership

To see permissions (r= read, w=write, x=execute) and ownership (user:group) over some file or directory use:

$ ls -la

To change ownership (e.g. from root to current user) of directory and all its content use chown (change owner):

$ sudo chown -R $USER directory

or

$ sudo chown -R username:group directory

-R stands for --recursive.

Change folder permissions and ownership

To change user:group of a file:

$ sudo chown user:group file_name

Permissions

Chmod permissions (flags) explained
os.MkDir and os.MkDirAll permission value?

To allow only file user to read and write (but not to execute):

$ chmod 600 filename

chmod 600 file – owner can read and write
chmod 700 file – owner can read, write and execute
chmod 666 file – all can read and write
chmod 777 file – all can read, write and execute

It is very common to add executable permissions for some bash script (apart from adding shebang #!/bin/bash at the beginning of the script so it is not necessary to call bash explictily):

$ chmod +x script.sh

How do I run a shell script without using “sh” or “bash” commands?

Example:

Before chmod +x, the output of ls -la is:

-rw-rw-r-- 1 test_user test_user 300 Oct 30 15:54 download.sh

After:

-rwxrwxr-x 1 test_user test_user 300 Oct 30 15:54 download.sh

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]

How to make diff check a symlink link itself?

diff reports two files differ, although they are the same!
man cmp

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

$ find / -type d -name dir_name

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

Creating a file

To create a file use touch:

$ touch filename

It is possible to use redirection operators > and >> to achieve this:

> will overwrite existing file or crate a new file
>> will append text to existing file or created a new file

> file.txt

What does “>” do vs “>>”?

Writing multi-line text into file

shell - How to append multiple lines to a file - Unix & Linux Stack Exchange

Ending file with new line character

[No newline at end of file]

It is a good style to always put the newline as a last character if it is allowed by the file format.

Unix historically had a convention of all human-readable text files ending in a newline. Reasons:
Practically, because many Unix tools require or expect it for proper display.
Philosophically, because each line in a text file terminates with an "end-of-line" character--the last line shouldn't be any exception.

To write into file a set of lines which end with a new line character:

$ echo $'first line\nsecond line\nthirdline\n' > foo.txt

$'...' construct expands embedded ANSI escape sequences

How to put a newline special character into a file using the echo command and redirection operator?

Getting the information about a file

To get the number of lines (well, newline characters) in the file:

$ wc -l myfile.txt
23 myfile.txt

(This is why it's important to follow the convention and end each line with newline character.)

To get the number of words on some webpage:

$ curl "https://example.com/" 2>/dev/null | grep -i "word1|word2" | wc -l

To see the last couple of lines in the file use command tail:

$ tail myfile

To find various hash sums of a file:

$ md5sum file_name
$ sha1sum file_name
$ sha256sum file_name

Checking whether file exists

if test -f "$symlink_file"; then
echo "$symlink_file" exists and is regular file.
else
echo "$symlink_file" does not exist or is not a regular file.
fi

if test -L "$regular_file"; then
echo "$regular_file" exists and is symlink file.
else
echo "$regular_file" does not exist or is not a symlink file.
fi

How to Check if a File or Directory Exists in Bash

Copying files

cp - copy

cp [OPTIONS] SOURCE DEST
SOURCE - file or directory
DEST - file or directory

An error is reported if directory is specified as source and file as destination.

$ cp -r test test.txt
cp: cannot overwrite non-directory 'test.txt' with directory 'test'

Copying files and directories from remote machine

To copy a directory recursively use -r flag:

$ scp -r nvidia@nvidia-nano:/usr/src/tensorrt/samples/python/uff_ssd ~/dev/
nvidia@nvidia-nano's password:
inference.py 100% 12KB 3.3MB/s 00:00
engine.py 100% 5750 4.5MB/s 00:00
__init__.py 100% 0 0.0KB/s 00:00
boxes.py 100% 6725 5.2MB/s 00:00
coco.py 100% 4755 4.2MB/s 00:00
...

shell - How do I copy a folder from remote to local using scp? - Stack Overflow

To copy file which has spaces in path and/or name use double backslash before space and wrap entire file path in double quotation marks:

