1. CSC 352– Unix Programming, Fall 2012
October 2012
Shell Programming (Highlights only), Chapter 13

2. Highlights of Shell Programming
Chapter 13 has a lot of information.
Perhaps a little too much for the first reading.
We will concentrate on the main programming constructs of immediate use.
These ideas carry over from programming in a language like C++.
The shell makes them handier because it is an interpreter. You can interact with the shell.
We will use bash.

3. The hash-bang line # starts a comment in the shell.
#! on an executable text file says that this is an interpreter’s source file, with the path of the interpreter to follow.
#!/bin/bash on the first line of a Bash script.
Hash-bang works for any interpreted language with # as the comment delimiter – shells, Perl, Python, etc.
Use “chmod +x FILENAME” to add execute permission.

4. Variables in bash (review)
VARIABLE=STRING, with no spaces around the =, assigns the STRING into VARIABLE.
In order to make it accessible to children processes, you must export the variable from the shell process that defined it. Use unset to remove a variable binding.
C library functions getenv & putenv manipulate these.
-bash-3.00$ v=1
-bash-3.00$ echo $v 1
-bash-3.00$ bash -c 'echo $v'
-bash-3.00$ export v

5. Shell read and built-in variables.
Use read VARIABLENAME to read the terminal into VARIABLENAME.
Use echo STRING or echo –e STRING to echo to stdout; -e interprets escapes such as \n \t.
Special variables:
$? is exitstatus of exit exitstatus; $0 is argv[0] (program name), $1 is argv[1] (first argument), etc.; shift shifts $2 down to $1, etc.;
$# is number of positional arguments; $* or give all command line arguments; set - - STRINGS sets command line arguments to whatever is in STRINGS.

6. Boolean expressions
Any successful (0) exit status is true, non-0 is false.
Shell’s test primitive tests Shell variables and files. Exit status is 0 for success. [ expression ] is shorthand for test expression.
-bash-3.00$ ls *.out
nohup.out
-bash-3.00$ test -f nohup.out ; echo $?
-bash-3.00$ test -f junk.out ; echo $? 1

7. Shell test or [ ] expressions
Numerical comparisons:
-eq, -ne, -gt, -ge, -lt, -le
String comparisons:
s1 = s2, s1 != s2, -n stg (not null), -z stg(is null), stg (assigned and non-empty), s1 == s2 (ksh and bash only).
Use “$v” or “${v}” if v may be empty; otherwise, a binary comparison operator won’t see empty string.
File tests:
-f, -r, -w, -x, -d, -s (size > 0), -e (exists), -L (symbolic link), f1 –nt f2 (newer than), f1 –ot f2 (older), f1 -ef f2 (linked to) for ksh and bash.

8. Boolean connectives && is logical AND, || is logical OR
Both are short-circuited.
&& quits on the first failure, || on the first success.
-bash-3.00$ ls *.out
nohup.out
-bash-3.00$ [ -f nohup.out ] && echo hello
hello
-bash-3.00$ [ -f junk.out ] && echo hello
-bash-3.00$ [ -f nohup.out ] || echo hello
-bash-3.00$ [ -f junk.out ] || echo hello

9. if control construct selects an execution path
Just like you are used to in C++, but
Use elif instead of else if, use fi to end an if construct.
-bash-3.00$ count=`ls *out |wc -l`
-bash-3.00$ echo $count 1
-bash-3.00$ if [ $count -eq 0 ]
> then
> echo none
> elif [ $count -eq 1 ]
> echo one
> else
> echo other
> fi
one

10. while iterates while a text is true
while uses do and done keywords, also break and continue
-bash-3.00$ set -- `date` # set command line arguments
-bash-3.00$ echo "COUNT $# DATA:
COUNT 6 DATA: Sun Mar 27 19:45:36 EDT 2011
-bash-3.00$ i=1
-bash-3.00$ while [ $# -gt 0 ]
> do
> echo "var $i: $1"
> i=`expr $i + 1`
> shift # shift position parameters to the left by one position, discard former $1, decrement $#
> done
var 1: Sun
var 2: Mar
var 3: 27
var 4: 19:49:10
var 5: EDT
var 6: 2011

11. for loops over a list of values
-bash-3.00$ ls -d s* splitstudents startup -bash-3.00$ for f in `ls -d s*` > do > echo "file: $f" > done file: splitstudents file: startup -bash-3.00$ find ~/unix -name "s*" -print /export/home/faculty/parson/unix/seeargs /export/home/faculty/parson/unix/seeargs/seeargs.c /export/home/faculty/parson/unix/seeargs.zip -bash-3.00$ for path in `find ~/unix -name "s*" -print` > echo "file `basename $path` in directory `dirname $path`" file seeargs in directory /export/home/faculty/parson/unix file seeargs.c in directory /export/home/faculty/parson/unix/seeargs file seeargs.zip in directory /export/home/faculty/parson/unix

12. case similar to C++ switch-case construct
case expression in
pattern1) commands1 ;;
pattern2) commands2 ;;
. . .
*) defaultcommands ;;
esac
Patterns can be numeric values, regular expressions.

13. Here document (<<)
Use << to redirect input from the script itself.
Make up a delimiter that is not in the data.
-bash-3.00$ wc -l << DELIM
> this is a
> three-line
> chuck of text
> DELIM 3

14. Shell functions
They take no parameters, work on variables in the shell process. Useful for repeated code.
-bash-3.00$ fcounter() {
> fcnt=`find . -type f -print |wc -l`
> echo "$fcnt regular files in and below `pwd`"
> }
-bash-3.00$ fcounter
1537 regular files in and below /export/home/faculty/parson/process_incoming

15. aliases
Alias is useful for changing complex command strings into simple command aliases.
It is also good for pointing to a specific path.
-bash-3.00$ alias
alias vi='vim'

16. eval and exec
eval takes a string and evaluates it as a program fragment.
-bash-3.00$ echo -e "Enter a command that I will read as data: \c" ; read cmd
Enter a command that I will read as data: date
-bash-3.00$ eval $cmd
Sun Mar 27 20:25:09 EDT 2011
exec replaces the current shell with the exec’d program.