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COMP2004
Programming Practice
2002 Summer School


Kevin Pulo
School of Information Technologies
University of Sydney

More Unix Tools

• Keeping track of code versions
• More Unix shell scripting
  • Including common Unix utilities

Revision Control

• Keeping track of changes to code
• Not using one leads to
  • Accidently deleting all your code
  • Breaking your working code
• Often also supports group work
  • Merging changes
  • Access control

Simple revisioning

• cp -r src src.bak1
• cp -r src src.bak2
• ...
• Extremely simple
  • Possibly adequate for small code
• Wastes space - copies all src / obj files
  • Better to just save differences
• Easy to accidentally delete a backup
• Doesn't support groupwork

PRCS

• Easy to setup and use revision control
• Using it will make coding easier
• There's information on it in the tute
• ~sholden/pub/prcs/bin/prcs
• CVS is a more complicated/powerful system

The shell

• The shell is actually a programming language
• It has if, for, while, etc statements
• It has variables and operators
• Can run other programs and manipulate their input/output
• Many common tasks often done by other Unix utilities

Simple commands I

• cd- Change directory
• cp- Copies files/dirs
• mv- Moves (renames) files/dirs
• rm- Removes (deletes) files/dirs

• Common parameters:
  • -f = force
  • -r = recursive
  • -i = interactive (ask before

overwriting)

Simple commands: echo

• echo - Prints parameters to stdout

bash$ echo "Hello World" Hello World
bash$

• Options (for bash's echo):
  • -n- No trailing newline
  • -e- Allow \n, \t, etc

Simple commands: cat

• cat - Prints files (or stdin) to stdout

bash$ cat hello.txt Hello World
bash$

Redirection

• Output:
  • echo "Hello World" > hello.txt

• Appending:
  • echo -ne "Hello\t" > hello.txt
  • echo "World" >> hello.txt

• Input:
  • cat < hello.txt

Redirection caveat

• Eg: Suppose a program called prog:
  • Reads from file.txt
  • Does some processing
  • Outputs to stdout
• prog file.txt > file.txt
• Contents of file.txt are deleted by the shell before prog is run!
• Thus prog reads an empty file
• And then outputs nothing
• So file.txt stays empty

Redirection caveat

• Solution: use a temporary file instead

prog file.txt > file.tmp
mv -f file.tmp file.txt

Exit Status

• Returned by a program when it exits
• That's why main() returns an int
• 0 is value for success (use EXIT_SUCCESS in C++)
• Everything else is failure
• cmp exits with 0 for identical files
• We can use this to help out testing

If in shell

• If tests the exit value of a command
• 0 is true, everything else is false
• Recall that
  • Our testing output is in outfile
  • Correct output is in test.out

if cmp test.out outfile; then
echo "test passed"
else
echo "test failed"
fi

elif

if cmp test.out outfile; then
echo "test passed"
elif diff -i test.out outfile; then
echo "test passed (only just)"
else
echo "test failed"
fi

• The -i option to diff ignores case
  • ie: a more forgiving comparison

Exit in shell

• Can control the exit status of a shell script with exit
• Eg:
  • exit
  • exit 0
  • exit 1
• Useful when the command in an if statement is another shell script:

if another_script; then
...
fi

Testing variables

• String-based
  • if [ "$somevar" = "hello" ]; then
  • if [ "$somevar" != "hello" ]; then
• Numeric-based
  • if [ "$num" -lt 42 ]; then
  • if [ "$num" -ge 42 ]; then
• File-based
  • if [ -r "$fname" ]; then
  • if [ "$fname" -nt "file.txt" ]; then
• Many more, for info: man test

Putting stdout into variables

• Sometimes want to put stdout from a program into a shell variable
• Use backticks:

echo "f1.h f2.h f3.h" > file.lst
filelist=`cat file.lst`
cat $filelist
cat `cat file.lst`

• bash has alternate syntax

cat $(cat file.lst)

Shell arithmetic

• Use a command called expr

bash$ expr 3 + 8 \* 2 19
bash$

• bash has alternate syntax, instead of
  • prog `expr 3 + 8 \* 2`
• can use
  • prog $((3 + 8 * 2))
• which is equivalent to
  • prog 19

while

• Standard counting loop:

count=1
while [ $count -le 10 ]; do
echo "$count"
count=$(($count + 1))
done

while

• Infinite loop:

while :; do
if [ -e "file.txt" ]; then
break
fi
sleep 1
done

Command line parameters

• The command line parameters are $1, $2, $3, ..., $9
• bash also supports ${10}, ${11}, ...
• Name of the shell script is $0
• Number of parameters is $# (no $0)
• shift - deletes $1, makes $2 be $1, $3 be $2, etc
• shift n - shifts n times

Example parameters

• The shell script sample:

#!/gnu/usr/bin/bash
while [ $# -ge 1 ]; do
echo "$1"
shift
done

bash$ sample a bc defgh a
bc
defgh

case

case "$1" in
-d*)
cmd="diff"
;;
-c*)
cmd="cmp"
;;
*)
exit 1
;;
esac
$cmd file1.txt file2.txt

case

• Using diff:
  • compare -d
  • compare -diff
  • compare -djofiwni
• Using cmp:
  • compare -c
  • compare -cmp
  • compare -compare
• Cause fail exit:
  • compare -e
  • compare 42
  • compare

Testing script

• Testing script so far:

#!/gnu/usr/bin/bash
prog < test.in > outfile
if cmp test.out outfile; then
echo "test passed"
else
echo "test failed"
fi

Pipes

• You can redirect one program's output to another's input
• This removes the need for outfile

if prog < test.in | cmp - test.out; then
echo "test passed"
else
echo "test failed"
fi

Multiple Tests

• Multiple tests are always needed
• We can automate them as well
• Put all our tests in a directory (tests)
• Have the input files named xyz.in
• Have the output files name xyz.out
• Where xyz is the test name
• We can use the shell to run them all

Running Multiple Tests

#!/gnu/usr/bin/bash
for infile in tests/*.in; do
name=`basename $infile .in`
if prog < $infile | cmp - \
tests/$name.out; then
echo "$name : passed"
else
echo "$name : failed"
fi
done

sed

• The Stream EDitor
• sed file.txt
  • Same as cat file.txt
• sed -e '10q' file.txt
  • Output first 10 lines
• sed -n -e '10,20p' file.txt
  • Output lines 10 to 20
• sed -e 's/foo/bar/g' file.txt
  • Replace all 'foo' with 'bar'
• More info: man sed

awk

• Extremely useful utility
• Yet another programming language
• Still good for simple tasks
• Excellent at extracting columns
• GNU gawk has many good extensions
• More info: man awk or man gawk
• Eg: output logins for summer students:

awk -F: '/summer semester/ {print $1}' \
/etc/passwd

Other useful utilities

• sort- sorts lines in a file
• uniq- removes duplicate lines
• head- output first n lines
• tail- output last n lines
• cut- extracts columns of characters
• join, paste - merges files by columns

• For all, get more info with
  • man command

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