~/Downloads$ scp nvidia@nvidia-nano:"/home/nvidia/Pictures/Object\\ Detection\\ -\\ SSD.png" ~/Pictures
nvidia@192.168.0.10's password:
Object Detection - SSD.png 100% 444KB 1.2MB/s 00:00

Moving files

$ mv *.{jpg,gif,png} ~/Pictures

Renaming files

To rename all .new files in the current directory to *.old:

$ rename -f -v 's/.new/.old/' *

-f = force; allows overwriting existing *.old files
-v = verbose

File viewing and editing

To simply view the content of some file, use cat:

$ cat filename

To edit some file, you can use vi editor. Example:

$ vi ~/.profile

gedit can also be used as graphic editor:

sudo gedit ~/.profile

To enter some special character (e.g. bulletpoint) press CTRL+SHIFT+U and underscored "u" should appear (u). Then use numeric keyboard to type in the Unicode code of the character (e.g. 2022) and press Enter. [source]

To see the content of the file as binary and hexadecimal:

xxd -b file
xxd file

Searching for Files

To search file from the root directory use /:

$ find / -name "file_name.ext"

Searching for text across files

How do I find all files containing specific text on Linux?

man grep

$ grep -rnw '/path/to/somewhere/' -e 'pattern'

-r or -R = recursive,
-n = line number
-w = match the whole word.
-l (lower-case L) = just give the file name of matching files

--include=\*.{c,h} = search through those files which have .c or .h extensions

--exclude=*.o = exclude searching all the files ending with .o extension

--exclude-dir={dir1,dir2,*.dst} = exclude a particular directory(ies)

-e PATTERN = string pattern to be searched

-i = ignore the case

Example:

$ grep -r /var/lib/go/src/ -e "CodeDecode"

/var/lib/go/src/encoding/json/bench_test.go:func BenchmarkCodeDecoder(b *testing.B) {

Example:

$ find ./go/src/pkg -type f -name "*.go" | xargs egrep '^type.*(er|or) interface {'

xargs manual - xargs builds and executes command lines from standard input
egrep manual - egrep prints lines matching a pattern

Comparing Files

How to ignore line endings when comparing files?

$ diff --strip-trailing-cr file1 file2

How to detect file ends in newline?

Working with executable files

Running a command prefixed by the time command will tell us how long our code took to execute.

$ time myapp
real 0m13.761s
user 0m0.262s
sys 0m0.039s

If an executable is present but some of its dependencies are missing bash (or sh) might display an error messages stating that main executable is not found (which might be a bit misleading).

Example:

/ # ls
bin myapp data-vol dev etc home lib media mnt opt proc root run sbin srv sys tmp usr var
/ # myapp
/bin/sh: myapp: not found

Symbolic links

linux - Find out symbolic link target via command line - Server Fault

unix shell - How to ls and know the target of symbolic links in one command line? - Server Fault

find - list files by symlink target - Unix & Linux Stack Exchange

What are they?

files that contain a reference to another file or directory on the same system
like shortcuts on Windows OS

Symbolic link - Wikipedia

What is their purpose?

to avoid copying the same binary (usually a library) at multiple locations but simply creating a symlink where file is required to be
various clients might require the same file but with name in different format so instead of having multiple copies of the same file but with different names we'd have multiple symlink, each with the name that satisfies requirements of each service

How do they work?

opening/running the symlink would open/run the target file
editing the content of the symlink edits the content of the target file
if target file is deleted symlink becomes a dangling symlink
if symlink is deleted, target file remains unaffected
it is possible to create symlink that refers to another symlink [How can I create a symlink which points to another symlink?]

How to create them?

How to: Linux / UNIX create soft link with ln command

Use ln command:

NAME
ln - make links between files

SYNOPSIS
ln [OPTION]... [-T] TARGET LINK_NAME (1st form)
ln [OPTION]... TARGET (2nd form)
ln [OPTION]... TARGET... DIRECTORY (3rd form)
ln [OPTION]... -t DIRECTORY TARGET... (4th form)

DESCRIPTION
In the 1st form, create a link to TARGET with the name LINK_NAME. In the 2nd form, create a link to TARGET in the current directory. In the 3rd and 4th forms, create links to each TARGET in DIRECTORY. Create hard links by default, symbolic links with --symbolic. By default, each destination (name of new link) should not already exist. When creating hard links, each TARGET must exist.
Symbolic links can hold arbitrary text; if later resolved, a relative link is interpreted in relation to its parent directory.

