IT 244 – Chapter 10
Programming the Bourne Again Shell:
Part1 – Overview
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Debugging Shell Scripts
o Place echo statements along the way.
o Use the –x bash option
 bash –x yourScript arglist
 This prints a trace: which is a printout of each command
before it actually runs it. All expansions will already have been
done before the line is printed.
 The PS4 prompt is used to show command line
printouts, so this can be set to make it obviously
different to program output.
Control Structures
o if, if/then, if/then/else, if/then/else/elif
o for, for in,
o while (do done), until (do done)
o case
o break, continue
Input during a run
o select, uses the PS3 variable as prompt
o Here document – uses redirection – double less than with begin and
end characters – text is all “here”
o File descriptors
 exec n> outfile, or duplicate another by exec n>&m
 exec m< infile, or duplicate another by exec m<&n
 Close using exec n>&-, or exec m<&Functions – rehashed
o Function funcName() { <functionBody> }
o Call this from anywhere in the script, whether outside of a function, in
another function, or a function calling itself, by giving the function
name.
o Arguments are passed just as on the command line. If you have a
variable with a space-delimited string, that can be given as the
argument, and set will automatically be called on it to produce $1 - $n.
Parameters and Variables
o Local vs global
 The default is local. No child process can see the variable.
 export makes a variable global, in that the value of the variable
will be copied to any child processes
 The child making a change does not reflect with the parent
copy.
Functions are not processes, so they have access to variables
even if they are not defined within their own block. No need
for them to be exported.
 Whether a variable is local or global, bash makes sure that any
variables in your script will not conflict with variables in
another script that uses yours, since it does not allow the child
process to change the value of a variable owned by the parent.
 Functions can have local variables by using the declare or
typeset command.
o A variable or parameters can be used to run a command merely by
preceding it with a $ as the first token on a command line.
o Parameters:
 Special Parameters:
 $$ – PID# for this process
 $? – Exit status of the last command
 Positional Parameters:
 $# – Number of command line arguments.
 $0 – Name of this calling program (script)
o The exact relative or absolute path used to make
the call (except ~ is substituted by full path)
 $1 - $n – Command line arguments
o Any larger than $9 must be surrounded by curly
braces ( ${10} )
o shift – Command to shift arguments one to the
left (promotes arguments).
o set – Can be used to initialize command line
arguments
 Combining command substitution or
pathname expansion with set.
 Convenient way to get input data.
 $* and $@ – Represent all command line arguments
o One is a space delimited string, and the other is
an array. (See issue on p444)
 Expanding null and unset variables – expands to a null string.
 You can:
o Use a default value ( :- ) - Uses but doesn’t assign
 ${name:-defaultVal}
o Assign a default value ( := ) - Uses and assigns
 ${name:=defaultVal)
 In both cases, defaultVal can contain
variables and other expansions.
 To use the assign without actually setting
the value into something else, start the
command with a : and a space – ( :
${name:=defaultVal} ). Otherwise, you
must use this as an expansion
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o Display an error message ( :? )
 ${name:?someErrorMsg}
Some Built-ins
o type – Gives the type of a command, whether builtin or not, and
whether hashed or not. Also gives the path if not builtin. If the string
is a shell keyword, like if, or an alias, this will also be reported.
 Note: type tells whether a command is hashed. What this
means is that a non-builtin command, when called the first
time, is searched for using the PATH variable. The path is then
plugged into a hash table, with the name of the command as
the key, so that the next time the command is called, the shell
will know where to look. This plugging of the hash happens
the first time for each session.
o read
 Without the –p option, you echo a prompt (using –n) and then
call read followed by a variable name. read the waits for
keyboard input and places that into the variable. It waits for a
newline to retrieve the value.
 read also has a –p option that takes a prompt string an then
waits for keyboard input after printing the prompt.
 If you give more than one variable on the read command line, it
was read each token (delimited by the given character(s)) into
each variable. If there are less variables than tokens, it will fill
the last one with the rest, separated by one space, no matter
how many spaces were on the command line.
 Options:
 -a aname - Will create an array called aname of all
tokens read.
 -d delim – Will change the input termination delimeter
from newline to delim. This means that, instead of
waiting for the newline in order to assign values to the
variables given, the input will be immediately assigned
when the alternate delimiter is received. Therefore, if
there is only one token before the alternate delimiter,
no more than one variable will be assigned to.
o A subsequent read will then get the next string
from the keyboard if it exists.
 -n num - Read that number of characters and then
returns.
o A subsequent read will then get the next string
from the keyboard if it exists.
 - p prompt – Prints a prompt and waits for keyboard
input.
 -s – silent so that it will not echo input back to the
display. This can be used for password entry.
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o exec
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o trap
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-u n – sets n as the file descriptor for read to take input
from.
Executes commands or redirects file descriptors
Command Execution:
 exec command argList
 Overlays the current process space with the new
command. The current process will terminate
execution, and the newly overlayed command (or shell
script) will begin execution in that space.
File Descriptor Redirects:
 exec < infile, or exec > outfile - Redirecting standard
in and out, and opens infile or outfile for input or
output.
 exec n< infile or exec m> outfile – Creates new file
descriptors and opens infile or outfile for input or
output.
Catch a system signal
 You can use this to make sure Ctrl-C, Ctrl-D, Ctrl-Z, and
other system signals do not stop your process. Or to
clean up properly before stopping.
Expression evaluation
o Arithmetic
 let <arithmetic assignment expression>
 This allows for assigning a value from an expression on
the right hand side of an “=” operator to a variable on
the left hand side.
o Example: let a=a+1
o Note: there can be no spaces in the assignment
expression.
o Note: to put more than one assignment in a let
statement, separate them with a space.
 Example: let a=a+1 b=b-3 c=a+b
 ((<arithmetic assignment expression>))
 This is a type of shorthand for the let statement above.
o Example: (( a=a+1 ))
o Note: to put more than one assignment in side
the double-parenthesis operators, separate them
using a comma.
 Example: (( a=a+1, b=b-3, c=a+b ))
o Logical (Conditional)
 [[ <conditional statement ]]
 Conditional statements evaluate whether the value of the left
side of a “=” operator is the same as, not the same as, less than,
or greater than the value of the right side.
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 Example: [[ $a = $b ]]
You can evaluate more than one conditional by using the &&
operator (AND) or the || operator (OR)
 Example: [[ $a = $b && $a = $c ]]