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UNIX Bourne Shell Scripting

UNIX Bourne Shell Scripting

These are the workshop notes I used to use for teaching a course on Bourne shell (sh) scripting. I haven’t taught the class for some time now, but the material is all still current since the Bourne shell hasn’t changed. Often you’ll find that these notes are a bit short of description. This is because they were intended by be a lecture aid.

What do you need to know to follow this course? These notes were originally written as a second class in UNIX. The first class taught how to use the basic UNIX commands (like sed, grep and find) and this class teaches how to combine these tools to accomplish bigger tasks.

In addition to the material in this course you might be interested in the Korn shell (ksh) and the Bourne again shell (bash), both of which are excellent shells that enchance the original Bourne shell. These alternate shells are upwardly-compatible with the original Bourne shell, meaning that a script written for sh can run in ksh or bash. However, there are additional features in bash and ksh that are not available in the Bourne shell.

The focus of this guide is to get you to understand and run some Bourne shell scripts. On several pages there are example scripts for you to run. On most of these pages there is a link you can click on (with the right mouse button) and download the script to your computer and run it.

Shell scripting skills have many applications, including:

Ability to automate tasks, such as
Backups
Administration tasks
Periodic operations on a database via cron
Any repetetive operations on files
Increase your general knowledge of UNIX
Use of environment
Use of UNIX utilities
Use of features such as pipes and I/O redirection
For example, I recently wrote a script to make a backup of one of the subdirectories where I was developing a project. I quickly wrote a shell script that uses /bin/tar to create an archive of the entire subdirectory and then copy it to one of our backup systems here at SDSC and store it under a subdirectory named according to today’s date.

As another example, I have some software that runs on UNIX that I distribute and people were having trouble unpacking the software and getting it running. I designed and wrote a shell script that automated the process of unpacking the software and configuring it. Now people can get and install the software without having to contact me for help, which is good for them and good for me, too!

For shell script experts one of the things to consider is whether to use the Bourne shell (or ksh or bash), the C shell, or a richer scripting language like perl or python. I like all these tools and am not especially biased toward any one of them. The best thing is to use the right tool for each job. If all you need to do is run some UNIX commands over and over again, use a Bourne or C shell script. If you need a script that does a lot of arithmetic or string manipulation, then you will be better off with perl or python. If you have a Bourne shell script that runs too slowly then you might want to rewrite it in perl or python because they can be much faster.

Historically, people have been biased toward the Bourne shell over the C shell because in the early days the C shell was buggy. These problems are fixed in many C shell implementations these days, especially the excellent ‘T’ C shell (tcsh), but many still prefer the Bourne shell.

There are other good shells available. I don’t mean to neglect them but rather to talk about the tools I am familiar with.

If you are interested also in learning about programming in the C shell I also have a comparison between features of the C shell and Bourne shell.

Table of Contents:

Review of a few Basic UNIX Topics (Page 1)
Storing Frequently Used Commands in Files: Shell Scripts (Page 6)
More on Using UNIX Utilities (Page 9)
Performing Search and Replace in Several Files (Page 11)
Using Command-line Arguments for Flexibility (Page 14)
Using Functions (Page 30)
Miscellaneous (Page 38)
Trapping Signals (Page 43)
Understanding Command Translation (Page 50)
Writing Advanced Loops (Page 59)
Creating Remote Shells (Page 67)
More Miscellaneous (Page 73)
Using Quotes (Page 75)

Section 1: Review of a few Basic UNIX Topics

Shell scripting involves chaining several UNIX commands together to accomplish a task. For example, you might run the ‘date’ command and then use today’s date as part of a file name. I’ll show you how to do this below.

Some of the tools of the trade are variables, backquotes and pipes. First we’ll study these topics and also quickly review a few other UNIX topics.

Variables

Topics covered: storing strings in variables
Utilities covered: echo, expr
To try the commands below start up a Bourne shell:
/bin/sh
A variable stores a string (try running these commands in a Bourne shell)
name=”John Doe”
echo $name
The quotes are required in the example above because the string contains a special character (the space)
A variable may store a number
num=137
The shell stores this as a string even though it appears to be a number
A few UNIX utilities will convert this string into a number to perform arithmetic
expr $num + 3
Try defining num as ‘7m8’ and try the expr command again
What happens when num is not a valid number?
Now you may exit the Bourne shell with
exit

