Core Features of Bash

• Command History: Executed commands are stored in ~/.bash_history. This file contains records from previous sessions. Commands from the current active session are held in memory and written to the file upon successful logout.
• Tab Completion:
  • Command completion: Press Tab after the first word of a command string.
  • File completion: Press Tab after the second word or later.
  • Double-tapping Tab at a prompt lists all available executable commands in the environment.
  • Installing bash-completion extends this functionality to include options and parameters for specific commands.
• Aliases: Shortcuts for command strings. For example, alias lm='ls -al' allows lm to function identically to ls -al.
• Job Control: Menaging foreground and background processes.
• Shell Scripting: Writing executable program scripts.
• Wildcards: Pattern matching for filenames. For instance, ls -l /usr/bin/X* lists files starting with 'X' in /usr/bin.

Inspecting Commands with type

type [-tpa] command_name

• Without options, type identifies if a command is a shell builtin or an external binary.
• -t: Outputs a single word: file (external), alias, or builtin.
• -p: Displays the full path if the command is an external file.
• -a: Lists all occurrences of the command found in PATH, including aliases.

Executing Commands

To split a long command across multiple lines, use the backslash \ to escape the Enter key. The > symbol indicates the continuation prompt.

# Splitting a long copy command into two lines
[user@host ~]$ cp /var/spool/mail/root /etc/crontab \
> /etc/fstab /root

Useful Keyboard Shortcuts

Key Combination	Action
Ctrl + U	Delete text from cursor to beginning of line
Ctrl + K	Delete text from cursor to end of line
Ctrl + A	Move cursor to the start of the line
Ctrl + E	Move cursor to the end of the line

Managing Variables

Displaying and Setting Variables (echo, unset)

Rules for assignment:

1. Use = to assign values (e.g., var=value).
2. No spaces allowed around the = sign.
3. Variable names must start with a letter or underscore, followed by alphanumeric characters.
4. Use "" for strings needing variable expansion (e.g., $var). Use '' for literal strings.
5. Escape special characters with \.
6. Use backticks `command` or $(command) for command substitution.
7. Append to a variable using $var or ${var} (e.g., var=${var}:new_content).
8. Use export to make a variable available to child processes (environment variable).
9. System variables are typically UPPERCASE; user variables are lowercase.
10. Remove a variable with unset var_name.

Environment Variables (env, set, export)

• env: Lists current environment variables.
  • HOME: User's home directory.
  • SHELL: Current shell path (e.g., /bin/bash).
  • PATH: Search paths for executables, separated by colons :.
  • LANG: System language/locale settings.
  • HISTSIZE: Number of history entries to keep.
  • RANDOM: Generates random integers between 0 and 32767.
• set: Displays all variables (including local and environment variables).
• export: Converts a local variable into an environment variable.

Customizing the Prompt (PS1): You can modify the PS1 variable to change the prompt appearance.

Code	Description
\u	Username
\h	Hostname (short)
\H	Hostname (full)
\w	Current working directory (full path)
\W	Current working directory (basename)
\t	Time (24-hour HH:MM:SS)
\$	# for root, $ for others
\!	History number of the command

Return Values:

• $?: Holds the exit status of the last executed command (0 for success, non-zero for failure).
• $$: Process ID (PID) of the current shell.

Locales

• locale -a: Lists all supported locales.
• locale: Shows current locale settings.
• LANG and LC_ALL are the primary variables controlling system language. System defaults are often configured in /etc/locale.conf.

Input, Arrays, and Declarations

Reading Input (read)

read [-pt] variable_name

• -p: Display a prompt string.
• -t: Set a timeout in seconds.

Declaring Types (declare)

declare [-aixr] variable_name

• By default, variables are strings.
• -a: Define as an array.
• -i: Define as an integer (math is limited to integer operations).
• -x: Export variable (same as export).
• +x: Unexport variable.
• -r: Make variable read-only (cannot be unset or changed).

System Limits (ulimit)

ulimit [-SHacdfltu] [limit]

• -S: Soft limit (warning threshold).
• -H: Hard limit (absolute maximum).
• -a: List all current limits.
• -f: Maximum file size (in blocks).
• -n: Max number of open file descriptors.
• -u: Max number of user processes.

String Manipulation

Deletion and Replacement

Assuming path="/usr/local/bin:/usr/bin:/bin":

Syntax	Description
${path#*:}	Remove shortest match of *:* from front
${path##*:}	Remove longest match of *:* from front
${path%:*}	Remove shortest match of :* from back
${path%%:*}	Remove longest match of :* from back
${path/bin/sbin}	Replace first occurrence of bin with sbin
${path//bin/sbin}	Replace all occurrences of bin with sbin

Default Values and Substitution

Syntax	Behavior if var is unset	Behavior if var is null	Behavior if var is set
${var:-default}	Use default	Use default	Use var
${var:=default}	Set var to default and use it	Set var to default and use it	Use var
${var:+alternate}	Use nothing	Use nothing	Use alternate
${var:?message}	Print message to stderr	Print message to stderr	Use var

Aliases and History

Managing Aliases

• alias: List current aliases.
• alias hi='history | head': Create a new alias.
• unalias hi: Remove an alias.

History Commands

history [n] [-c] [-raw] [file]

• n: Show last n commands.
• -c: Clear history list.
• -a: Append current session history to file.
• -w: Write current history to file.

Execution shortcuts:

• !n: Execute command number n from history.
• !string: Execute the last command starting with string.
• !!: Execute the previous command.

Shell Environment and Symbols

Common Control Keys

Key Combination	Action
Ctrl + C	Interrupt/Send SIGINT to current process
Ctrl + D	End of File (EOF) / Logout
Ctrl + Z	Suspend current process (background)
Ctrl + S	Stop screen output
Ctrl + Q	Resume screen output

Wildcards

Symbol	Meaning
*	Matches 0 or more characters
?	Matches exactly one character
[abc]	Matches any single character inside brackets
[a-z]	Matches any single character in range
[^abc]	Matches any single character NOT inside brackets

Special Characters

Symbol	Meaning
#	Comment line
\	Escape character
`	`
;	Command separator
&	Run command in background
>	Redierct output (overwrite)
>>	Redirect output (append)
2>	Redirect standard error
$(...)	Command substitution

Piping and Text Processing

cut

Extracts specific sections of lines.

• cut -d ':' -f 1,3 /etc/passwd: Use colon as delimiter, extract fields 1 and 3.
• cut -c 1-5 data.txt: Extract characters 1 through 5 from each line.

grep

Searches for patterns in files.

grep [-inv] 'pattern' file

• -i: Ignore case.
• -n: Show line numbers.
• -v: Invert match (show non-matching lines).
• --color=auto: Highlight matched pattern.