Friday 22 March 2019

Creating a minimal Docker image for Go application

Let's create a minimal Docker image which contains an arbitrary Go application. As an example application, we can use "Hello world":


package main
import (
func main() {
   fmt.Println("Hello, world!")

To build it and place the executable in bin directory we need to run go build:

$ go build -o bin/hello-world cmd/main.go

To test the executable, let's run it:

$ ./bin/hello-world 
Hello, world!

We want to create a Docker image which, when started, runs this binary. We first have to add Dockerfile - a file which defines how will Docker image be created. Creating a Docker image is like creating a lasagne: we take a base layer and then keep adding new layers on top of each other. Dockerfile specifies what will be the base Docker image (base layer), which application has to reside in it, what is its environment and dependencies that have to be installed and also how will that app be executed (or, what shall be executed when that image is launched).

In our case, we only want to have a single binary in the container and we want it to be launched. For this use case, our Dockerfile can be like this:


FROM scratch
COPY bin/hello-world app/
CMD ["/app/hello-world"]

FROM scratch specifies that empty image (0 bytes!) shall be used as a base layer (or...we can say that there is no base layer).

COPY copies files or directories from source in the host to destination in the container. Working directory on host can be specified via context argument to docker build command. Current directory is used by default and in our case that's go-docker-hello-world. Our binary will be copied here from bin directory on host into the app directory in the container. If destination has to be directory, a slash (/) hast to be added after the destination name. If we didn't add slash, COPY would have copied our binary into the root directory of the container and would have renamed it to app.

CMD contains the name of the executable that has to be run upon container's launch. We need to use an array format (square brackets) as Docker then uses the first argument as the entry point (process that is executed first) and subsequent elements are its arguments. If we used "/app/hello-world" instead of ["/app/hello-world"] Docker would have tried to pass the name of the executable as an argument to /bin/sh but as base image is empty, shell is not present and we'd get an error when running the container:
docker: Error response from daemon: OCI runtime create failed: container_linux.go:344: starting container process caused "exec: \"/bin/sh\": stat /bin/sh: no such file or directory": unknown.
Let's create the container:

go-docker-hello-world$ docker build -t helloworld ./
Sending build context to Docker daemon 2.055MB
Step 1/3 : FROM scratch
Step 2/3 : COPY bin/hello-world app/
---> 7c0c34e6ad64
Step 3/3 : CMD ["/app/hello-world"]
---> Running in b3f5695b79c5
Removing intermediate container b3f5695b79c5
---> 171dbd862be1
Successfully built 171dbd862be1
Successfully tagged helloworld:latest

-t applies a tag (name) to the container which can be used later in container managing commands (it is easier to use some descriptive name rather than container ID which is just an array of numbers).

./ specifies the context (the current working directory) for commands in the Dockerfile.

Let's verify that it appears in the list of images:

$ docker images
helloworld latest 171dbd862be1  42 minutes ago   2MB

Let's inspect it to verify that entry point is indeed our application:

$ docker inspect 171dbd862be1
        "Id": "sha256:171dbd862be107306bcad870587f8961c00566b946a4d2717ccbf3863492ca2c",
        "RepoTags": [
        "RepoDigests": [],
        "Parent": "sha256:7c0c34e6ad64538ff493910efd6046043b6fa28e78015be6333fcd2e880122d4",
        "Comment": "",
        "Created": "2019-03-22T16:15:30.7049579Z",
        "Container": "b3f5695b79c5add5e86af2ea02b893bd5ed35381221cca1fbf84dd6ea401b69e",
        "ContainerConfig": {
            "Hostname": "b3f5695b79c5",
            "Domainname": "",
            "User": "",
            "AttachStdin": false,
            "AttachStdout": false,
            "AttachStderr": false,
            "Tty": false,
            "OpenStdin": false,
            "StdinOnce": false,
            "Env": [
            "Cmd": [
                "#(nop) ",
                "CMD [\"/app/hello-world\"]"
            "ArgsEscaped": true,
            "Image": "sha256:7c0c34e6ad64538ff493910efd6046043b6fa28e78015be6333fcd2e880122d4",
            "Volumes": null,
            "WorkingDir": "",
            "Entrypoint": null,
            "OnBuild": null,
            "Labels": {}
        "DockerVersion": "18.09.3",
        "Author": "",
        "Config": {
            "Hostname": "",
            "Domainname": "",
            "User": "",
            "AttachStdin": false,
            "AttachStdout": false,
            "AttachStderr": false,
            "Tty": false,
            "OpenStdin": false,
            "StdinOnce": false,
            "Env": [
            "Cmd": [
            "ArgsEscaped": true,
            "Image": "sha256:7c0c34e6ad64538ff493910efd6046043b6fa28e78015be6333fcd2e880122d4",
            "Volumes": null,
            "WorkingDir": "",
            "Entrypoint": null,
            "OnBuild": null,
            "Labels": null
        "Architecture": "amd64",
        "Os": "linux",
        "Size": 1997502,
        "VirtualSize": 1997502,
        "GraphDriver": {
            "Data": {
                "MergedDir": "/var/lib/docker/overlay2/86d02e448ac1c650f65d6eb30b21eeea2f13f176918ccd6af3440c0d89336b19/merged",
                "UpperDir": "/var/lib/docker/overlay2/86d02e448ac1c650f65d6eb30b21eeea2f13f176918ccd6af3440c0d89336b19/diff",
                "WorkDir": "/var/lib/docker/overlay2/86d02e448ac1c650f65d6eb30b21eeea2f13f176918ccd6af3440c0d89336b19/work"
            "Name": "overlay2"
        "RootFS": {
            "Type": "layers",
            "Layers": [
        "Metadata": {
            "LastTagTime": "2019-03-22T16:15:30.835954727Z"