Page 1

I/O Redirection

Topics covered: specifying the input or capturing the output of a command in a file
Utilities covered: wc, sort
The wc command counts the number of lines, words, and characters in a file
wc /etc/passwd
wc -l /etc/passwd
You can save the output of wc (or any other command) with output redirection
wc /etc/passwd > wc.file
You can specify the input with input redirection
wc < /etc/passwd Many UNIX commands allow you to specify the input file by name or by input redirection sort /etc/passwd sort < /etc/passwd You can also append lines to the end of an existing file with output redirection wc -l /etc/passwd >> wc.file

Page 2

Backquotes

Topics covered: capturing output of a command in a variable
Utilities covered: date
The backquote character looks like the single quote or apostrophe, but slants the other way
It is used to capture the output of a UNIX utility
A command in backquotes is executed and then replaced by the output of the command
Execute these commands
date
save_date=`date`
echo The date is $save_date
Notice how echo prints the output of ‘date’, and gives the time when you defined the save_date variable
Store the following in a file named backquotes.sh and execute it (right click and save in a file)
#!/bin/sh
# Illustrates using backquotes
# Output of ‘date’ stored in a variable
Today=”`date`”
echo Today is $Today
Execute the script with
sh backquotes.sh
The example above shows you how you can write commands into a file and execute the file with a Bourne shell
Backquotes are very useful, but be aware that they slow down a script if you use them hundreds of times
You can save the output of any command with backquotes, but be aware that the results will be reformated into one line. Try this:
LS=`ls -l`
echo $LS

Page 3

Pipes

Topics covered: using UNIX pipes
Utilities covered: sort, cat, head
Pipes are used for post-processing data
One UNIX command prints results to the standard output (usually the screen), and another command reads that data and processes it
sort /etc/passwd | head -5
Notice that this pipe can be simplified
cat /etc/passwd | head -5
You could accomplish the same thing more efficiently with either of the two commands:
head -5 /etc/passwd
head -5 < /etc/passwd For example, this command displays all the files in the current directory sorted by file size ls -al | sort -n -r +4 The command ls -al writes the file size in the fifth column, which is why we skip the first four columns using +4. The options -n and -r request a numeric sort (which is different than the normal alphabetic sort) in reverse order Page 4 awk Topics covered: processing columnar data Utilities covered: awk The awk utility is used for processing columns of data A simple example shows how to extract column 5 (the file size) from the output of ls -l ls -l | awk '{print $5}' Cut and paste this line into a Bourne shell and you should see a column of file sizes, one per file in your current directory. A more complicated example shows how to sum the file sizes and print the result at the end of the awk run ls -al | awk '{sum = sum + $5} END {print sum}' In this example you should see printed just one number, which is the sum of the file sizes in the current directory. Page 5 Section 2: Storing Frequently Used Commands in Files: Shell Scripts Shell Scripts Topics covered: storing commands in a file and executing the file Utilities covered: date, cal, last (shows who has logged in recently) Store the following in a file named simple.sh and execute it #!/bin/sh # Show some useful info at the start of the day date echo Good morning $USER cal last | head -6 Shows current date, calendar, and a six of previous logins Notice that the commands themselves are not displayed, only the results To display the commands verbatim as they run, execute with sh -v simple.sh Another way to display the commands as they run is with -x sh -x simple.sh What is the difference between -v and -x? Notice that with -v you see '$USER' but with -x you see your login name Run the command 'echo $USER' at your terminal prompt and see that the variable $USER stores your login name With -v or -x (or both) you can easily relate any error message that may appear to the command that generated it When an error occurs in a script, the script continues executing at the next command Verify this by changing 'cal' to 'caal' to cause an error, and then run the script again Run the 'caal' script with 'sh -v simple.sh' and with 'sh -x simple.sh' and verify the error message comes from cal Other standard variable names include: $HOME, $PATH, $PRINTER. Use echo to examine the values of these variables Page 6 Storing File Names in Variables Topics covered: variables store strings such as file names, more on creating and using variables Utilities covered: echo, ls, wc A variable is a name that stores a string It's often convenient to store a filename in a variable Store the following in a file named variables.sh and execute it #!/bin/sh # An example with variables filename="/etc/passwd" echo "Check the permissions on $filename" ls -l $filename echo "Find out how many accounts there are on this system" wc -l $filename Now if we change the value of $filename, the change is automatically propagated throughout the entire script Page 7 Scripting With sed Topics covered: global search and replace, input and output redirection Utilities covered: sed Here's how you can use sed to modify the contents of a variable: echo "Hello Jim" | sed -e 's/Hello/Bye/' Copy the file nlanr.txt to your home directory and notice how the word 'vBNS' appears in it several times Change 'vBNS' to 'NETWORK' with sed -e 's/vBNS/NETWORK/g' < nlanr.txt You can save the modified text in a file with output redirection sed -e 's/vBNS/NETWORK/g' < nlanr.txt > nlanr.new
Sed can be used for many complex editing tasks, we have only scratched the surface here

