1. What Is Shell

Shell, aka Command line, is a program that takes keyboard strokes and passes them to the operating system to carry out.

2. File System

Files are organized in a tree-like pattern of directories.

Commands

• pwd (Current Working Directory)
• cd (Change Directory)
  • cd -: go back to previous working directory.
• ls (List Directory Contents)

ls Long Format

Pathnames

• absolute: starting with /.
• relative: to current working directory.
  • .
  • ..

3. Exploring the System

Commands

• ls
  • -l
  • -t: sort by file’s modification time.
  • -A: list almost all hidden files.
• file: determine file type.
• less: view file contents.
  • g: to beginning
  • G: to end
  • /characters: search

Commands are often followed by one or more options that modify their behavior, and further, by one or more arguments, the items upon which the command acts.

command -options arguments

4. Manipulating Files and Directories

• cp: copy
  • cp item1 item2
  • cp item… directory
  • r: required when copying directories
• mv: move or rename
  • mv item1 item2
  • mv item… directory
• mkdir: create dir
  • mkdir dir…
• rm: remove
  • r: required when deleting a directory
• ln: create links
  • ln file link
  • hard links: additional name(s) for same data part on disk (inode).
  • ln -s item link
  • symbolic links: text pointer to target file.
  • most file operations are carried out on the link’s target, not the link itself. rm is an exception.

Wildcards

5. Working with Commands

Commands

• type: display command type.
• which: display executable’s location.
• man: display manual page.
  • man ${section} ${search_term}
• whatis: brief description of a command.
• alias
  • alias ${name}=’string’
  • use unalias command to remove.

Shell Command Type

1. An executable program.
2. A shell built-in command.
3. A shell function.
4. An alias.

Help Notation

Square brackets indicates optional items; vertical bar indicates mutually exclusive items.

cd [-L|[-P[-e]]] [dir]

This notation says that the command cd may be followed optionally by either a “-L” or a “-P” and further, if the “-P” option is specified the “-e” option may also be included followed by the optional argument “dir”.

Man Page Organization

1. User commands
2. Programming interfaces kernal system calls
3. Programming interfaces to the C library
4. Special files such as device nodes and drivers
5. File formats
6. Grames and amusements such as screen savers
7. Miscellaneous
8. System administration commands

bash doc in /usr/share/doc/bash/.

Put more than one command on a line by separating each command with a semicolon character.

command1; command2; command3...

6. I/O Redirection

Commands

• cat: concatenate and print files.
• sort
• uniq: report or filter out repeated lines in a file.
• grep: print lines matching a pattern.
  • i: ignore case
  • v: print lines that do not match the pattern.
• wc: word, line, character, and byte countq
  • lines, words, bytes count
• head: display the first 10 lines of a file.
• tail: display the last 10 lines of a file.
  • n: specify numbers, default is 10.
  • f: don’t stop when EOF is reached, but wait for additional data.
• tee: duplicate standard input

  • ls /usr/bin

    tee ls.txt

    grep zip

To redirect standard output:

• >: overwrite a file.
  • ls /usr/bin > output.txt
• >>: append to the end of a file.
  • ls /usr/bin » output.txt

To redirect standard error:

• 2>:
  • ls /usr/bin 2> output.txt
• 2>>:

To redirect both standard output and error to the same file:

• &>
• &>>
  • ls -l /bin/usr &» ls-output.txt

To supress error messages from a comand:

• /dev/null
  • ls -l /bin/usr 2> /dev/null

In the absence of filename arguments, cat copies standard input to standard output. To create a short text file:

cat > some_file.txt
This is the content of text file.(Ctrl-d, Ctrl-c)

Pipelines

The standard output of one command can be piped into the standard input of another:

command1 | command2

• command1 > file: redirection operator connets a command with a file.
• command1 | command2: pipeline operator connects t he output of one command with the input of a second command.

