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Using
The Shell
Using The Shell
A Bit
of History
The
Command Line
Metacharacters
Shell
variables
Command
arguments
Command
output as arguments
I/O
redirection
Other
features
A Bit
of History
- The book refers to "the shell"
- Not "the Bourne shell,"
- Not "the C-shell" (aka “csh”)
- Not "the Bourne-Again shell" (aka. "bash")
- "The shell" here means "Bourne shell"
- The primary shell around at the time the book was written
- The C-shell did exist at the time the book was written
- It is mentioned briefly on p. 100
- csh added many useful interactive features
- command history
- job control
- the ability to edit previous commands
- Why focus on Bourne Shell?
- It's standard
- It's available everywhere
- It's the shell most Unix system scripts are written in
- There are some serious flaws that make the C-shell less suitable for programming
- Many newer shells, such as bash, zsh, and ksh, offer the better syntax of Bourne Shell along with the better interactive features of C-shell
The
Command Line
- The shell interperets each line of text as a whole and then executes the specified commands with the specified arguments
- Generally it's one command per line
- Commands may also be separated by ';'
- Commands may be grouped into a subshell
- Using parentheses
- Can be rather useful
- The output of the subshell can be piped
- The whole subshell can be put into the background
- You can do some limited scoping of variables
- Commands can be put into the background with '&'
- "In the background" means the shell doesn't wait for the command to complete
- Useful for long jobs
- The '|' character creates a pipe
- The output of the first command is the input of the second command
- Examples
- What is the output of
- date; who | wc
- (date; who) | tee save | wc
cat save - (sleep 5; echo boo) & echo date
- sleep 5; echo boo; date
- sleep 5; echo boo &
- (sleep 5; echo boo) &
Metacharacters
- Many characters are "special" to the shell and are interpreted before any commands are run
- These are collectively known as "metacharacters"
- Table 3.1 on p. 76 gives a complete list
- String matching metacharacters
- *
- Matches zero or more characters, except for an initial '.'
- echo *
- echo a*
- echo *.*
- echo *a*
- ?
- Matches a single character
- echo ?
- echo abcd?
- [...]
- Matches a single character from the list enclosed in brackets
- echo [ax]*
- echo abc[0-9]
- What if you want to match a metacharacter itself?
- Use a ' \'
- Use single quotes (like in the line above)
- You can even escape the newline
Shell
variables
- Variables are assigned with the syntax var=value
- There must be no whitespace around the '='
- The shell treats whitespace as significant
- Variables do not need to be declared before use
- There are no data types
- Variables are referenced with the syntax $var
- Variables are specific to their shell
- Not passed to subshells
- Unless exported
- Not passed up from subshells
- The '.' construct lets you read in a file and run it in the current shell
- Useful for things like .profile
- Or loading some default values from a file
Command
arguments
- The arguments to a shell command are placed into special variables
- The first is $1
- The second is $2
- etc. through $9
- The command itself is $0
- $* and $@ refer to all of the arguments, with some differences we'll cover when we cover shell scripting
Command
output as arguments
- The output of one command can become a parameter for another
- By using backquotes '`'
- (The character next to '1' on most keyboards)
- echo "The time is `date`, you know"
- Here is what Prof. Bakken wants on his tombstone below the usual stuff:
% echo “GO
COUGS!”
% echo “The Huskies
still suck as of `date`”
- The alert reader will notice that the above could also be done without the backquotes command output:
% echo “GO
COUGS!”
% echo –n
“The Huskies still suck as of ”; date
I/O
redirection
- The Bourne shell offers a great deal of control over I/O
- >
- Sends standard out (stdout) to the named file
- >>
- Appends stdout to the named file
- <
- Takes standard in (stdin) from the named file (rather than the terminal)
- File descriptors
- All files that are open in a shell have a number assigned to them
- That number can be used to refer to that particular file
- Remember that everything is a file, including stdin and stdout
- stdin is 0
- stdout is 1
- standard error (stderr) is 2
- Made a separate thing so that errors can go to the terminal even if the command output is redirected
- A file other than stdout (i.e stderr) can be redirected with the syntax grep x * 2>/tmp/error.out
- There can't be a space between the file descriptor and the redirect
- 2>>file does what you would expect
- Merging output
- time ls /bin >/tmp/ls.out 2>&1
- The merge must be done after the redirection
- The parsing happens left-to-right
- This sort of redirection is one of the things that C-shell is weak in
Some
Useful Commands (and commonly used options)
· od (-bc) : peek at contents in files
· ps (-aux): see what processes are running
· uptime: see how long computer has been up, what load is, etc.
· top: see the most resource-hogging processes
· cat (-n): dump out (concatenate) contents of files
· kill (-9 or %1 etc): kill a running process (that you own)
Other
features
We'll discuss more shell features (including control structures like if, for, and while) when we talk about shell scripting