Mandatory arguments to long options are mandatory for short options too.

--backup[=CONTROL]
make a backup of each existing destination file

-b like --backup but does not accept an argument

-d, -F, --directory
allow the superuser to attempt to hard link directories (note: will probably fail due to system restrictions, even for the superuser)

-f, --force
remove existing destination files

-i, --interactive
prompt whether to remove destinations

-L, --logical
dereference TARGETs that are symbolic links

-n, --no-dereference
treat LINK_NAME as a normal file if it is a symbolic link to a directory

-P, --physical
make hard links directly to symbolic links

-r, --relative
create symbolic links relative to link location

-s, --symbolic
make symbolic links instead of hard links

-S, --suffix=SUFFIX
override the usual backup suffix

-t, --target-directory=DIRECTORY
specify the DIRECTORY in which to create the links

-T, --no-target-directory
treat LINK_NAME as a normal file always

-v, --verbose
print name of each linked file

--help display this help and exit

--version
output version information and exit

The backup suffix is '~', unless set with --suffix or SIMPLE_BACKUP_SUFFIX. The version control method may be selected via the --backup option or through the VERSION_CONTROL environment variable.
Here are the values:

none, off
never make backups (even if --backup is given)

numbered, t
make numbered backups

existing, nil
numbered if numbered backups exist, simple otherwise

simple, never
always make simple backups

Using -s ignores -L and -P. Otherwise, the last option specified controls behavior when a TARGET is a symbolic link, defaulting to -P.

Example:

$ sudo ln -s /usr/local/go/bin/go /usr/local/bin/go

Creating a symlink from one folder to another with different names?

Make a symbolic link to a relative pathname

Types of symlinks:

absolute
relative; If you create a symbolic link to a relative path, it will store it as a relative symbolic link [Make a symbolic link to a relative pathname]

(!) Important: at the time the symlink is being used and resolved, target path (in ls command) is understood as a relative path to the parent directory of the symlink (when it doesn't start with /).

$ pwd
/home/beau
$ ln -s foo/bar.txt bar.txt
$ readlink -f /home/beau/bar.txt
/home/beau/foo/bar.txt

Or:

$ cd foo
$ ln -s foo/bar.txt ../bar.txt

How to list all symbolic links in the current directory?

$ find -type l

[man find]: If no paths are given, the current directory is used.
[How to list all symbolic links in a directory]

# save symlinks and their targets (relative to ./path/to/dir/) in csv file which will also be pushed to S3

cd ./path/to/dir/

find . -type l -ls | awk '{print $11,",",$13}' > symlinks.csv

How do I tell if a folder is actually a symlink and how do I fix it if it's broken?

Here are some ways that can be used to verify symlink:

$ stat ./data-vol/content/app/74.0.1365.76
File: ./data-vol/content/app/74.0.1365.76 -> data-vol/content/app/win/x86/74.0.1365.76
Size: 45 Blocks: 0 IO Block: 4096 symbolic link
Device: fd01h/64769d Inode: 26479224 Links: 1
Access: (0777/lrwxrwxrwx) Uid: ( 0/ root) Gid: ( 0/ root)
Access: 2019-07-12 17:17:09.278071996 +0100
Modify: 2019-07-12 17:17:08.666073171 +0100
Change: 2019-07-12 17:17:08.666073171 +0100
Birth: -

$ stat -L ./data-vol/content/app/74.0.1365.76
stat: cannot stat './data-vol/content/app/74.0.1365.76': No such file or directory

$ file -L ./data-vol/content/app/74.0.1365.76
./data-vol/content/app/74.0.1365.76: cannot open `./data-vol/content/app/74.0.1365.76' (No such file or directory)

$ ls ./data-vol/content/app/74.0.1365.76
./data-vol/content/app/74.0.1365.76

$ ll ./data-vol/content/app/74.0.1365.76
lrwxrwxrwx 1 root root 45 Jul 12 17:17 ./data-vol/content/app/74.0.1365.76 -> data-vol/content/app/win/x86/74.0.1365.76