(l seems a shortcut to ls -lh)

7. Seeing the World as the Shell Sees It

Commands

• echo: write arguments to the standard output

Expansion

Use echo to print expanded argument(s).

echo *
echo D*
echo [[:upper:]]*
echo /usr/*/share

• ~ (tilde exapnsion): expands to home directory.
  • ~tom: tom’s home directory
• $((expression)): arithmetic expansion
  • echo $((2 + 2))
• {list or range} (brace expansion):
  • Number_{1..5}
  • {01..015}
  • a{A{1,2},B{3,4}}b

• $PARAMETER: parameter expansion

• $(command): command substitution
  • ls -l $(which cp)

Quoting

• double quotes: supress part of expansions. unify a string with spaces and newlines into a single argument.
  • echo “$(cal)”

• single quotes: supress all expansions.

• use \ to escape special meaning characters, this is done in double quotes.

8. Advanced Keyboard Tricks

Commands

• clear
• history

Cursor Movement Commands

• Ctrl-a: to the beginning of line.
• Ctrl-e: to the end of line.
• Ctrl-l: clear the screen, same as clear command.
• Alt-f: move forward one word.
• Alt-b: move backward one word.

Modifying Text

• Ctrl-d: delete the character at the cursor location.

Cut & Paste

• Ctrl-u: kill the whole line.
• Ctrl-k: kill text from the cursor location to the end of line.
• Ctrl-y: yank text from the kill-ring and insert it at the cursor location.

History

• history: whole search
  • !number: repeat list item number.

• ctrl-r: incremental search

• script: makes a typescript of everything printed on your terminal.

9. Permissions

• id: show user identity.
• chmod: change permissions of a file or directory (owner or superuser).
  • octal: chmod 600 foo.txt
  • symbolic: chmod u=rw,go-rwx
• umask: remove default permissions.
• sudo: execute a command as another user.
  • /etc/sudoers file defines specific commands that particular users are permitted to execute.
  • sudo -l: view privileges granted by sudo.
• chown: change file owner / group (superuser previleges required).
• passwd: change password.
  • passwd [user]

Owner, Group And Everybody Else

A user may own files and directories. Users can belong to a group who are given access to files and directories by their owners. Owner may also grant some set of access rights to everybody else.

• /etc/passwd: where users defined.
• /etc/group: where groupds defined.
• /etc/shadow: where users’ password defined.

Read, Write and Execution Access

-rw-rw-r-- 1 me me 0 2008-03-06 14:52 foo.txt

First 10 characters of above string is the file attribute.

The first one is file type:

• -: regular file.
• d: directory.
• l: symbolic link.
• c: character special file, like terminal.
• b: block special file, like a hard drive.

Rest of them is file mode, representing the read, write and execute permissions for file’s owner, group and everybody else.

Permission Attributes:

| Attribute | Files | Directories | | ——– | ——- | ——- | | r | open and read. | list contents (x) | | w | write and truncate, but cannot be renamed or deleted. | files within can be created, deleted, and renamed (x) | | x | treated as a program and execute. for program files written in scripting languages (r) | enter into | | s (replace x) | setuid | setgid | | t | _ | sticky bit |

10. Processes

Processes are how Linux organizes the different programs waiting for their turn at the CPU.

• ps:
  • ps x: shoow all of our processes regardless of any terminal they are controlled by.
• top: real-time sorted information about processes.
• jobs:
• bg:
• fg:
• kill: terminate or signal a process.
  • kill [-signal] PID…
  • If not signal is specified on the commandline, then TERM (Terminate) signal is sent by default.
• killall:
• shutdown:

Processes have:

• PID
• memory assigned
• owner
• user ids
• effective user ids

Process States

For ps aux, we have below column headers:

Common Process Actions

• Ctrl-c: interrupt a process. (politely asked the program to terminate.)
  • INT: Interrupt signal.
• Put in background: command &
  • [1] 91594: job No.1, PID 91594
  • jobs: list the jobs that have been launched.
• Return to foreground: fg %jobspec
  • If there is only 1 job, jobspec is optional.
• Pause a process: Ctrl-z
  • fg %jobspec or bg %jobspec a program to make it continue.
  • TSTP: Terminal Stop signal.

  Common Signals