Page 8

Section 3: More on Using UNIX Utilities

Performing Arithmetic

Topics covered: integer arithmetic, preceding ‘*’ with backslash to avoid file name wildcard expansion
Utilities covered: expr
Arithmetic is done with expr
expr 5 + 7
expr 5 \* 7
Backslash required in front of ‘*’ since it is a filename wildcard and would be translated by the shell into a list of file names
You can save arithmetic result in a variable
Store the following in a file named arith.sh and execute it
#!/bin/sh
# Perform some arithmetic
x=24
y=4
Result=`expr $x \* $y`
echo “$x times $y is $Result”

Page 9

Translating Characters

Topics covered: converting one character to another, translating and saving string stored in a variable
Utilities covered: tr
Copy the file sdsc.txt to your home directory
The utility tr translates characters
tr ‘a’ ‘Z’ < sdsc.txt This example shows how to translate the contents of a variable and display the result on the screen with tr Store the following in a file named tr1.sh and execute it #!/bin/sh # Translate the contents of a variable Cat_name="Piewacket" echo $Cat_name | tr 'a' 'i' This example shows how to change the contents of a variable Store the following in a file named tr2.sh and execute it #!/bin/sh # Illustrates how to change the contents of a variable with tr Cat_name="Piewacket" echo "Cat_name is $Cat_name" Cat_name=`echo $Cat_name | tr 'a' 'i'` echo "Cat_name has changed to $Cat_name" You can also specify ranges of characters. This example converts upper case to lower case tr 'A-Z' 'a-z' < file Now you can change the value of the variable and your script has access to the new value Page 10 Section 4: Performing Search and Replace in Several Files Processing Multiple Files Topics covered: executing a sequence of commands on each of several files with for loops Utilities covered: no new utilities Store the following in a file named loop1.sh and execute it #!/bin/sh # Execute ls and wc on each of several files # File names listed explicitly for filename in simple.sh variables.sh loop1.sh do echo "Variable filename is set to $filename..." ls -l $filename wc -l $filename done This executes the three commands echo, ls and wc for each of the three file names You should see three lines of output for each file name filename is a variable, set by "for" statement and referenced as $filename Now we know how to execute a series of commands on each of several files Page 11 Using File Name Wildcards in For Loops Topics covered: looping over files specified with wildcards Utilities covered: no new utilities Store the following in a file named loop2.sh and execute it #!/bin/sh # Execute ls and wc on each of several files # File names listed using file name wildcards for filename in *.sh do echo "Variable filename is set to $filename..." ls -l $filename wc -l $filename done You should see three lines of output for each file name ending in '.sh' The file name wildcard pattern *.sh gets replaced by the list of filenames that exist in the current directory For another example with filename wildcards try this command echo *.sh Page 12 Search and Replace in Multiple Files Topics covered: combining for loops with utilities for global search and replace in several files Utilities covered: mv Sed performs global search and replace on a single file sed -e 's/application/APPLICATION/g' sdsc.txt > sdsc.txt.new
The original file sdsc.txt is unchanged
How can we arrange to have the original file over-written by the new version?
Store the following in a file named s-and-r.sh and execute it
#!/bin/sh
# Perform a global search and replace on each of several files
# File names listed explicitly
for text_file in sdsc.txt nlanr.txt
do
echo “Editing file $text_file”
sed -e ‘s/application/APPLICATION/g’ $text_file > temp
mv -f temp $text_file
done
First, sed saves new version in file ‘temp’
Then, use mv to overwrite original file with new version

Page 13

Section 5: Using Command-line Arguments for Flexibility

What’s Lacking in the Scripts Above?

Topics covered: looping over files specified with wildcards
Utilities covered: no new utilities
File names are hard-coded inside the script
What if you want to run the script but with different file names?
To execute for loops on different files, the user has to know how to edit the script
Not simple enough for general use by the masses
Wouldn’t it be useful if we could easily specify different file names for each execution of a script?

Page 14

What are Command-line Arguments?

