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Command-line Tools

Why Learn the Command Line?

The terminal or the command line is a powerful tool that can be used to perform a wide variety of tasks. It can be used to navigate through files and folders, run programs, and automate tasks. In robotics, the ability to use the command line effectively can come in very handy when logging into a robot remotely or debugging a robot that is not responding. Since complex robots often have full fledged operating systems running on them, the ability to work directly with the command line can be very useful.

Key Concepts

What is the command line?

The command line is a text interface for your computer. It's a program that takes in commands, which it passes on to the computer's operating system to run. From the command line, you can navigate through files and folders on your computer through a text-based interface and run programs without a graphical user interface (GUI).

What is a shell?

A shell is a program whose primary purpose is to read commands and run other programs.

Some of the most popular shells are:

  • Bourne Shell (Sh)
  • Bourne Again Shell (Bash)
  • Zsh
  • Fish

Shell scripts are files that contain shell commands. They are used to automate tasks and can be run from the command line. Typically, shell scripts have the .sh extension. The first line of a shell script is called the shebang and it tells the shell which program to use to run the script. For example, the shebang #!/bin/bash tells the shell to use the Bash shell to run the script.

Example of a shell script

#!/bin/bash

echo "Hello, world!"

What is a terminal emulator?

A terminal emulator is a program that opens a window and lets you interact with the shell. There are several different terminal programs that vary across operating systems and desktop environments. Some of the most popular terminal emulators are:

  • GNOME Terminal
  • iTerm2
  • xterm
  • Konsole

Basic Commands

ls

ls lists the contents of a directory.

ls

cd

cd changes the current working directory.

# Change to the home directory

cd

# Change to the root directory

cd /

# Change to the parent directory

cd ..

# Change to the previous directory

cd -

# Change to the directory named "foo"

cd foo

# Change to the directory named "foo" in the current directory

cd ./foo

# Change to the directory named "foo" in the parent directory

cd ../foo

# Change to the directory named "foo" in the root directory

cd /foo

pwd

pwd prints the current working directory.

# Print the current working directory

pwd

mkdir

mkdir creates a new directory.

# Create a new directory named "foo"

mkdir foo

touch

touch creates a new file.

# Create a new file named "foo"

touch foo

cp

cp copies a file or directory.

# Copy the file "foo" to the file "bar"

cp foo bar

# Copy the file "foo" to the directory "bar"

cp foo bar/

# Copy the directory "foo" to the directory "bar"

cp -r foo bar

mv

mv moves a file or directory.

# Move the file "foo" to the file "bar"

mv foo bar

# Move the file "foo" to the directory "bar"

mv foo bar/

# Move the directory "foo" to the directory "bar"

mv foo bar

rm

rm removes a file or directory.

# Remove the file "foo"

rm foo

# Remove the directory "foo"

rm -r foo

cat

cat prints the contents of a file to the terminal.

# Print the contents of the file "foo"

cat foo

less

less prints the contents of a file to the terminal one page at a time.

# Print the contents of the file "foo"

less foo

head

head prints the first 10 lines of a file to the terminal.

# Print the first 10 lines of the file "foo"

head foo

tail

tail prints the last 10 lines of a file to the terminal.

# Print the last 10 lines of the file "foo"

tail foo

grep

grep searches for a pattern in a file.

# Search for the pattern "foo" in the file "bar"

grep foo bar

find

find searches for files in a directory hierarchy.

# Search for files named "foo" in the current directory

find . -name foo

# Search for files named "foo" in the current directory and its subdirectories

find . -name foo -print

# Search for files named "foo" in the current directory and its subdirectories and delete them

find . -name foo -delete

locate

locate searches for files in a database.

# Search for files named "foo" in the database

locate foo

which

which searches for a program in the directories specified by the PATH environment variable.

# Search for the program "foo" in the directories specified by the `PATH` environment variable

which foo

man

man displays the manual page for a program.

man foo

alias

alias creates an alias for a command.

# Create an alias for the command "ls" named "l"

alias l="ls"

echo

echo prints a string to the terminal.

# Print the string "foo" to the terminal

echo foo

export

export sets an environment variable.

# Set the environment variable "FOO" to the value "bar"

export FOO=bar

source

source executes a file in the current shell.