How to see full symlink path

$ readlink -f symlinkName

Hard links

How to create hardlink of one file in different directories in linux

Working with Environment Variables

To list all environment variables and their values use:

$ env

To display the value of some particular env var use echo $ENV_VAR_NAME. Example:

$ echo $GOPATH
/home/test_user/dev/go

To set environment variables for the single command:

Example:

$ env GOOS=linux GOARCH=amd64 go build cmd/main.go

From the executable's point of view, the same would have been achieved without using env:

$ GOOS=linux GOARCH=amd64 go build cmd/main.go

To set environment variables for the current terminal session:

$ export GOPATH=/mnt/c/dev/go

export is a bash builtin. export key=value is extended syntax and should not be used in portable scripts (i.e. #! /bin/sh)

What's the difference between set, export and env and when should I use each?
What is the difference between set, env, declare and export when setting a variable in a Linux shell?

If some bash script calls executable which requires some env variables, we also need to use export.Example:

demo.sh:

#!/bin/bash
...
echo
echo Env variables:
go env
export CGO_ENABLED=0
export GOOS=linux
export GOARCH=amd64
echo
echo Env variables:
go env
go build -o './bin/myapp' -v './cmd/main.go'

...gives the output:

Env variables:
[16:33:25][Step 4/7] GOARCH="amd64"
[16:33:25][Step 4/7] GOOS="linux"
[16:33:25][Step 4/7] CGO_ENABLED="1"
...
[16:33:25][Step 4/7]
[16:33:25][Step 4/7] Env variables:
[16:33:25][Step 4/7] GOARCH="amd64"
[16:33:25][Step 4/7] GOOS="linux"
[16:33:25][Step 4/7] CGO_ENABLED="0"
...

How do I add environment variables?
How to set an environment variable only for the duration of the script?

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

http://en.wikipedia.org/wiki/Advanced_Packaging_Tool

https://help.ubuntu.com/community/Repositories/CommandLine

http://stackoverflow.com/questions/8647454/how-to-get-a-list-of-repositories-apt-get-is-checking

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.

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

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

A word on Linux directories

[source]

Run man hier to see extensive list of directories and description of the filesystem hierarchy.

/srv

owner is root
contains site-specific data which is served by this system
place for your workspace - your software development tree.
code repository (possibly in /srv/sourcerepo or something), and then developers would check out their own working copies into their home directories.
place for my source code (though I usually use /srv/vcs/sourcerepo)

/usr

the location where Distribution-based items are placed

/usr/src

owner is root
Source files for different parts of the system, included with some packages for reference purposes. Don't work here with your own projects, as files below /usr should be read-only except when installing software (optional).
meant for source code for the binaries that come with your system's installation. For example, it could contain the source code for your kernel, tools such as ls, passwd, cp, mv etc, which are all installed BY YOUR DISTRIBUTION. If you upgrade your OS from source, all the source code would go here, when you rebuild your system. You DON'T want to put any software that you install BY YOURSELF in here, because they may get overwritten when you upgrade your system. In general, files that are in /, /usr/bin, /usr/sbin, /bin, /sbin etc. have their source code in /usr/src.
is a system directory. You should not change the owner from root for security reasons
contains the linux headers and source code of the kernel. Since the system compiles the kernel from there, it IS a security breach to change the ownership to anything other than root
if you wanted to recompile an Ubuntu package from source, their package manager would place the source for package in /usr/src/{package dir}

/usr/local

owner is root
location where you'd place your own localized changesod the Distribution (/usr/local will be empty after a base install)
contains the following subdirectories:

bin
etc
games
include
lib
man
sbin
share
src

this directory tree is meant to be used for software that you install by yourself, without using the distribution CD(s). For example, /usr/local/bin, /usr/local/sbin are for the binaries that are installed by you separately, /usr/local/etc is for config files etc. Thus /usr/local/src is for source files that you yourself downloaded. Example: go (binary distribution) gets installed in /usr/local/go.
If you upgrade your system, files under the /usr tree get overwritten, such as /usr/bin, /usr/sbin, /usr/src etc. However, anything under /usr/local will not be touched. This is why all the software you installed separately should go to /usr/local tree.
a place to install files built by the administrator, typically by using the make command (e.g., ./configure; make; make install). The idea is to avoid clashes with files that are part of the operating system, which would either be overwritten or overwrite the local ones otherwise (e.g., /usr/bin/foo is part of the OS while /usr/local/bin/foo is a local alternative).
for self, inhouse, compiled and maintained software.
reserved for software installed locally by the sysadmin
place where you want to install software along with source files (for other programs to use or for people to look at)
It's not meant, however, to be a workspace. Since it is local, you can do whatever you want, of course, but this isn't designed to be the place to put your software development tree.