Topics covered: specifying command-line arguments
Utilities covered: no new utilities
Command-line arguments follow the name of a command
ls -l .cshrc /etc
The command above has three command-line arguments
-l (an option that requests long directory listing)
.cshrc (a file name)
/etc (a directory name)
An example with file name wildcards:
wc *.sh
How many command-line arguments were given to wc? It depends on how many files in the current directory match the pattern *.sh
Use ‘echo *.sh’ to see them
Most UNIX commands take command-line arguments. Your scripts may also have arguments

Page 15

Accessing Command-line Arguments

Topics covered: accessing command-line arguments
Utilities covered: no new utilities
Store the following in a file named args1.sh
#!/bin/sh
# Illustrates using command-line arguments
# Execute with
# sh args1.sh On the Waterfront
echo “First command-line argument is: $1”
echo “Third argument is: $3”
echo “Number of arguments is: $#”
echo “The entire list of arguments is: $*”
Execute the script with
sh args1.sh -x On the Waterfront
Words after the script name are command-line arguments
Arguments are usually options like -l or file names

Page 16

Looping Over the Command-line Arguments

Topics covered: using command-line arguments in a for loop
Utilities covered: no new utilities
Store the following in a file named args2.sh and execute it
#!/bin/sh
# Loop over the command-line arguments
# Execute with
# sh args2.sh simple.sh variables.sh
for filename in “$@”
do
echo “Examining file $filename”
wc -l $filename
done
This script runs properly with any number of arguments, including zero
The shorter form of the for statement shown below does exactly the same thing
for filename
do

Don’t use
for filename in $*
Fails if any arguments include spaces
Also, don’t forget the double quotes around $@

Page 17

If Blocks

Topics covered: testing conditions, executing commands conditionally
Utilities covered: test (used by if to evaluate conditions)
This will be covered on the whiteboard
See Chapter 8 of the book

Page 18

The read Command

Topics covered: reading a line from the standard input
Utilities covered: no new utilities
stdin is the keyboard unless input redirection used
Read one line from stdin, store line in a variable
read variable_name
Ask the user if he wants to exit the script
Store the following in a file named read.sh and execute it
#!/bin/sh
# Shows how to read a line from stdin
echo “Would you like to exit this script now?”
read answer
if [ “$answer” = y ]
then
echo “Exiting…”
exit 0
fi

Page 19

Command Exit Status

Topics covered: checking whether a command succeeds or not
Utilities covered: no new utilities
Every command in UNIX should return an exit status
Status is in range 0-255
Only 0 means success
Other statuses indicate various types of failures
Status does not print on screen, but is available thru variable $?
Example shows how to examine exit status of a command
Store the following in a file named exit-status.sh and execute it
#!/bin/sh
# Experiment with command exit status
echo “The next command should fail and return a status greater than zero”
ls /nosuchdirectory
echo “Status is $? from command: ls /nosuchdirectory”
echo “The next command should succeed and return a status equal to zero”
ls /tmp
echo “Status is $? from command: ls /tmp”
Example shows if block using exit status to force exit on failure
Store the following in a file named exit-status-test.sh and execute it
#!/bin/sh
# Use an if block to determine if a command succeeded
echo “This mkdir command fails unless you are root:”
mkdir /no_way
if [ “$?” -ne 0 ]
then
# Complain and quit
echo “Could not create directory /no_way…quitting”
exit 1 # Set script’s exit status to 1
fi
echo “Created directory /no_way”
Exit status is $status in C shell

Page 20

Regular Expressions

Topics covered: search patterns for editors, grep, sed
Utilities covered: no new utilities
Zero or more characters: .*
grep ‘provided.*access’ sdsc.txt
sed -e ‘s/provided.*access/provided access/’ sdsc.txt
Search for text at beginning of line
grep ‘^the’ sdsc.txt
Search for text at the end of line
grep ‘of$’ sdsc.txt
Asterisk means zero or more the the preceeding character
a* zero or more a’s
aa* one or more a’s
aaa* two or more a’s
Delete all spaces at the ends of lines
sed -e ‘s/ *$//’ sdsc.txt > sdsc.txt.new
Turn each line into a shell comment
sed -e ‘s/^/# /’ sdsc.txt

