COMP421
Unix Environment for Programmers
Lecture 05: Shell Scripts_______________________________________

Jeff Wiegley, Ph.D.
Computer Science
jeffw@csun.edu

09/12/2005

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Motivation____________________________________________

• The shell is most often used for “single” command entry
• “single” may include multiple, piped commands.
  • who | cut -c1-8 | sort -u | pr -l1 -8 -w78 -t¹
• The shell can also do more complicated programming chores:

  for file in ‘ls -1‘; do       if grep -q quote $file; then         echo "File $file contains what you seek"       fi     done

• Such constructions can become very long and tedious to retype.²

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Shell Scripts:_________________________________________

It would be much easier to wrap up long, complicated sequences in single “script” that can be executed and edited on demand.

Such files are called “shell scripts”. They are interpreted and are the basis for rapid prototyping and task automation in Unix.

They can be very powerful and are used as the foundation for controlling how operating system services are stopped and started.

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The $PATH to enlightenment:_____________________

• Shell scripts must be interpreted by a shell.
• When a command is typed the interpreting shell searches the directories listed in the shell/environment variable “$PATH” for the program.
• Typing: “$ loggedin” probably will result in a “command not found” error because the current directory is not part of $PATH by default.
• There are three ways to overcome the problem:
  1. Add “.” to the $PATH variable.

     PATH=$PATH:.   export PATH

  2. Specify an absolute path instead

     $ ./loggedin

  3. Pass the filename as an argument to a shell.

     $ /bin/sh loggedin  [spawns a new shell process]   $ . loggedin        [Current shell does interpretation]

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Shell tricks:___________________________________________

There are few handy syntax tricks to know when dealing with the Shell.

• I/O redirection is possible.

  {>,<,&>,#>}

• The “command” !! means execute the last command line entered.
• The “command” !foo means execute the last command line entered that started with “foo”.
• The ‘;’ terminates a command and therefore allows multiple commands to be given on the same line.³⁴

  echo hello; echo there

• Commands can be continued across lines by using the ‘\’ character followed immediately by an end of line.

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Which shell?:_________________________________________

• More than one shell has been written and adopted into widespread use.
  • Bourne shell (sh)
  • C shell (csh)
  • Korn shell (ksh)
  • tcsh
  • Bourne-again shell (bash)
• Not all of the shells have 100% compatible syntax and features.
  • For instance, ksh and bash can do arithmetic functions with x=$((x+1)) sh and csh cannot.⁵

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Variables: a cursed blessing:________________________

• Blessing: Variables in shell are simple; comprised only of “strings”.
  • They do not have to be declared and are all global.⁶
• Curse: They are very limited. The shell relies on other utility programs such as sed to manipulate the data.
• Variables are set through assignment.
  • $ name=value
  • $ mylist="jane mike jim kim"
  • $ set mylist="jane mike jim kim"
• There cannot be any whitespace around the assignment character “=”.
• Variables contents can be printed with the “echo” command.
  • $ echo "$mylist"

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Conditional branchings :____________________________

Conditional branching is handled a number of ways:

1. if statements

   if conditional
   then
     One or more commands...
   else
     One or more commands
   fi
   

2. boolean construction

   cp src dest || echo "failed to copy"

3. case statements, the syntax for which is covered later.

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Conditionals :________________________________________

The only “test” that the shell knows about is the exit value of a command.⁷

For example if the grep command finds any matches then it exits with a return value of 0, otherwise it exits with a non-zero value.

This makes prototyping fast and efficient:

if grep -q file1 file2 filen   then      echo "Something was found"   else      echo "Give up!"   fi

This makes testing the contents of a variable rather challenging. how do you do $mylist == "hello"?

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All hail test!, the almighty tester!________________

• /usr/bin/test is a program that does just testing.
• Uses command line arguments to specify what and how to test.
• returns a 0 if the test suceeeded, non-zero otherwise.⁸

  if test "$mylist" = "hello"; then       echo "They’re the same!!"     fi

• The command run in this example is test "$mylist" = "hello"
• test is very powerful and includes all the possible comparisons one would want. Including boolean operators and arithmetic comparisons which convert their string arguments to integers.

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All hail []! the almighty… imposter?:_____________

• It gets very boring and tedious to keep writing test for every conditional you want.
• test also doesn’t look elegant or like anything similar to ( ... ) which is common syntax in other programming languages.
• So shells have a shortcut. You can use [ ... ] instead of test.

  if [ "$mylist" = "hello" ]; then           echo "They’re the same!!"         fi

• [ ... ] functions identical to test.
• You must have whitespace before and after these delimiters.

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Loops:_________________________________________________

There are two looping constructs that test the exit status of command

1. until, which executes a command until its command returns 0:

   until who | grep "ˆbarb"; do     sleep 60   done

2. while, which executes its command until its command returns non-zero. (Same syntax)

A third looping construct exists called a for loop that does not test an exit status but rather processes a list of things:

for varname in "one" "two" "three" "four"; do           echo $varname         done

If the “in list” portion is omitted then the for loop iterates over the command line arguments.

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Automatic variables:_________________________________

Certain variables exist automatically.

$@: consists of the entire command line passed to the shell.

$0: the command name used to invoke the shell.

$1…$9: The first nine individual command line arguments.⁹

$? the exit value of the last command executed.

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