Finally, let's run the container:

$ docker run  helloworld
Hello, world!

When I built once natively, on Ubuntu, a similar, small app from scratch, I got the following error when I ran its container:

ERROR: for my_app  Cannot start service carl: OCI runtime create failed: container_linux.go:345: starting container process caused "exec: \"my_app\": executable file not found in $PATH": unknown

The problem seemed to be related to app being dynamically linked to some of shared libraries on my dev Linux machine so when binary was copied over to empty (scratch) Docker container, binary could not find them so threw such error. I assume this was the reason for such error as solution was to explicitly disable dynamic linking when building my app:

$ CGO_ENABLED=0 go build cmd/main.go 

Indeed, using CGO_ENABLED flag makes a difference. 

Default is dynamic linking:

$ go build cmd/main.go 
$ file main
main: ELF 64-bit LSB executable, x86-64, version 1 (SYSV), dynamically linked, interpreter /lib64/l, not stripped

Static linking has to be explicitly set:

$ CGO_ENABLED=0  go build cmd/main.go 
$ file main
main: ELF 64-bit LSB executable, x86-64, version 1 (SYSV), statically linked, not stripped

Useful Linux commands



# - 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.

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

User management

Linux User Management | My Public Notepad

File & Directory Ownership

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

$ ls -la


$ sudo ls -la ./database_data/
total 128
drwx------ 19   999 root    4096 Apr 20 16:13 .
drwxrwxr-x  7 bojan bojan   4096 Apr 20 16:12 ..
drwx------  6   999 docker  4096 Apr 20 16:12 base

$ sudo ls -la ./database_data/ | awk '{print $3, $4, $9}'
999 root .
bojan bojan ..
999 docker base

$3 is user, $4 is group.

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


$ 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


Chmod permissions (flags) explained
os.MkDir and os.MkDirAll permission value?
c - What does mode_t 0644 mean? - Stack Overflow

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

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


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

-rw-rw-r-- 1 test_user test_user  300 Oct 30 15:54


-rwxrwxr-x 1 test_user test_user  300 Oct 30 15:54

Working with directories

To list the content of the directory:


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

$ 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
< 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/:

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

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 create a new file

> file.txt

What does “>” do vs “>>”?

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?

Difference between printf and echo:

$ printf "hello \n"
$ printf "hello " // note that new line is not appended automatically
hello $ echo "hello \n"
hello \n
$ echo "hello"
$  // new line is appended automatically

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 "" 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

If file is Windows executable, it is possible to examine it with:

$ exiftool somefile.exe 

To install exiftool:

$ sudo apt install libimage-exiftool-perl

linux - viewing dll and exe file properties/attributes via the command line - Unix & Linux Stack Exchange

Checking whether file exists

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

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

How to Check if a File or Directory Exists in Bash

Copying files

cp - copy

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 to/from remote machine


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"

To find any file or directory which contains some string in their name, recursively, starting from the current directory:

$ find . -name "*STRING*" 

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


$ grep -r /var/lib/go/src/ -e "CodeDecode"
/var/lib/go/src/encoding/json/bench_test.go:func BenchmarkCodeDecoder(b *testing.B) {


$ 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).


/ # 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

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 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.


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

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

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 apkComparison 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

# 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

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 stable etcher" | sudo tee /etc/apt/sources.list.d/balena-etcher.list

Trust's GPG key:

sudo apt-key adv --keyserver --recv-keys 379CE192D401AB61

Update and install:

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


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"


$ 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 files are and execute:

$ unrar x -e file.part1.rar

ZIP format unzipping into the specified directory:

$ unzip -d bookmarks-dev-413

$ unzip -d bazel bazel-1.2.1-dist

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


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/'

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


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


$ printf my_string | base64


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
       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"

or print the name of the current terminal process:

$ echo $0

Robert Muth: Better Bash Scripting in 15 Minutes

Bash script should start with shebang e.g.


Escaping and preserving special characters in bash

Bash wildcards: ? (question mark) and * (asterisk) |
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



$ 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.
      -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
                           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"
    echo "no files found"

Create a directory if this does not exist:

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

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



$ 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]}