Page 21

Greed and Eagerness

Attributes of pattern matching
Greed: a regular expression will match the largest possible string
Execute this command and see how big a string gets replaced by an underscore
echo ‘Big robot’ | sed -e ‘s/i.*o/_/’
Eagerness: a regular expression will find the first match if several are present in the line
Execute this command and see whether ‘big’ or ‘bag’ is matched by the regular expression
echo ‘big bag’ | sed -e ‘s/b.g/___/’
Contrast with this command (notice the extra ‘g’)
echo ‘big bag’ | sed -e ‘s/b.g/___/g’
Explain what happens in the next example
echo ‘black dog’ | sed -e ‘s/a*/_/’
Hint: a* matches zero or more a’s, and there are many places where zero a’s appear
Try the example above with the extra ‘g’
echo ‘black dog’ | sed -e ‘s/a*/_/g’

Page 22

Regular Expressions Versus Wildcards

Topics covered: clarify double meaning of asterisk in patterns
Utilities covered: no new utilities
Asterisk used in regular expressions for editors, grep, sed
Different meaning in file name wildcards on command line and in find command and case statement (see below)
regexp wildcard meaning

.* * zero or more characters, any type
. ? exactly one character, any type
[aCg] [aCg] exactly one character, from list: aCg
Regexps can be anchored to beginning/ending of line with ^ and $
Wildcards automatically anchored to both extremes
Can use wildcards un-anchored with asterisks
ls *bub*

