Summary of the book.

Operating the Shell

Editing key in Bash is controlled by (1) shopt command, like shopt -o emacs (2) stty command
Command history in bash:
- !x - Run the last command line that begin with letter x
- !! - Run the last command line
- !-2 - Run the 2nd command line back from the history
- !456 - Run the command line number 456 of the output of history
- history 10 - Show the last 10 command lines
- history -p !d - Show the command line that match the criteria of beginning with letter d
- history -d 456 - Delete the entry number 456 in the command line history
- !?frag - Run the last command that contains string fragment frag
- ^str1^str2^ - Run the last command again with replacing str1 with str2

Shell Customization

bind -P - Show the current command line editing key bindings
bind -l - List all possible keyboard functions
bind -q <function> - Query the key binding of a particular function
bind -u <function> - Remove (unbind) a key binding from a particular function, if there are several, remove the first one
bind -r <key seq> - Remove a binding by its key sequence
bind "<key seq>:<function>" - Assign a key binding to a particular function
bind -v / bind -V - Show the keyboard options
bind "set <option> on|off" - Turn on off a keyboard option
bind -s/ bind -S - List all currently defined keyboard macros

Alternatively: inputrc

Script basics

Suspend a script and wait until a signal: suspend
Reading keyboard input and put into $VAR: read VAR
- With prompt: read -p "Enter a value: " VAR
- By default, read understands backslash escaping of characters. To read as raw (i.e. backspace is a character), use: read -r VAR
- With 5 sec timeout for reading input: read -t 5 VAR
- With a limit of 10 characters max (no need Enter if reached): read -n 10 VAR
- If no variables is given, by default read will store the value into $REPLY
Redirecting stderr and stdin:
- command > file.output >&2
- command &> file.output

Variables

Declare a variable as integer: declare -i VAR
- If a string is assigned to a variable declared as integer, the value will be 0
Show the variable declaration: declare -p VAR
Turn off the integer declaration: declare +i VAR
Create variable with constant (read-only) content: declare -r VAR="value"
- Writing a read-only variable will generage an error
- To turn off the read-only attribute of an variable, use: declare +r VAR
Decalre an array: declare -a VAR
- With initilization: declare -a VAR[0]="value0" VAR[1]="value1"
- Use with: echo ${VAR[0]}
- Set with: VAR[0]="value0"
- Assign multivalues: VAR=("value0" "value1" "value2")
- Assign multivalues with subscript: VAR=([3]="value3" [2]="value2" [1]="value1")
- Entire array as one string separated with $IFS: $VAR[*]
- Entire array as separate strings: $VAR[@]
- Return number of items in the array: ${#VAR[*]} or ${#VAR[@]}

Expressions

Pattern recognition (globbing):

if [[ $VAR = A* ]]; then ... fi
if [[ $VAR = A?? ]]; then ... fi
if [[ $VAR = [ABC]* ]]; then ... fi
if [[ $VAR = [A-Z]* ]]; then ... fi
if [[ $VAR = [^a-z]* ]]; then ... fi
if [[ $VAR = [!a-z]* ]]; then ... fi

Extended globbing (enabled by: shopt -s extglob):
- ?(pattern) - Matches zero or one occurrence of given patterns
- *(pattern) - Matches zero or more occurrence of given patterns
- +(pattern) - Matches one or more occurrences of the given patterns
- @(pattern) - Matches exactly one of the given patterns
- !(pattern) - Matches anything except one of the given patterns
- Separate list of patterns by using vertical bar (|)
- Example: if [[ $VAR = @(Hello|Hallo) ]]; then ... fi
- Common character classes:
  - [:alnum:] - Alphanumeric
  - [:alpha:] - Alphabetic
  - [:ascii:] - ASCII chars
  - [:blank:] - Space or tab
  - [:cntrl:] - Control characters
  - [:digit:] - Decimal digits
  - [:graph:] - Non-blank
  - [:lower:] - Lowercase
  - [:print:] - Non-control
  - [:prunt:] - Punctuation characters
  - [:space:] - Whitespace
  - [:upper:] - Uppercase
  - [:xdigit:] - Hexadecimal digits
Turn off globbing: shopt -s -o noglob
Substitute the globbing into null string if not matched: shopt -s nullglob
Disable case sensitivity: shopt -s nocaseglob
Turn off special treatment of loading dot in filenames: shopt -s dotglob
Treat a list of string as filenames rather than globbing expressions: GLOBIGNORE=<string1>:<string2>:<string3>:...
Dollar sign substitutions:
- ANSI C Escape: $'<string>'
  Treat the string as ANSI C expression, allows the following escapes:
  - \a = Bell (alarm)
  - \b = Backspace
  - \cC = Control-C
  - \e = Escape character
  - \r = Carriage return
  - \t = Horizontal tab
  - \v = Vertical tab
  - \\ = Literal backslash
  - \' = Single quote
  - \nnn = ASCII character in octal value (up to 3 digits)
  - \xnnn = ASCII character in hexadecimal value (up to 3 digits)
- Locale translation: $"<string>"
  Translate the string into the character set of the current locale
- Variable name matching: ${!<VAR>*}
  Returns a list of variable names that start with the specified string
- Variable length: ${#VAR}
  Returns the length of the variable’s content
- Default values: ${VAR:-value}
  Return value if the variable is empty
- Assign default: ${VAR:=value}
  If the variable is empty, return the value and assign it to the variable as well
- Existence check: ${VAR:?message}
  If the variable does not exist, print the message and Bash exists
- Overriding default: ${VAR:+value}
  If the variable is not empty, set it to value and return this value. If it is empty, just return that value and keep the variable empty.
- Substrings: ${VAR:n}
  Return the value started from the n-th character, first character as 0th.
- Substrings: ${VAR:n:m}
  Return the value started from the n-th character, first character as 0th. But the returned value should not exceed m characters
- Substring removal: ${VAR#pattern}
  Return the (right-hand-side) substring after removing the globbing pattern from VAR, with smallest possible matching
- Substring removal: ${VAR##pattern}
  Return the (right-hand-side) substring after removing the globbing pattern from VAR, with largest possible matching
- Replace substring: ${VAR/pattern/replacement}
  Replace the first occurance. If # in the pattern, it matches the beginning; if $ in the pattern, it matches the end
- Replace substring: ${VAR//pattern/replacement}
  Replace all the occurances
- Delete substring: ${VAR/pattern}
- Delete all occurances of substring: ${VAR//pattern}
- Command substitution: $(<command line>)
- Arithmetic expression substitution: $((<arith expression>))