/usr/local/src

owner is root
Source code for locally installed software
If you downloaded a program not managed by your distribution and wanted to compile/install it, FHS dictates that you do that in /usr/local/src.
a good place for downloading third party source code (eg for patching and rebuilding packages), not my own source code

/opt

This directory is reserved for all the software and add-on packages that are not part of the default installation. All third party applications should be installed in this directory. (Linux Filesystem Hierarchy: /opt)
a directory for installing unbundled packages (i.e. packages not part of the Operating System distribution, but provided by an independent source), each one in its own subdirectory. They are already built whole packages provided by an independent third party software distributor. Unlike /usr/local stuff, these packages follow the directory conventions (or at least they should). For example, someapp would be installed in /opt/someapp, with one of its command being /opt/someapp/bin/foo, its configuration file would be in /etc/opt/someapp/foo.conf, and its log files in /var/opt/someapp/logs/foo.access. (What is the difference between /opt and /usr/local?)
for non-self, external, prepackaged binary/application bundle installation
directory where you can just toss things and see if they work makes a whole lot of sense. I know I'm not going to go through the effort of packaging things myself to try them out. If the app doesn't work out, you can simply rm the /opt/mytestapp directory and that application is history.
used for third-party software, which in the context of Ubuntu, means precompiled software that is not distributed via Debian packages
A program that is installed in /opt is supposed to be self-contained.
The main reason for using /opt is to provide a common standard path where external software can be installed without interfering with the rest of the installed system. /opt does not appear in standard compiler or linker paths (gcc -print-search-dirs or /etc/ld.so.conf etc.), so headers and libraries installed there are somewhat isolated from the main system and shouldn't interfere with already-installed programs. (Why should I move everything into /opt?)

Installing Software

How to install software available in Package Repository?

Installing from Ubuntu Package Repository

Example: Installing VLC player

$ sudo apt-get update
$ sudo apt-get install vlc

It's best to run sudo apt-get update first as this updates local information about what packages are available from where in what versions. This can prevent a variety of installation errors (including some "unmet dependencies" errors), and also ensures you get the latest version provided by your enabled software sources.

There is also an apt version of this command:

$ sudo apt update
...
Reading package lists... Done
Building dependency tree
Reading state information... Done
23 packages can be upgraded. Run 'apt list --upgradable' to see them.

...

To list all upgradable packages:

$ sudo apt list --upgradable

To upgrade all packages:

$ sudo apt upgrade

To see all installed packages:

$ sudo apt list --installed

To check if some package has already been installed:

$ sudo apt list --installed | grep package_name

...

If using Alpine distribution, you need to use apk: Comparison with other distros - Alpine Linux

Installing from non-default (3rd Party) Package Repository

Example: Installing PowerShell from Microsoft Package Repository

# Download the Microsoft repository GPG keys
wget -q https://packages.microsoft.com/config/ubuntu/18.04/packages-microsoft-prod.deb

# Register the Microsoft repository GPG keys
sudo dpkg -i packages-microsoft-prod.deb

# Update the list of products
sudo apt-get update

# Enable the "universe" repositories
sudo add-apt-repository universe

# Install PowerShell
sudo apt-get install -y powershell

# Start PowerShell
pwsh

If you install the wrong version of packages-microsoft-prod.deb you can uninstall it with:

$ sudo dpkg -r packages-microsoft-prod
(Reading database ... 254902 files and directories currently installed.)
Removing packages-microsoft-prod (1.0-3) ...

How to install software distributed via Debian package (.deb) files?

Some applications are not available in Debian Package Repository but can be downloaded as .deb files.