Page 23

Getting Clever With Regular Expressions

Topics covered: manipulating text matched by a pattern
Utilities covered: no new utilities
Copy the file animals.txt to your home directory
Try this sed command, which changes the first line of animals.txt
sed -e “s/big \(.*\) dog/small \1 cat/” animals.txt
Bracketing part of a pattern with \( and \) labels that part as \1
Bracketing additional parts of a pattern creates labels \2, \3, …
This sed command reverses the order of two words describing the rabbit
sed -e “s/Flopsy is a big \(.*\) \(.*\) rabbit/A big \2 \1 rabbit/” < animals.txt Page 24 The case Statement Topics covered: choosing which block of commands to execute based on value of a string Utilities covered: no new utilities The next example shows how to use a case statement to handle several contingencies The user is expected to type one of three words A different action is taken for each choice Store the following in a file named case1.sh and execute it #!/bin/sh # An example with the case statement # Reads a command from the user and processes it echo "Enter your command (who, list, or cal)" read command case "$command" in who) echo "Running who..." who ;; list) echo "Running ls..." ls ;; cal) echo "Running cal..." cal ;; *) echo "Bad command, your choices are: who, list, or cal" ;; esac exit 0 The last case above is the default, which corresponds to an unrecognized entry The next example uses the first command-line arg instead of asking the user to type a command Store the following in a file named case2.sh and execute it #!/bin/sh # An example with the case statement # Reads a command from the user and processes it # Execute with one of # sh case2.sh who # sh case2.sh ls # sh case2.sh cal echo "Took command from the argument list: '$1'" case "$1" in who) echo "Running who..." who ;; list) echo "Running ls..." ls ;; cal) echo "Running cal..." cal ;; *) echo "Bad command, your choices are: who, list, or cal" ;; esac The patterns in the case statement may use file name wildcards Page 25 The while Statement Topics covered: executing a series of commands as long as some condition is true Utilities covered: no new utilities The example below loops over two statements as long as the variable i is less than or equal to ten Store the following in a file named while1.sh and execute it #!/bin/sh # Illustrates implementing a counter with a while loop # Notice how we increment the counter with expr in backquotes i="1" while [ $i -le 10 ] do echo "i is $i" i=`expr $i + 1` done Page 26 Example With a while Loop Topics covered: Using a while loop to read and process a file Utilities covered: no new utilities Copy the file while2.data to your home directory The example below uses a while loop to read an entire file The while loop exits when the read command returns false exit status (end of file) Store the following in a file named while2.sh and execute it #!/bin/sh # Illustrates use of a while loop to read a file cat while2.data | \ while read line do echo "Found line: $line" done The entire while loop reads its stdin from the pipe Each read command reads another line from the file coming from cat The entire while loop runs in a subshell because of the pipe Variable values set inside while loop not available after while loop Page 27 Interpreting Options With getopts Command Topics covered: Understand how getopts command works Utilities covered: getopts getopts is a standard UNIX utility used for our class in scripts getopts1.sh and getopts2.sh Its purpose is to help process command-line options (such as -h) inside a script It handles stacked options (such as -la) and options with arguments (such as -P used as -Pprinter-name in lpr command) This example will help you understand how getopts interprets options Store the following in a file named getopts1.sh and execute it #!/bin/sh # Execute with # # sh getopts1.sh -h -Pxerox file1 file2 # # and notice how the information on all the options is displayed # # The string 'P:h' says that the option -P is a complex option # requiring an argument, and that h is a simple option not requiring # an argument. # # Experiment with getopts command while getopts 'P:h' OPT_LETTER do echo "getopts has set variable OPT_LETTER to '$OPT_LETTER'" echo " OPTARG is '$OPTARG'" done used_up=`expr $OPTIND - 1` echo "Shifting away the first \$OPTIND-1 = $used_up command-line arguments" shift $used_up echo "Remaining command-line arguments are '$*'" Look over the script getopts looks for command-line options For each option found, it sets three variables: OPT_LETTER, OPTARG, OPTIND OPT_LETTER is the letter, such as 'h' for option -h OPTARG is the argument to the option, such as -Pjunky has argument 'junky' OPTIND is a counter that determines how many of the command-line arguments were used up by getopts (see the shift command in the script) Execute it several times with sh getopts1.sh -h -Pjunky sh getopts1.sh -hPjunky sh getopts1.sh -h -Pjunky /etc /tmp Notice how it interprets -h and gives you 'h' in variable OPT_LETTER Now you can easily implement some operation when -h is used Notice how the second execution uses stacked options Notice how the third execution examines the rest of the command-line after the options (these are usually file or directory names) Page 28 Example With getopts Topics covered: interpreting options in a script Utilities covered: getopts The second example shows how to use if blocks to take action for each option Store the following in a file named getopts2.sh and execute it #!/bin/sh # # Usage: # # getopts2.sh [-P string] [-h] [file1 file2 ...] # # Example runs: # # getopts2.sh -h -Pxerox file1 file2 # getopts2.sh -hPxerox file1 file2 # # Will print out the options and file names given # # Initialize our variables so we don't inherit values # from the environment opt_P='' opt_h='' # Parse the command-line options while getopts 'P:h' option do case "$option" in "P") opt_P="$OPTARG" ;; "h") opt_h="1" ;; ?) echo "getopts2.sh: Bad option specified...quitting" exit 1 ;; esac done shift `expr $OPTIND - 1` if [ "$opt_P" != "" ] then echo "Option P used with argument '$opt_P'" fi if [ "$opt_h" != "" ] then echo "Option h used" fi if [ "$*" != "" ] then echo "Remaining command-line:" for arg in "$@" do echo " $arg" done fi Execute it several times with sh getopts2.sh -h -Pjunky sh getopts2.sh -hPjunky sh getopts2.sh -h -Pjunky /etc /tmp Can also implement actions inside case statement if desired Page 29 Section 6: Using Functions Functions Sequence of statements that can be called anywhere in script Used for Good organization Create re-usable sequences of commands Page 30 Define a Function Define a function echo_it () { echo "In function echo_it" } Use it like any other command echo_it Put these four lines in a script and execute it Page 31 Function Arguments Functions can have command-line arguments echo_it () { echo "Argument 1 is $1" echo "Argument 2 is $2" } echo_it arg1 arg2 When you execute the script above, you should see Argument 1 is arg1 Argument 2 is arg2 Create a script 'difference.sh' with the following lines: #!/bin/sh echo_it () { echo Function argument 1 is $1 } echo Script argument 1 is $1 echo_it Barney Execute this script using sh difference.sh Fred Notice that '$1' is echoed twice with different values The function has separate command-line arguments from the script's Page 32 Example With Functions Use functions to organize script read_inputs () { ... } compute_results () { ... } print_results () { ... } Main program very readable read_inputs compute_results print_results Page 33 Functions in Pipes Can use a function in a pipe ls_sorter () { sort -n +4 } ls -al | ls_sorter Function in pipe executed in new shell New variables forgotten when function exits Page 34 Inherited Variables Variables defined before calling script available to script func_y () { echo "A is $A" return 7 } A='bub' func_y if [ $? -eq 7 ] ; then ... Try it: is a variable defined inside a function available to the main program? Page 35 Functions -vs- Scripts Functions are like separate scripts Both functions and scripts can: Use command-line arguments echo First arg is $1 Operate in pipes echo "test string" | ls_sorter Return exit status func_y arg1 arg2 if [ $? -ne 0 ] ... Page 36 Libraries of Functions Common to store definitions of favorite functions in a file Then execute file with . file Period command executes file in current shell Compare to C shell's source command Page 37 Section 7: Miscellaneous Here Files Data contained within script cat << END This script backs up the directory named as the first command-line argument, which in your case in $1. END Terminator string must begin in column one Variables and backquotes translated in data Turn off translation with \END Page 38 Example With Here File Send e-mail to each of several users for name in login1 login2 login3 do mailx -s 'hi there' $name << EOF Hi $name, meet me at the water fountain EOF done Use <<- to remove initial tabs automatically Page 39 Set: Shell Options Can change Bourne shell's options at runtime Use set command inside script set -v set +v set -xv Toggle verbose mode on and off to reduce amount of debugging output Page 40 Set: Split a Line Can change Bourne shell's options set -- word1 word2 echo $1, $2 word1, word2 Double dash important! Word1 may begin with a dash, what if word1 is '-x'? Double dash says "even if first word begins with '-', do not treat it as an option to the shell Page 41 Example With Set Read a line from keyboard Echo words 3 and 5 read var set -- $var echo $3 $5 Best way to split a line into words Page 42 Section 8: Trapping Signals What are Signals? Signals are small messages sent to a process Process interrupted to handle signal Possibilities for managing signal: Terminate Ignore Perform a programmer-defined action Page 43 Common Signals Common signals are SIGINTR sent to foreground process by ^C SIGHUP sent when modem line gets hung up SIGTERM sent by kill -9 Signals have numeric equivalents 2 SIGINTR 9 SIGTERM Page 44 Send a Signal Send a signal to a process kill -2 PID kill -INTR PID Page 45 Trap Signals Handling Signals trap "echo Interrupted; exit 2" 2 Ignoring Signals trap "" 2 3 Restoring Default Handler trap 2 Page 46 Where to Find List of Signals See file /usr/include/sys/signal.h Page 47 User Signals SIGUSR1, SIGUSR2 are for your use Send to a process with kill -USR1 PID Default action is to terminate process Page 48 Experiment With Signals Script that catches USR1 Echo message upon each signal trap 'echo USR1' 16 while : ; do date sleep 3 done Try it: does signal interrupt sleep? Page 49 Section 9: Understanding Command Translation Command Translation Common translations include Splitting at spaces, obey quotes $HOME -> /users/us/freddy
`command` -> output of command
I/O redirection
File name wildcard expansion
Combinations of quotes and metacharacters confusing
Resolve problems by understanding order of translations

