The Shell

Boiled down:

Command Line Syntax

• Command [arg1] [arg2] ... [argn] RETURN
• Arguments numbered; command is arg0
• Option/flags - modify the effect of the command

Command Editing

• ctrl-u - erase a line
• ctrl-w - erase a word
• ctrl-h - erase a character
• ctrl-r

Processing a Command

• Shell is running, interprets the command
• Finds the command by checking (in order) $PATH
• Shell execs the command and goes to sleep
• When command finished, shell wakes up, issues prompt and waits for another command
• To change your shell - chsh

Stdin and Stdout

• Standard input (stdin) has fileID 0
• Standard output (stdout) has file ID 1
• Standard error (stderr) has file ID 2
• The who command reports the filename of your window (tty01)
• The cat command copies stdin to stdout
$ cat

will keep echoing everything you type until ctrl-d is entered

Redirection

• > redirects stdout
• > will overwrite the file
• < redirects stdin
• >> redirects stdout and appends it
• /dev/null (a bit bucket) is a place you can redirect output you do not want
• to redirect stderr to stdout
  command 2 >&1
• to redirect stdout to stderr
  command 1 >&2

Pipes

• connects stdout of one command to stdin of the next command?
  $ tr str1 str2
  translates the matched character in str1 to the corresponding character in str2. (1st character in string1translated to 1st character in string2)
• Filters modify a stream of input and produce a stream of output (eg. sort)
• The tee command single input, 2 outputs
  $ who | tee file | grep string output from who goes to file and into grep

Special Characters

• *
• ?
• &
• |
• ;
• "
• '
  single quote - don't interpret anything
• "
  double quote - for strings, won't process spaces, but will process $ as a variable name
• Try echo $PATH, echo "$PATH", echo '$PATH'
• Try ls "*", ls '*'
• '
  accent mark - to run a command
• [ ]
• ( )
• $
• < >
• { }
• /
• \
• To use a special character precede it with a backslash \
• /dev/null
• /dev/tty
• Use ` ` for command substitution (to the left of the 1)

Shell variables

• myvar=abc
  sets myvar to abc, cannot use spaces
• unset myvar
  removes a variable
• readonly myvar
  cannot give value to myvar

All work herein is subject to copyright. Original content to Dr. Deborah Whitfield, text content (Your unix/Linux) to Prentice Hall publishing.