The shell is one of the main interfaces to UNIX that a Systems Administrator uses. The shell is one of the main interfaces to UNIX that a Systems Administrator uses. - Interpreter
- Programming language
- Process groups of commands stored in a file called shell scripts.
- Like other languages, shells have
- Variables
- Control flow commands
There are many different types There are many different types - Bourne shell, sh The original shell from AT&T.
- Korn Shell, ksh
- A superset of Bourne shell that lets you edit the command line.
- C shell, csh Shell for BSD UNIX. Which uses C syntax and has many conveniences.
- modern updates – bash, tcsh modern updates of the Bourne and C shell
- bash is the default Linux shell.
- Most examples used in this lecture are from bash.
Shells are just executable programs. Shells are just executable programs. - Different shells use different syntax and provide different services.
You can start any shell at anytime You exit a shell with Each user has an entry in the /etc/passwd file which includes the name of the shell to execute.
As a command interpreter, the shell performs the following tasks. As a command interpreter, the shell performs the following tasks. - Wait for the user to enter a command
- Parse the command line
- This is the step this lecture concentrates on
- Find the executable file for the command
- a shell function
- a built-in shell command
- an executable program.
- If the command can't be found generate an error message
- If it is found, fork off a child process to execute the command
- Wait until the command is finished
- Return to step 1
When parsing the command line, bash performs three major steps When parsing the command line, bash performs three major steps - I/O redirection
- Expansion
- Tilde ~, Variable, Command, Arithmetic, Filename , Brace {} etc.
- Remove quote characters
Much of the above process is achieved using characters with special meanings.
When you enter a command, the shell searches for special characters, and it then performs some special tasks to replace the special characters. When you enter a command, the shell searches for special characters, and it then performs some special tasks to replace the special characters. - Example:
- echo * will not display a *.
- To actually use a special character (e.g. *) as itself you have to quote it
- using a pair of single quotes. echo '*'
- using a single back slash. Echo \*
Check the actual command after the replacing: - Turn on: set –x
- Turn off: set +x
Keeping track of special characters and their meanings can be a hassle Keeping track of special characters and their meanings can be a hassle Exercise – run the following command - [root@faile 6]# echo a \" is a special character
- [root@faile 6]# echo a “ is a special character
- [root@faile 6]# echo a “’’” is a special character
- [root@faile 6]# echo showing multiple \\\\\\\\\ is tricky
- [root@faile 6]# echo then again maybe not '\\\\'
- [root@faile 6]# echo then again maybe not "\\\\"
Any command line consists of Any command line consists of - Command and any number of arguments
- All separated by white space
- When the shell parses the command line it removes the white space
Control operators are a collection of characters (; & && || ) which change the operation of the command line. Control operators are a collection of characters (; & && || ) which change the operation of the command line. - The ; character can be used to place more than one command on a single line.
- Example: ls ; echo now in root directory ; cd / ; ls
- Place the command in the background: &
&& characters tell the shell to execute the 2nd command only if the first command has an exit status 0 (and) - && characters tell the shell to execute the 2nd command only if the first command has an exit status 0 (and)
- Example:
- grep root /etc/passwd && echo it works
- grep not_there /etc/passwd && echo it works
- || characters tell shell to execute the 2nd command only if the first command has an exit status of not 0
- Example:
- grep root /etc/passwd || echo it does not work
- grep not_there /etc/passwd || echo it does not work
- Example: On Linux, /etc/init.d/sshd has a line
- $SSHD $OPTIONS && success || failure
How to type a long command How to type a long command - Continue a Command using \ at the end of line
- NEWLINE is the end of a command
- \ escape the meaning of the next character
Group Commands () - User parentheses to group commands
- The shell create a subshell for each group
- Each subshell has its own env
- Ex:
- #pwd; (cd /); pwd
- #pwd; cd /; pwd
- (cd $1; tar –cf - . ) | (cd $2 ; tar –xvf - )
A way to send input and output (I/O) to different places. A way to send input and output (I/O) to different places. Used to - Read data from a file
- Write data to a file
- EX: ls –lR > all.my.files
- Join Unix commands together
- EX: grep “/bin/sh” /etc/passwd | cut –f1 –d:
Every process has a table of file descriptors (one for each file). Every process has a table of file descriptors (one for each file). Every process has three standard file descriptors which are used by default.
Log in to wopr.csl.mtu.edu Log in to wopr.csl.mtu.edu - Start a bash
- %bash
- Create a work dir and change to it
- Generate the “ls –R /etc” output to etc.files
- Save the error output from the above command to file errors also.
- $ls –lR /etc > etc.files 2> errors
- Put output and error output in the samefile
- $ls –R / > output.and.errors 2>& 1
Here documents – commonly used in script - $cat << the_end
- >This is a test of output of ls
- >`ls`
- >the_end
If no input file is provided, some commands will wait for the input from keyboard, until ctrl-D (EOF) is pressed.