Page 50

Experiment With Translation

Try wildcards in echo command
echo b*
b budget bzzzzz
b* translated by sh before echo runs
When echo runs it sees
echo b budget bzzzzz
Echo command need not understand wildcards!

Page 51

Order of Translations

Splits into words at spaces and tabs
Divides commands at
; & | && || (…) {…}
Echos command if -v
Interprets quotes
Performs variable substitution

Page 52

Order of Translations (continued)

Performs command substitution
Implements I/O redirection and removes redirection characters
Divides command again according to IFS
Expands file name wildcards
Echos translated command if -x
Executes command

Page 53

Exceptional Case

Delayed expansion for variable assignments
VAR=b*
echo $VAR
b b_file
Wildcard re-expanded for each echo

Page 54

Examples With Translation

Variables translated before execution
Can store command name in variable
command=”ls”
$command
file1 file2 dir1 dir2…
Variables translated before I/O redirection
tempfile=”/tmp/scriptname_$$”
ls -al > $tempfile

Page 55

Examples (continued)

Delayed expansion of wildcards in variable assignment
Output of this echo command changes when directory contents change (* is re-evaluated each time the command is run)
x=*
echo $x
Can view values stored in variables with
set
Try it: verify that the wildcard is stored in x without expansion

Page 56

Examples (continued)

Wildcards expanded after redirection (assuming file* matches exactly one file):
cat < file* file*: No such file or directory Command in backquotes expanded fully (and before I/O redirection) cat < `echo file*` (contents of file sent to screen) Page 57 Eval Command Forces an extra evaluation of command eval cat \< file* (contents of matching file) Backslash delays translation of < until second translation Page 58 Section 10: Writing Advanced Loops While loops Execute statements while a condition is true i=0 while [ $i -lt 10 ] do echo I is $i i=`expr $i + 1` done Page 59 Until loops Execute statements as long as a condition is false until grep "sort" dbase_log > /dev/null
do
sleep 10
done
echo “Database has been sorted”
Example executes until grep is unsuccessful

Page 60

Redirection of Loops

Can redirect output of a loop
for f in *.c
do
wc -l $f
done > loop.out
Loop runs in separate shell
New variables forgotten after loop
Backgrounding OK, too