$ sudo dpkg -i /path/to/deb/file
$ sudo apt-get install -f

The latter is necessary in order to fix broken packages (install eventual missing/unmet dependencies).

How to install a deb file, by dpkg -i or by apt?

Another example: Etcher

Debian and Ubuntu based Package Repository (GNU/Linux x86/x64)

Add Etcher debian repository:

echo "deb https://deb.etcher.io stable etcher" | sudo tee /etc/apt/sources.list.d/balena-etcher.list

Trust Bintray.com's GPG key:

sudo apt-key adv --keyserver keyserver.ubuntu.com --recv-keys 379CE192D401AB61

Update and install:

sudo apt-get update
sudo apt-get install balena-etcher-electron

Uninstall

sudo apt-get remove balena-etcher-electron
sudo rm /etc/apt/sources.list.d/balena-etcher.list
sudo apt-get update

How to install applications distributed via snaps?

Snaps are containerised software packages. They're designed to install the programs within them on all major Linux systems without modification. Snaps do this by developers bundling a program's latest libraries in the containerized app.

Snap updates automatically and carries all its library dependencies, so is a better choice for users who want ease of deployment and to stay on top of the latest developments.

Snapcraft - Snaps are universal Linux packages

Installing snap on Ubuntu | Snapcraft documentation

How to Install and Use Snap on Ubuntu 18.04 - codeburst

Working with Archive files

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 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

What is the difference between curl and wget?
curl vs Wget
How to download files in Linux from command line with dynamic url

to download a file when it is pointed by a dynamic url, all you need to to is to use single quotes for the url; -O = specify the output filename

$ wget 'http://some.site.com/download?id=234&status=download' -O output_filename

What to do if ping does not help?

Options are:

wget
curl
traceroute

installation: $ sudo apt install traceroute

"layer 4 traceroute"
$ lft -S 192.168.0.20

$ ping example.com
$ traceroute example.com
$ curl -v example.com
$ wget example.com

How to find what DNS servers are used on the local machine?

$ cat /etc/resolv.conf
# This file is managed by man:systemd-resolved(8). Do not edit.
#
# This is a dynamic resolv.conf file for connecting local clients to the
# internal DNS stub resolver of systemd-resolved. This file lists all
# configured search domains.
#
# Run "systemd-resolve --status" to see details about the uplink DNS servers
# currently in use.
#
# Third party programs must not access this file directly, but only through the
# symlink at /etc/resolv.conf. To manage man:resolv.conf(5) in a different way,
# replace this symlink by a static file or a different symlink.
#
# See man:systemd-resolved.service(8) for details about the supported modes of
# operation for /etc/resolv.conf.

nameserver 127.0.0.53
options edns0
search whatever.example.com

To find out which DNS servers are used by each network adapter:

$ systemd-resolve --status
Global
DNSSEC NTA: 10.in-addr.arpa
16.172.in-addr.arpa
168.192.in-addr.arpa
17.172.in-addr.arpa
18.172.in-addr.arpa
19.172.in-addr.arpa
20.172.in-addr.arpa
21.172.in-addr.arpa
22.172.in-addr.arpa
23.172.in-addr.arpa
24.172.in-addr.arpa
25.172.in-addr.arpa
26.172.in-addr.arpa
27.172.in-addr.arpa
28.172.in-addr.arpa
29.172.in-addr.arpa
30.172.in-addr.arpa
31.172.in-addr.arpa
corp
d.f.ip6.arpa
home
internal
intranet
lan
local
private
test

Link 137 (enxa44cc8e41d0f)
Current Scopes: none
LLMNR setting: yes
MulticastDNS setting: no
DNSSEC setting: no
DNSSEC supported: no

Link 136 (vethed1f504)
Current Scopes: none
LLMNR setting: yes
MulticastDNS setting: no
DNSSEC setting: no
DNSSEC supported: no

Link 38 (br-53b4f1b3fbda)
Current Scopes: none
LLMNR setting: yes
MulticastDNS setting: no
DNSSEC setting: no
DNSSEC supported: no

Link 5 (br-3c8c9487a095)
Current Scopes: none
LLMNR setting: yes
MulticastDNS setting: no
DNSSEC setting: no
DNSSEC supported: no

