Unix Shell Programming

Command Separation & Grouping

Characters that separate commands

• '\n' (newline) initiates execution of the command preceding it.
  Note: if you type '\' before pressing the <ENTER> key, the newline is quoted, and treated like a space
• ';' separates commands on a single line
• '|' combines separate commands into a single command called a pipeline
• '&' runs the preceding command in the background

Operator precedence

• '|' has highest precedence
• '&' is next
• ';' has lowest precedence
• groupings made explicitly with parentheses

Command Separation & Grouping

Examples

• date ; who
• date ; who | wc
  (date ; who) | wc
  (date ; who) | tee save | wc
• (sleep 5 ; date) & date
• echo At the tone the time will be 'date'

Variables

Variables contain values, set explicitly or by the system

• varname=value sets the variable value
• $varname or ${varname} represents the value of the variable
• you can view a variable's value with echo
• variables may be exported to all subshells with export

Examples

• echo $SHELL
• TERM=vt100 ; export TERM
• PATH = "$HOME/mybin:$PATH"
• VISUAL=$(whence emacs)
• PS1='! $PWD> '

Read-Only Shell Variables

Some variables may only be read by shell scripts

• Command line arguments
• $1, ..., $9
• $* and $@
• $#
• PID numbers
• $$
• $!
• Exit status
• $?

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Control-Flow Commands

Conditionals

Conditionals test the value of a condition, i.e. the exit status of a command

• if command executed successfully, exit status = 0 (TRUE)
• boolean expressions in [[ ]] construct may also be evaluated
• if several commands follow "if", final exit status used
• boolean operators may combine results
• &&
• ||

String comparisions

• str = pat # two strings match
• str != pat # two strings do not match
• str < pat # str precedes pat in alphabetical ordering
• str > pat # str follows pat in alphabetical ordering
• -n str # str is not null (length > 0)
• -z str # str is null (length = 0)

File checking

• -a file # file exists
• -d file # file is a directory
• -f file # file is an ordinary file
• -r file # user has read permissions on file
• -s file # file is not empty
• -w file # user has write permissions on file
• -x file # user has execute/search permissions on file
• -O file # user owns the file
• -G file # user has same group ID as file
• file1-nt file2 # file1 is newer than file2
• file1 -ot file2 # file1 is older than file2

Integer comparisions

• -lt # less than
• -le # less than or equal to
• -eq # equal to
• -ge # greater than or equal to
• -gt # greater than
• -ne # not equal to

if then

Syntax:

if test-command
    then
        commands
fi

Example:

if [[ $# = 0 ]]
    then
        echo "You need to indicate an input file"
        exit 1
fi

if then else

Syntax:

if test-command
    then
        commands
    else
        commands
fi

Example:

if [[ $# = 0 ]]
    then
        print "Enter a file name: \c"
        read fname
    else
        fname=$1
fi

if then elif

Syntax:

if test-command
    then
        commands
    elif test-command
        then
            commands
    else
        commands
fi

Example:

if [[ $1 < $2 ]] ; then
    print "$1 < $2"
elif [[ $1 > $2 ]] ; then
    print "$1 > $2"
else
    print "$1 = $2"
fi

for in

Syntax:

for loop-index in argument-list
do
   commands
done

Example:

for filename in *.c
do
    cp $filename ${filename}.$1
done

for

Syntax:

for loop-index
do
   commands
done

Example:

for filename
do
    print "********** $filename **********"
    cat $filename
done