Page 61

Continue Command

Used in for, while, and until loops
Skip remaining statements
Return to top of loop
for name in *
do
if [ ! -f $name ] ; then
continue
fi
echo “Found file $name”
done
Example loops over files, skips directories

Page 62

Break Command

Used in for, while, and until loops
Skip remaining statements
Exit loop
for name in *
do
if [ ! -r $name ] ; then
echo “Cannot read $name, quitting loop”
break
fi
echo “Found file or directory $name”
done
Example loops over files and directories, quits if one is not readable

Page 63

Case Command

Execute one of several blocks of commands
case “string” in
pattern1)
commands ;;
pattern2)
commands ;;
*) # Default case
commands ;;
esac
Patterns specified with file name wildcards
quit) …
qu*) …

Page 64

Example With Case

Read commands from keyboard and interpret
Enter this script ‘case.sh’
echo Enter a command
while read cmd
do
case “$cmd” in
list) ls -al ;;
freespace) df . ;;
quit|Quit) break ;;
*) echo “$cmd: No such command” ;;
esac
done
echo “All done”
When you run it, the script waits for you to type one of:
list
freespace
quit
Quit
Try it: modify the example so any command beginning with characters “free” runs df

Page 65

Infinite Loops

Infinite loop with while
while :
do

done
: is no-op, always returns success status
Must use break or exit inside loop for it to terminate

Page 66

Section 11: Forking Remote Shells

Remote Shells

Rsh command
rsh hostname “commands”
Runs commands on remote system
Must have .rhosts set up
Can specify different login name
rsh -l name hostname “commands”

Page 67

Examples With rsh

Check who’s logged on
rsh spooky “finger”
Run several remote commands
rsh spooky “uname -a; time”
Executes .cshrc on remote system
Be sure to set path in .cshrc instead of .login

Page 68

Access Control with .Rhosts

May get “permission denied” error from rsh
Fix this with ~/.rhosts on remote system
Example: provide for remote shell from spunky to spooky
spunky % rlogin spooky
spooky % vi ~/.rhosts
(insert “spunky login-name”)
spooky % chmod 600 ~/.rhosts
spooky % logout
spunky % rsh spooky uname -a
spooky 5.5 sparc SUNW,Ultra-1
May also rlogin without password: security problem!

Page 69

Remote Shell I/O

Standard output sent to local host
rsh spooky finger > finger.spooky
Standard input sent to remote host
cat local-file | rsh spooky lpr –

Page 70

Return Status

Get return status of rsh
rsh mayer “uname -a”
echo $?
Returns 0 if rsh managed to connect to remote host
Returns 1 otherwise
Invalid hostname
Permission denied

Page 71

Remote Return Status

What about exit status of remote command?
Have to determine success or failure from stdout or stderr

Page 72

Section 12: More Miscellaneous

Temporary Files

Use unique names to avoid clashes
tempfile=$HOME/Weq_$$
command > $tempfile
$$ is PID of current shell
Avoids conflict with concurrent executions of script
Do not use /tmp!

Page 73

Wait Command

Wait for termination of background job
command &
pid=$!
(other processing)
wait $pid
Allows overlap of two or more operations

Page 74

Section 13: Using Quotes

Quotes

Provide control of collapsing of spaces and translation of variables
Try it: run three examples
No quotes (variables translated, spaces collapsed)
echo Home: $HOME
Home: /users/us/freddy
Double quotes (no collapsing)
echo “Home: $HOME”
Home: /users/us/freddy
Single quotes (no translation or collapsing)
echo ‘Home: $HOME’
Home: $HOME
Try it: single quotes within double quotes
echo “Home directory ‘$HOME’ is full…”

Page 75

Metacharacters

Characters with special meaning to shell
” ‘ ` $ * [ ] ?
; > < & ( ) \ Avoid special meaning with quoting echo 'You have $20' Backslash like single quotes Applies only to next character echo You have \$20 Page 76 Examples With Quotes Bad command line: grep dog.*cat file Shell tries to expand dot.*cat as file name wildcard Use quotes to avoid translation grep 'dog.*cat' file Single quotes OK in this case because we don't need variable translation Page 77 More Examples With Quotes Read name and search file for name read name grep "$name" dbase Single quotes not OK because we need variable translation Page 78 Searching for Metacharacters Bad command line: search for dollar sign grep "Gimme.*$20" file Problem: shell translates variable $20 Solution: use single quotes grep 'Gimme.*$20' file

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