UNIX treats everything (devices, processes, the kernel) as files UNIX treats everything (devices, processes, the kernel) as files Sending information to these files sends the information to the device. - Example:
- Send the contents of a file to terminal 1
- Send a file to the bit bucket in the sky
- $cat file > /dev/null
- $ls –lR /etc > etc.files 2> /dev/null
# # #15 2 * * * /usr/sbin/quot /fortran > /fortran/disk.usage 2>&1 # # check quotas once every hour # quotacheck is run on demand by e-mailing quotacheck@cslserver from login # scripts now so only run quotacheck once a day # #30 3 * * * /usr/sbin/quotacheck /fortran > /dev/null 2>&1 # # delete old apache logs and compress new ones # 0 1 * * * /usr/bin/find /var/adm-httpd -type f -mtime +14 -name "*log*gz" -exec /usr/bin/rm {} \; > /dev/null 2>&1 5 1 * * * /usr/bin/find /var/adm-httpd -type f -mtime +1 -name "*log*" \! -name "*gz" -exec /usr/bin/gzip {} \; > /dev/null 2>&1 # # check number of files in /var/spool/mqueue for CEC nagios monitor # 0,2,4,6,8,10,12,14,16,18,20,22,24,26,28,30,32,34,36,38,40,42,44,46,48,50,52,54,56,58 * * * * /usr/bin/ls /var/spool/mqueue | /usr/bin/wc -l | /usr/bin/awk '{print $1}' - >/usr/host/nagios/mqf 2>/dev/null
A shell defines variables to control the behavior of your UNIX session - Pre-defined variables
- User defined variables
- Name - must start with a letter or underscore character
- followed by 0 or more letters, numbers or underscores
Define a variable or Assign a new value Define a variable or Assign a new value - variable_name=[value]
- no space around =
- Bash: % name=“david jones”
- Tcsh: % set name=“values”
Using the value - %echo ${name}
- %echo $name
Check what variables are set - The set command (actually a built-in shell command) will show all the current shell variables and their values.
To Unset a variable
By default, user defined variables are local. By default, user defined variables are local. Environment variables are passed to sub-processes. Local variables are not. To promote a local variables to environment variables - Bash:
- $CLASSPATH=$NETSCAPE_HOME/classes
- $export CLASSPATH
- Tcsh:
- $setenv CLASSPATH $NETSCAPE_HOME/classes
To check environment variables - Tcsh: setenv
- Bash: export –n
Environment variables should be set at login time in startup files. Environment variables should be set at login time in startup files. Bourne shell - System wide /etc/profile
- .profile in your home dir
- Example: /etc/profile in undergrad lab.
Make the change take effect Make the change take effect - Log out and log back in
- Running .profile with . (DOT) built-in
- Ex: $. .profile
- Command runs the script as the part of the current process
- Changes will affect the login shell.
- If without the first ., the new variable would be in effect only in the subshell running the script.
- You can change the values of the environment variables just like you would to any other variable variable_name=[value]
- What will happen if you do:
Change your prompt for the bash Change your prompt for the bash - Start a bash %bash
- Find the value for the environment variable $PS1
- ‘%man bash ‘ and looking for PS1
- Set PS1 to be like [username@hostname:pwd%]
About $PATH - Login to icu0.csl.mtu.edu as youself
- What is the value of PATH?
- Which startup files set its value?
- $man tcsh – looking for startup and shutdown section
- /etc/csh.cshrc
- /etc/csh.login
- ~/.tcshrc
- ~/.cshrc
- ~/.history
- ~/.login
Example Example - dinbig:~$ myName=
- dinbig:~$ echo my name is ${myName:-"NO NAME"}
- my name is NO NAME
- dinbig:~$ echo my name is $myName
- my name is
- dinbig:~$ echo my name is ${myName:="NO NAME"}
- my name is NO NAME
- dinbig:~$ echo my name is $myName
- my name is NO NAME
dinbig:~$ herName= dinbig:~$ herName= dinbig:~$ echo her name is ${herName:?"she hasn't got a name"} bash: herName: she hasn't got a name dinbig:~$ echo her name is ${herName:?} bash: herName: parameter null or not set
Command substitution allows you to insert the output of one command into the command line of another. Command substitution allows you to insert the output of one command into the command line of another. - command substitution uses back quote character `.
- BE CAREFUL: Many people confuse the back quote with the single quote character. They are different.
Exercise: - %echo “there are `wc –l /etc/passwd` lines in the passwd file.
- Create a file called hostname.log
- Create a file named yyyy_mm_dd.log
filename substitution or globbing filename substitution or globbing - A way of specifying a number of filenames with a small number of characters.
- Some special characters are used to specify which filenames to match.
- *
- Any string including the null string
- ?
- [ ]
- Matches any one character within the [ ] Two characters separated by a - indicates a range If ! or ^ are the first character then any character NOT between [ ] are matched.
The ~ (tilde) character expands to either user's home directory, current working directory or previous working directory depending on following character. The ~ (tilde) character expands to either user's home directory, current working directory or previous working directory depending on following character. - Examples
- echo my home directory is ~
- echo working directory is ~+
- echo previous working directory is ~-
bash supports a number of other expansions which can be useful bash supports a number of other expansions which can be useful - brace expansion
- Similar to pathname expansion
- Example
- ls -ld /etc/rc.d/{init,rc1,rc2}.d
- arithmetic expansion
- Evaluation of arithmetic expressions.
