196 lines
8.6 KiB
Plaintext
196 lines
8.6 KiB
Plaintext
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Command: sh, ., break, case, cd, continue, eval, exec, exit, export,
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for, if, read, readonly, set, shift, trap, umask, wait,
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while - shell
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Syntax: sh [-eiknqstvxu] [-c str] [file]
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Flags: -c Execute the commands in str
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-e Quit on error
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-i Interactive mode; ignore QUIT, TERMINATE, INTERRUPT
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-k Look for name=value everywhere on command line
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-n Do not execute commands
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-q Change qflag from sig_ign to sig_del
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-s Read commands from standard input
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-t Exit after reading and executing one command
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-v Echo input lines as they are read
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-x Trace
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-u Unset variables
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Example: sh script # Run a shell script
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Sh is the shell, which forms the user's main interface with the
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system. On startup, the shell reads /etc/profile and $HOME/.profile, if
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they exist, and executes any commands they contain. The Minix shell has
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most of the features of the V7 (Bourne) shell, including redirection of
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input and output, pipes, magic characters, background processes, and
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shell scripts. A brief summary follows, but whole books have been
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written on shell programming alone.
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Some of the more common notations are:
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date # Regular command
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sort <file # Redirect stdin (standard input)
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sort <file1 >file2 # Redirect stdin and stdout
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cc file.c 2>error # Redirect stderr
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a.out >f 2>&1 # Combine standard output and standard error
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sort <file1 >>file2 # Append output to file2
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sort <file1 >file2 & # Background job
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(ls -l; a.out) & # Run two background commands sequentially
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sort <file | wc # Two-process pipeline
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sort <f | uniq | wc # Three-process pipeline
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ls -l *.c # List all files ending in .c
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ls -l [a-c]* # List all files beginning with a, b, or c
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ls -l ? # List all one-character file names
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ls \? # List the file whose name is question mark
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ls '???' # List the file whose name is three question
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marks
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v=/usr/ast # Set shell variable v
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ls -l $v # Use shell variable v
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PS1='Hi! ' # Change the primary prompt to Hi!
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PS2='More: ' # Change the secondary prompt to More:
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ls -l $HOME # List the home directory
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echo $PATH # Echo the search path
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echo $? # Echo exit status of previous command in
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decimal
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echo $$ # Echo shell's pid in decimal
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echo $! # Echo PID of last background process
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echo $# # Echo number of parameters (shell script)
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echo $2 # Echo second parameter (shell script)
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echo "$2" # Echo second parameter without expanding
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spaces
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echo $* # Echo all parameters (shell script)
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echo $@ # Echo all parameters (shell script)
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echo "$@" # Echo all parameters without expanding
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spaces
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The shell uses the following variables for specific purposes:
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SHELL the path of the current shell
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HOME the default value for the cd(1) command
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PATH the directories to be searched to find
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commands
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IFS the internal field separators for command
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strings
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PS1 the primary shell prompt
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PS2 the secondary shell prompt
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There are various forms of substitution on the shell command line:
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`...` Command string between back-quotes is
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replaced by its output
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"..." Permits variable substitution between quotes
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'...' Inhibits variable substitution between quotes
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$VAR Replaced by contents of variable VAR
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${VAR} Delimits variable VAR from any following
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string
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The expressions below depend on whether or not VAR has ever been set.
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If VAR has been set, they give:
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${VAR-str} Replace expression by VAR, else by str
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${VAR=str} Replace expression by VAR, else by str and
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set VAR to str
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${VAR?str} Replace expression by VAR, else print str and
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exit shell
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${VAR+str} Replace expression by str, else by null
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string
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If a colon is placed after VAR, the expressions depend on whether or not
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VAR is currently set and non-null.
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The shell has a number of built-in commands:
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: return true status
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. fn execute shell script fn on current path
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break [n] break from a for, until or while loop; exit n
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levels
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continue [n] continue a for, until or while loop; resume
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nth loop
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cd [dir] change current working directory; move to
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$HOME
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eval cmd rescan cmd, performing substitutions
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eval rescan the current command line
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exec cmd execute cmd without creating a new process
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exec <|> with no command name, modify shell I/O
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exit [n] exit a shell program, with exit value n
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export [var] export var to shell's children; list exported
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variables
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pwd print the name of the current working
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directory
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read var read a line from stdin and assign to var
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readonly [var] make var readonly; list readonly variables
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set -f set shell flag (+f unsets flag)
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set str set positional parameter to str
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set show the current shell variables
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shift reassign positional parameters (except ${0})
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one left
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times print accumulated user and system times for
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processes
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trap arg sigs trap signals sigs and run arg on receipt
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trap list trapped signals
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umask [n] set the user file creation mask; show the
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current umask
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wait [n] wait for process pid n; wait for all
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processes
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The shell also contains a programming language, which has the following
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operators and flow control statements:
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# Comment The rest of the line is
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ignored
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= Assignment Set a shell variable
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&& Logical AND Execute second command only if
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first succeeds
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|| Logical OR Execute second command only if
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first fails
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(...) Group Execute enclosed commands
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before continuing
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for For loop (for ... in ... do ... done)
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case Case statement ((case ... ) ... ;; ... esac)
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esac Case statement end
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while While loop (while ... do ... done)
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do Do/For/While loop start (do ... until ...)
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done For/While loop end
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if Conditional statement (if ... else ... elif
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... fi)
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in For loop selection
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then Conditional statement start
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else Conditional statement alternative
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elif Conditional statement end
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until Do loop end
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fi Conditional statement end
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