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introduce the shell interpreter
POSIX.1-2008 XCU 2.1
shell reads input from:
a specified file,
'-c' option argument, or
stdin (interactive mode)
shell splits input lines into tokens and parses them
shell expands words
shell performs I/O redirection
shell executes functions, built-in commands, files, etc.
parameters and variables
positional parameters
POSIX.1-2008 XCU 2.5.1
decimal numbers greater than 0, e.g. $1 and $2
multiple-digit numbers should be enclosed in braces
assigned with:
shell arguments when shell starts
function arguments when a function is called
can be reassigned using 'set' built-in command
special parameters
POSIX.1-2008 XCU 2.5.2
@
all positional parameters
*
all positional parameters
#
number of positional parameters
?
exit status of most recent command pipeline
-
current option flags
$
process ID of shell
!
process ID of most recent background command
0
name of shell or shell script
shell variables
POSIX.1-2008 XCU 2.5.3
HOME
pathname of user's home directory
IFS
delimiters for field splitting
default: <space> <tab> <newline>
LINENO
line number of current script, if any
PATH
list of paths which should be searched for commands
PS1
prompt value to be expanded and written to standard error
default value: "$ "
PWD
absolute pathname of the current working directory
word expansions
tilde expansion
POSIX.1-2008 XCU 2.6.1
tilde-prefix: unquoted tilde at the beginning of a word
followed by all unquoted characters until first unquoted slash
login name is all characters in tilde-prefix following tilde
if login name is empty (tilde-prefix == "~")
expanded to value of HOME
else
expanded to home directory associated with login name
parameter expansion
POSIX.1-2008 XCU 2.6.2
${expression} or $expression
expression:
parameter
parameter:-word
parameter:=word
parameter:?word or parameter:?
parameter:+word
#parameter
parameter%word
parameter%%word
parameter#word
parameter##word
command substitution
POSIX.1-2008 XCU 2.6.3
$(command)
or
`command`
expanding:
shell executes command in a subshell
shell replaces substitution with stdout of the command
newlines within command are allowed
nesting
`command1 \`command2 foo\` bar`
$(command1 $(command2 foo) bar)
arithmetic expansion
POSIX.1-2008 XCU 2.6.4
$((expression))
expanding:
shell processes expression
shell replaces substitution with value of the expression
arithmetic:
signed long integer precision
decimal, octal, and hexadecimal constants
e.g. 42, 052, 0x2A
operators:
( ), unary +, unary -, +, -, *, /, %
~, !, <<, >>, &, ^, |
<, <=, >, >= ==, !=, &&, ||, expr ? expr : expr
=, +=, -=, *=, /=, %=, <<=, >>=, &= ^=, |=
shell variables
if x has an integer value, then the following are equivalent:
$((x + 1)) $(($x + 1))
field splitting
POSIX.1-2008 XCU 2.6.5
IFS is a set of delimiters, at which unquoted fields are split
pathname expansion
POSIX.1-2008 XCU 2.6.6
POSIX.1-2008 XCU 2.13
performed unless 'set -f' was run
patterns
? matches any single character
* matches multiple (0 or more) characters
[ opens RE bracket expansion
if pattern does not match any existing files
pattern string is left unchanged
redirection
input redirection
[n]<word
n: optional file descriptor number (default: 0, stdin)
word: name of file
output redirection
[n]>word
[n]>|word
n default: 1, stdout
appending redirected output
[n]>>word
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