- Examples
Why shell script? Why shell script? - Simply and quickly initiate a complex series of tasks or a repetitive procedure.
Creating a simple shell script - A shell script is a file that contains commands that the shell can execute.
- Any commands you enter in response to a shell prompt.
- A utility
- A compiled program
- Another shell script
- Control flow commands
Make the file executable Make the file executable - When you create a shell script using a editor, does it have execute permission typically?
- Example
- [ruihong@dafinn ~/cs3451]$ ./test
- ./test: Permission denied.
- [ruihong@dafinn ~/cs3451]$ ls -l test
- -rw------- 1 ruihong csdept 22 Jan 28 09:33 test
- [ruihong@dafinn ~/cs3451]$ chmod +x test
- [ruihong@dafinn ~/cs3451]$ ./test
- this is a test
Enter the script filename as the command Enter the script filename as the command - The shell forks a process
- Which creates a duplicate of the shell process (subshell)
- The new process attempt to exec the command
- If the command is a executable program
- Exec succeeds
- System overlays the newly created subshell with the executables programs
- If the the command is a shell script
- Exec failed
- The command is assumed to be a shell script
- The subshell runs the commands in the shell one after another
Pass the shell script as a parameter to a shell program - Run a shell script which does not have execution permission
- Run the script with different shell other than your interactive shell
Put special characters on the first line of a shell script Put special characters on the first line of a shell script - To tell OS checks what kind of file it is before attempting to exec it
- To tell which utility to use (sh, csh, tcsh, …)
- The firsts two character of a script are #!
- Then followed by the absolute pathname of the program that should execute the script
- sh-2.05b$ more /etc/init.d/sshd
- #!/bin/bash
- #
- # Init file for OpenSSH server daemon
- #
Comments make shell scripts easier to read and maintain Comments make shell scripts easier to read and maintain Pound sign (#) start a comment line until the end of that line, except - #! In the first line.
- Or inside quotes
A shell parameter is associated with a value that is accessible to the user. A shell parameter is associated with a value that is accessible to the user. - Shell variables
- Keyword shell variables
- Has special meaning to the shell
- Being created and initialized by the startup file
- User created variables (create and assign value)
- Local variables
- Environment variables
- Positional parameters
- Special parameters
The command name and arguments The command name and arguments - Why are they called positional parameters?
- Because they can by referenced by their position on the command line
- $0 : Name of the calling program
- $1 - $9 : Command-line Arguments
- The first argument is represented by $1
- The second argument is represented by $2
Example:
How to access the parameter after $9? How to access the parameter after $9? - shift
- Built-in command shift promotes each of the command-line arguments.
- The first argument ( which was $1) is discarded
- The second argument ( which was $2) becomes $1
- The third becomes the second
- And so on
Repeatedly using shift is a convenient way to loop over all the command-line arguments
Example:
Initialize arguments outside of the command line Initialize arguments outside of the command line - set (sh/ksh only)
- Set the positional parameters starting from $1, …
Use quote for variable reference - Example:
- what’s the difference if $1 is null
- $ display_4args $1 a b c d
- $ display_4args “$1” a b c d
- What will happen if a is null
- if [ $a = 3 ]; then
- echo a is 3
- fi
Special parameters give you some useful values Special parameters give you some useful values - Number of the Command-line arguments
- Return status of the execution of shell commands
- Etc..
Special parameters’ value can not be changed directly, like positional parameters
Value of Command-line arguments: $* and $@ Value of Command-line arguments: $* and $@ - $* and $@ represent all the command_line arguments ( not just the first nine)
- “$*” : treat the entire list of arguments as a single argument
- “$@” : produce a list of separate arguments.
sh-2.05b$ more for_test sh-2.05b$ more for_test echo "using \$@ " for arg in "$@" do echo "$arg" done echo "using \$* " for arg in "$*" do echo "$arg" Done sh-2.05b$ ./for_test 1 2 3 using $@ 1 2 3 using $* 1 2 3
The number of arguments: $# The number of arguments: $# The current shell’s PID number: $$ The PID number of last process that you ran in the background: $! - Ex:
- sh-2.05b$ sleep 1000 &
- [1] 11962
- sh-2.05b$ echo $!
- 11962
Exit status: $? Exit status: $? - When a process stops executing for any reason, it returns an exit status to its parent process.
- By convention,
- Nonzero represents a false value that the command failed.
- A zero value is true and means that the command was successful
- You can specify the exit status that a shell script returns by using the exit built-in followed by a number
- Otherwise, the exit status of the script is the exit status of the last command the script ran.
A shell is both a command interpreter and a programming language A shell is both a command interpreter and a programming language Local variables and environment variables Command line expansion - Parameter expansion
- variable expansion
- Command substitution
- pathname expansion
Special parameters
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