Debugging and Version Control

Checking a script for syntax errors without running it: bash -n script.sh
Terminates the shell script if a command returns an error code: shopt -s -o errexit
Terminates with an error if nonexistent (unset) variables are referenced: shopt -s -o nounset
Displays each command before it’s executed: shopt -s -o xtrace

Parameters and Subshells

Replacement for basename $0 : $BASENAME=${0##*/}
Walking through the script parameters: while-loop and shift

Better parameter handling: getopts-builtin

Example:

#!/bin/bash
getopts "hc:" SWITCH
printf "The first switch is %s Switch argument is %s Position of next parameter is %s\n" $SWITCH $OPTARG $OPTIND

Advanced parameter handling: getopt command (/usr/bin/getopt):

Subshell:
- Calling compound commands without a subshell: { command1 ; command2 ; }
- Calling compound commands with a subshell: ( command1: command 2 )
  - No semicolon after the last command in subshell style
  - Calling subshell will shield the compound commands from the current shell’s environment

Job control and Signals

In script, to stop itself can be done with:
```
suspend
kill -SIGSTOP $$
```
To enable job control in script (to make suspend work): shopt -s -o monitor
When Bash (non-interactive session) get SIGHUP, it will passing the signal to all sub-scripts, unless disown -h is used
When Bash in interactive session, setting shopt -s -o huponexit will pass SIGHUP to all its children process upon exit
Use of trap: trap <action command string> <signal> ...
- Run the specified string as command line when the specified signal is received
- View the current trap setting: trap -p <signal>
Run command when script exits: Use trap and the non-existent signal EXIT

Text file basics

Get file information (mtime, ctime, atime, etc.): stat <filename>
- In machine readable (one-line, space separated) format: state -t <filename>
- Specifying output format: statftime (another version of stat)
csplit - split a file into sections determined by context lines
expand - convert tabs into spaces
unexpand - Convert space back to tabs
Process substitution:
- Example: grep -f <(ls -1) /var/log/filelist.log
- Example: cat textfile > >(wc -l)
- <(list) will substitute on-the-fly with a FIFO, which can be read to obtain the output of list
- <(list) will substitute on-the-fly with a FIFO, which can be written to, and list will read the content as if it is from console input
File descriptors:
- exec 3< filename; read LINE <&3;
  Open filename as input and assign to file descriptor number 3
- 3< filename; read LINE <&3;
  Open filename as input and assign to file descriptor number 3. But once assigned, 3 cannot be reassigned nor the file can be reopened.
- exec 4<&3
  Copy an input file descriptor. If 4 is omitted, 0 is assumed (as for input file descriptor)
- exec 4<&3-
  Copy input file descriptor 3 to file descriptor 4, then close file descriptor 3.
- exec 4>&3
  Copy output file descriptor
- exec 4>log.txt ; command >&4
  Use of output file descriptor
- exec 3<>filename
  Open a file for both input and output. But if you output something at the middle, the content pending for input will be overwritten. Think of a file position pointer concept.
- exec 3<&-
  Close input file descriptor
- exec 4>&-
  Close output file descriptor
- /dev/fd/n
  Refer to file descriptor n
tail +n filename - Open a file, skip the first n lines and print the rest.
fold - wrap text lines
column - Similar to paste command, but create fixed-width columns.
merge - Three-way file merge

Text file processing

Example of find: find . "(" -name "*.txt" -or -name "*.sh" ) -type f
- modified in more than one day: -mtime +0
- accessed in last 10 to 60 minutes: -amin +9 -amin -61

Console Scripting

tty - Show the name of the terminal (e.g. /dev/tty1, /dev/pts1)
fgconsole - Print the console number (e.g. tty1 = 1), Report an error if not a console
Commands that can only run in console:
- showkey: Determines the keyboard data (e.g. reports the scan code, key code)
- setleds
- dumpkeys
- setterm

Network Programming

Open a socket: exec 3<> /dev/tcp/hostname/port

Bibliographic data

@book{
   title = "Linux Shell Scripting with Bash",
   author = "Ken O. Burtch",
   year = "2004",
   publisher = "Sams Publishing",
}