# Execute the file "foo" in the current shell

source foo

chmod

chmod changes the permissions of a file or directory.

# Give the user read, write and execute permissions, the group read and execute permissions and the others read and execute permissions to the file "foo"

chmod 755 foo

chown

chown changes the owner and group of a file or directory.

# Change the owner of the file "foo" to the user "bar" and the group "baz"

chown bar:baz foo

sudo

sudo executes a command as another user, usually the superuser.

# Execute the command "foo" as the superuser

sudo foo

su

su switches to another user.

# Switch to the user "foo"

su foo

passwd

passwd changes the password of a user.

passwd

exit

exit exits the current shell.

exit

clear

clear clears the terminal screen.

clear

history

history prints the command history.

history

date

date prints the current date and time.

date

uptime

uptime prints the system uptime.

uptime

uname

uname prints system information.

uname

df

df prints disk usage information.

df

du

du prints disk usage information for a file or directory.

# Print disk usage information for the file "foo"

du foo

# Print disk usage information for the directory "foo"

du foo

free

free prints memory usage information.

free

ps

ps prints information about running processes.

ps

kill

kill sends a signal to a process.

# Send the SIGTERM signal to the process with the PID 1234

kill -s SIGTERM 1234

top

top prints information about running processes and system resource usage.

top

bg

bg resumes a suspended process in the background.

# Resume the suspended process with the PID 1234 in the background

bg 1234

fg

fg resumes a suspended process in the foreground.

# Resume the suspended process with the PID 1234 in the foreground

fg 1234

jobs

jobs lists the jobs running in the background.

jobs

nohup

nohup runs a command that ignores the HUP signal.

# Run the command "foo" that ignores the `HUP` signal

nohup foo

killall

killall sends a signal to all processes with a given name.

# Send the SIGTERM signal to all processes with the name "foo"

killall -s SIGTERM foo

ifconfig

ifconfig prints network interface information.

ifconfig

ping

ping pings a host.

ping foo.com

traceroute

traceroute traces the route taken by packets across an IP network.

traceroute foo.com

wget

wget downloads files from the internet.

# Download the file "foo" from the URL "http://example.com/foo"

wget http://example.com/foo

curl

curl transfers data from or to a server.

# Transfer data from the URL "http://example.com/foo" to the file "foo"

curl http://example.com/foo -o foo

ssh

ssh connects to a remote server.

# Connect to the remote server "foo.com" as the user "bar"

ssh

# Connect to the remote server "foo.com" as the user "bar" and execute the command "baz"

ssh

# Connect to the remote server "foo.com" as the user "bar" and execute the command "baz" in the background

ssh

scp

scp copies files between hosts on a network.

# Copy the file "foo" from the remote server "bar.com" to the current directory

scp bar.com:foo .

# Copy the file "foo" from the current directory to the remote server "bar.com"

scp foo bar.com:

# Copy the directory "foo" from the remote server "bar.com" to the current directory

scp -r bar.com:foo .

# Copy the directory "foo" from the current directory to the remote server "bar.com"

scp -r foo bar.com:

rsync

rsync copies files between hosts on a network.

# Copy the file "foo" from the remote server "bar.com" to the current directory

rsync bar.com:foo .

# Copy the file "foo" from the current directory to the remote server "bar.com"

rsync foo bar.com:

# Copy the directory "foo" from the remote server "bar.com" to the current directory

rsync -r bar.com:foo .

# Copy the directory "foo" from the current directory to the remote server "bar.com"

rsync -r foo bar.com:

tar

tar creates or extracts an archive.

# Create an archive named "foo.tar" from the file "foo"

tar -cf foo.tar foo

# Extract the archive named "foo.tar" to the current directory

tar -xf foo.tar

gzip

gzip compresses or decompresses a file.

# Compress the file "foo"

gzip foo

# Decompress the file "foo.gz"

gzip -d foo.gz

gunzip

gunzip decompresses a file.

# Decompress the file "foo.gz"

gunzip foo.gz

zip

zip creates or extracts a zip archive.

# Create a zip archive named "foo.zip" from the file "foo"

zip foo.zip foo

# Extract the zip archive named "foo.zip" to the current directory

unzip foo.zip

unzip

unzip extracts a zip archive.

# Extract the zip archive named "foo.zip" to the current directory

unzip foo.zip

References