Command Languages

Introduction

The most important utility is the command processor, which usually is an interpreter for a command language of some kind (procedural, graphical, etc). Command languages are distinguished by the level of object they operate on: files and processes. They also have the typical constructs of programming languages: variables, conditionals and loops.

These are

• bash - a Unix shell
• command.com - the MSDOS shell
• ls, rm - Unix utilities working on files
• awk, perl - string processing languages
• sed - a string processing utility
• C, Ada - languages that work on machine-level entities such as bytes, characters, integers, floats.
• tr - a Unix utility that translates characters
• bc - a desk calculator working on numbers

Generic Command Language Operations

Contents

• Manage processes
• Redirect file I/O
• Execute utilities and builtin commands
• Have variables
• Have conditionals and loops

Manage processes

Redirect I/O

Execute commands

Variables

Loops and conditionals

Unix Command Language - bash

Contents

Process management

ls
rm tmp*

bash test_script

chmod a+x test_script

I/O redirection

Shell variables

From now on we only discuss the Bourne shell, Korn shell, zsh and bash. The csh and tcsh have different syntax.

Assignment to a shell variable is by

variable=value

To use the value of a variable, prefix it with a ``$''

Example

x1=fred
x2="a string"
x3=4
x4="$x1 $x2"
echo $x1 $x2 $x3 $x4

x=fred
echo ${x}dy

Command substitution (grave command)

file_list=`ls`
echo "The files are $file_list"
echo "The last file is"
last_file=`echo $file_list | 
  sed 's/.* //'`
echo $last_file

Arithmetic

x=2
y=`expr $x + 2`

x=3
let y=$x+4

Sequencing

ls; echo "files listed"

For loop

for vbl [in values]
do
        commands...
done

Example

for i in a b c
do
  echo $i
done

Example

cd ~/..
for i in os*
do
        echo "A student is $i"
done

for i in $*
do
    echo "an arg was $i"
done

for i
do
    echo "an arg was $i"
done

Exit codes

Every command succeeds or fails. eg. rm. may succeed at removing a file, or may fail to remove another file because of permission problems. An exit code holds this value.

The exit code is not visible. The exit code of the last command is stored in the shell variable ``?''. A value of zero stands for success, anything else for failure.

cd /tmp
echo > tmp$$
rm tmp$$
echo "Exit code of ok rm is $?"
rm tmp$$
echo "Exit code of failed rm is $?"

While command

while commands
do
        commands
done

Example

i=1
while test $i -le 20
do
        echo $i
        let i=$i+1
done

ls |
while read file
do
  echo "A file is $file"
done

Conditional statement

if commands
then
        commands
fi

if commands
then
        commands
else
        commands
fi

Example

if [ $# -lt 2 ]
then
        echo "Usage: $0 files..."
        exit 1
fi

Case statement

case value in
  pattern) commands;;
  ...
  pattern) commands;;
esac

Example

Quoting

sed s/.*//

Interpretation of a single character is turned off by prefixing it with a backslash `\' as in

echo the amount is \$20

Enclosing something in single quotes '...' turns off all interpretation, including interpretation of $, * and \.

Enclosing something in double quotes "..." allows $variable substitution but no other.

echo The cost is $20
echo The cost is \$20
echo "The cost is $20"
echo 'The cost is $20'

Single quotes in this sed command will turn off variable substitution. No quotes cause an error.

x="two words"
echo "two words are too many" |
  sed "s/$x//"

Miscellaneous examples

Example

sum=0
while read x
do
    let sum=$sum+$x
done
echo "The sum was $sum"

Example

script=$0
command=$1
for f in *
do
    if [ -d $f ]
    then
        cd $f
        $script $command
        cd ..
    else
        $command $f
    fi
done

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