Mastering Shell Scripting: Loops, Functions, and Debugging

Shell Programming – Part 2

Looping with a List: for Statement • for lets a set of instructions be repeated on a set of items in a list. • The general form is: Example: • for variablein listfor file in file1 file2 file3 • do do • commands echo $file has `wc –l` lines • done echo $file has `wc –c` chars • done • Whitespace separates items in the list. Use quotes if items have whitespace.

A for List’s Items May Come from Anywhere • for list items may come from variables or wildcards as in this example. • # This shell program renames files with • # a txt extension to files with a doc extension • # e.g., rick.txt  rick.doc • for file in *.txt • do • leftname=`basename $file .txt` • mv $file ${leftname}.doc • done • Wildcards are interpreted by the shell as filenames.

Looping While a Condition Is True: while Statement • while lets a set of instructions be repeated as long as a condition is true. • The general form is: Example: • while condition is truex=5 • do while [ $x –gt 0 ] ; do • commands ps –e; sleep 3 • done x=`expr $x – 1` • done # repeats 5 times • The continue statement lets you end one iteration of the loop and start • the next iteration immediately. • The break statement lets you completely exit the loop immediately.

Try These whileExamples • Problems 1 and 2 of the In-Class Lab: • Modify program so that it continuously runs until the user • selects the exit option. • Create a shell program so that a “database” file of users • and their telephone numbers are created.

Using setto Manipulate Positional Parameters • set -- command assigns each of the items in command • a positional parameter. • Example: set -- `date` implies $1 is Mon,$2 is Nov, $3 is 10, $4 • is09:40:35 , $5 is EST, and $6 is 2010. • Let’s write a shell script, mydate.sh, that takes outputs the date in • the format: • Today is Mon, the 28th day of the month of Nov in 2011.

Try this example: mytime.sh • Write a shell program, mytime.sh, that displays the time in the format: • The time is 15 minutes past the hour of 10. • Hints: • 1. Grab the time from the date command positional parameter. • 2. Save the time in a file. • 3. Use the cut command to get the hours and minutes of the time. • 4. Format the time as above using the hours and minutes.

Using shiftto Manipulate Positional Parameters • shift n transfers the contents of each positional parameter to • its immediate lower numbered one. If a number n is • present, this is done n times. • Example: set -- `date` implies $1 is Mon,$2 is Nov, $3 is 10, $4 • is09:40:35 , $5 is EST, and $6 is 2010. • shift • echo $1 $2 $3 displays$1asNov,$2as10 and $3 as • 09:40:34

myln.sh • #!/bin/sh • # This program creates multiple symbolic links to a • # single target. Usage: myln.sh target list of files • # • original=$1 # target file • [ ! –f $original] && { echo “$original not found” ; • exit 1 ; } • shift # left-most argument (target) is lost • for file in $* ; do • ln –sf $original $file • done

An Alternative to alias: Shell Functions • Shell functions execute a group of statements enclosed within {} • General form: • functionName() { # always a null parameter list • statements • return value# optional number, indicating • } # success or failure • Example: ls –l files | more  myls() { • > ls –l $* | more • > } • myls *.c

Debugging Shell Scripts • set can be used to produce debugging output: • set -overbose • set –o xtrace • verboseEchoes each command before running them to stderr • xtraceEchoes each command after command-line processing, after parameter and command substitution, and the other subsequent steps. • Each line it prints starts with + which is customizable through the built-in shell variable PS4. So you can set PS4 to "xtrace-> "

Some Final Tips • To use the output of a UNIX command elsewhere in script, type a $, enclose the command within parentheses (), and store the output in an environment variable, e.g., VAR1=$(command1|command2) • To use a value of an environment variable, put a $ in front of the variable name and to avoid ambiguities, enclose the variable name inside curly braces {}, e.g., ls ${VAR1} • See the text for more useful topics: trap, eval, exec, awk, sed • we didn’t cover in class.

Mastering Shell Scripting: Loops, Functions, and Debugging