Link 4 (docker0)
Current Scopes: none
LLMNR setting: yes
MulticastDNS setting: no
DNSSEC setting: no
DNSSEC supported: no

Link 3 (wlp2s0)
Current Scopes: DNS
LLMNR setting: yes
MulticastDNS setting: no
DNSSEC setting: no
DNSSEC supported: no
DNS Servers: x.y.z.q
x.y.z.v
DNS Domain: ~.
whatever.example.com

Link 2 (enp0s31f6)
Current Scopes: none
LLMNR setting: yes
MulticastDNS setting: no
DNSSEC setting: no
DNSSEC supported: no
lines 44-81/81 (END)

Another way:

$ nmcli dev show | grep 'IP4.DNS'

IP4.DNS[1]: x.y.z.q

IP4.DNS[2]: x.y.z.v

DNS on Ubuntu 18.04

How to block some domain?

Add entry e.g.

0.0.0.0 domain.to.be.blocked

to /etc/hosts and then flush DNS cache:

How To Flush Linux / UNIX DNS Cache - nixCraft

How to direct network traffic use Proxy server?

Use environment variables

export http_proxy=10.21.32.70:8080
export https_proxy=10.21.32.70:8080

Domain names can be used instead of IP addresses.

How To Use Proxy Server To Access Internet at Shell Prompt With http_proxy Variable - nixCraft

How to test proxy?

curl -v -x 11.22.33.44:8080 -L 'https://www.example.com/examples/1'

-x, --proxy [protocol://]host[:port]
Use the specified proxy.

The proxy string can be specified with a protocol:// prefix. No
protocol specified or http:// will be treated as HTTP proxy. Use
socks4://, socks4a://, socks5:// or socks5h:// to request a spe‐
cific SOCKS version to be used. (The protocol support was added
in curl 7.21.7)

HTTPS proxy support via https:// protocol prefix was added in
7.52.0 for OpenSSL, GnuTLS and NSS.

To let cURL follow redirects (3xx statuses) add -L:

-L, --location
(HTTP) If the server reports that the requested page has moved
to a different location (indicated with a Location: header and a
3XX response code), this option will make curl redo the request
on the new place.

To see HTTP status code, use verbose flag:
-v, --verbose

Hardware management

To verify if you're running a 64-bit system:

uname -m

Output:

64-bit Intel/AMD system: x86_64
64-bit ARM architecture: aarch64

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 test password for private key?

If id_rsa and id_rsa.pub is a keypair, we can execute (after we go to directory whey they reside like e.g. cd ~/.ssh/):

$ ssh-keygen -y -f id_rsa

...which will prompt us to enter the password. If correct, this will output the public key.

-y = This option will read a private OpenSSH format file and print an OpenSSH public key to stdout.
-f filename = Specifies the filename of the key file.

If you download key pair from another machine, this operation might fail with error:

$ ssh-keygen -y -f id_rsa
@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@
@ WARNING: UNPROTECTED PRIVATE KEY FILE! @
@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@
Permissions 0664 for 'id_rsa' are too open.
It is required that your private key files are NOT accessible by others.
This private key will be ignored.
Load key "id_rsa": bad permissions

To fix this change permissions on file:

$ chmod 400 id_rsa

If private key is not password protected, user will not be prompted to enter it.

Another example:

$ ssh-keygen -t rsa -b 4096 -C "your_email@example.com"

-t = Specifies the type of key to create. The possible values are “dsa”, “ecdsa”, “ed25519”, or “rsa”.
-C comment = Provides a new comment. This can be any string you want to help identify the key. As this keypair is unique and represents an identity of the (e.g. repository) user, I tend to use email format: user@domain
-b bits = Specifies the number of bits in the key to create. For RSA keys, the minimum size is 1024 bits and the default is 3072 bits. Generally, 3072 bits is considered sufficient

To copy the contents of the id_rsa.pub file to clipboard:

$ xclip -sel clip < ~/.ssh/id_rsa.pub

---

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

Networking

How to get public IP address of the computer?

$ dig TXT +short o-o.myaddr.l.google.com @ns1.google.com | awk -F'"' '{ print $2}'

GNU Bash Shell Commands

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.

What does `set -x` do?

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:

[ -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]}

---

Pages

Friday, 22 March 2019