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Linux Command Line introduction

Command Line introduction

For any thing that is repetitive or programmable, there likely is a relevant command. Ask your peers or search online before you start writing a script. Just remember that Unix was first introduced in late 1960s - there is likely to be a command for what you need

Starting trouble with command line (for those accustomed to GUI) is the sudden trouble of interacting with the computer using just text commands. After using for a week or so, things will seem very systematic and GUI feels ill suited for frequent tasks. With continuous use, recalling various commands becomes easier. Short-cuts, history, aliases and tab-completion help in the process

If you’ve used a scientific calculator, you’d know that it is handy with too many functionalities cramped into tiny screen and plethora of multi-purpose buttons. Commands and short-cuts pack much more punch than that on a terminal.

  • Commands presented here are GNU/Linux specific and generally behave similarly across distros
  • Commands in GNU/Linux usually have a few different options and syntax than POSIX specification
    • man and info pages of commands come in handy
  • If any command is not found in a particular distro, either it has to be manually installed or not available for that distro
  • The bash shell version 4+ is used throughout this material


File System

Before we dive into ocean of commands, lets get a brief on Linux file system. If you’ve used Windows, you would be familiar with C: D: etc.
In Linux, directory structure starts with / symbol, which is referred as the root directory

  • man hier gives description of the filesystem hierarchy. A few examples:
    • / This is the root directory. This is where the whole tree starts.
    • /bin This directory contains executable programs which are needed in single user mode and to bring the system up or repair it.
    • /home On machines with home directories for users, these are usually beneath this directory, directly or not. The structure of this directory depends on local administration decisions.
    • /tmp This directory contains temporary files which may be deleted with no notice, such as by a regular job or at system boot up.
    • /usr This directory is usually mounted from a separate partition. It should hold only sharable, read-only data, so that it can be mounted by various machines running Linux.
    • /usr/bin This is the primary directory for executable programs. Most programs executed by normal users which are not needed for booting or for repairing the system and which are not installed locally should be placed in this directory.
    • /usr/share This directory contains subdirectories with specific application data, that can be shared among different architectures of the same OS. Often one finds stuff here that used to live in /usr/doc or /usr/lib or /usr/man.

Absolute and Relative paths

Quoting wikipedia

An absolute or full path points to the same location in a file system regardless of the current working directory. To do that, it must contain the root directory.

By contrast, a relative path starts from some given working directory, avoiding the need to provide the full absolute path. A filename can be considered as a relative path based at the current working directory. If the working directory is not the file’s parent directory, a file not found error will result if the file is addressed by its name.

Further Reading


Command Line Interface

Command Line Interface (CLI) allows us interact with computer using text commands
For example: opening a Terminal, typing ls and pressing Enter key - the ls command lists the contents of a directory. To do the same thing in GUI, you double-click on the directory to view its content

Shell and Terminal are sometimes interchangeably used to mean the same thing - a prompt where user enters and executes commands. They are quite different

  • Shell command line interpreter
  • Terminal text input/output environment
  • cat /etc/shells to know which shells are available
  • echo "$SHELL" to know which is your login-shell


Command Help

  • man man get help about manual pages
    • usually displayed using less command, press q key to quit the man page and h key to get help
    • for GNU/Linux commands, the info command has more detailed documentation
  • man bash manual page for bash
    • gvim <(man bash) open the manual page in text editor instead of displaying in terminal
  • man -k printf search manual pages for printf
    • man -k is equivalent for apropos command
  • type Display information about command type
    • type cd cd is a shell builtin
    • type sed sed is /bin/sed
    • type ls ls is aliased to ls --color=auto
  • Use help for builtin commands
    • help help help page on help command
    • help -m cd display usage in pseudo-manpage format for cd command
    • help -d compgen use -d option to output short description for each topic
    • help display all shell commands that are defined internally
  • whatis display one-line manual page descriptions
    • whatis grep print lines matching a pattern
  • whereis locate the binary, source, and manual page files for a command
    • whereis awk awk: /usr/bin/awk /usr/bin/X11/awk /usr/share/awk /usr/share/man/man1/awk.1.gz
  • history Display or manipulate the history list

Further Reading


Do one thing and do it well

Unix Philosophy

Write programs that do one thing and do it well.

Write programs to work together.

Write programs to handle text streams, because that is a universal interface.

Examples given below are for demonstration purposes only, more detail in later chapters

Command Structure

only the command

  • clear clear the terminal screen
  • top display Linux processes

command with options

  • ls -l list directory contents, use a long listing format
  • df -h report file system disk space usage, print sizes in human readable format (e.g., 1K 234M 2G)

command with arguments

  • mkdir project create directory named ‘project’ in current working directory
  • man sort manual page for sort command
  • wget http://s.ntnu.no/bashguide.pdf download file from internet

command with options and arguments

  • rm -r project remove ‘project’ directory
  • find . -name '*log*' print files in current directory (and its sub-directories) whose name contains the string ‘log’

single quotes vs double quotes

  • single quotes preserves the literal value of each character within the quotes
  • double quotes preserves the literal value of all characters within the quotes, with the exception of ‘$’, ‘`’, ‘', and, when history expansion is enabled, ‘!’
  • Difference between single and double quotes

Example:

$ echo '$SHELL'
$SHELL
$ echo "$SHELL"
/bin/bash

Command Network

Redirecting output of a command

  • to another command
    • du -sh * | sort -h calculate size of files/folders, display size in human-readable format which is then sorted
  • to a file (instead of displaying on terminal)
    • grep -i 'pass' *.log > pass_list.txt writes/overwrites to file
    • grep -i 'error' *.log >> errors.txt appends to file

Redirecting input

  • wc -l < file.txt in this case, useful to get just the number of lines, without displaying file name

Redirecting error

  • xyz 2> cmderror.log assuming a non-existent command xyz, it would give an error and gets redirected to specified file

Redirecting output of command as input file

  • comm -23 <(sort file1.txt) <(sort file2.txt) allows to create named pipes, effectively avoiding need to create temporary files

Combining output of several commands

  • (head -5 ~/.vimrc ; tail -5 ~/.vimrc) > vimrc_snippet.txt multiple commands can be grouped in () and redirected as if single command output

Command substitution

  • sed -i "s|^|$(basename $PWD)/|" dir_list.txt add current directory path and forward-slash character at the start of every line
    • Note the use of double quotes

stdin, stdout and stderr

  • < or 0< is stdin filehandle
  • > or 1> is stdout filehandle
  • 2> is stderr filehandle
  • read more

More detailed discussion in Shell chapter