Linux Shell (5)

7/31/2019 Linux Shell (5)

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The Linux ShellsBasic Commands

Operating Systems Laboratory

Amir Saman Memaripour


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The if CommandThe built-in ifcommand runs a command and checks the

results. If the command was successful, it executesanother set of commands. The only commandnormally used with the ifcommand is test. There are

also other uses of ifcommand.

The syntax for the basic ifcommand is as follows:

if test arguments ; then

statements to run

fi

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The if CommandThe keyword then is treated as a separate command,

requiring a semicolon to separate it from the ifcommand. The keyword fi appears after the finalcommand to be executed inside the if.

For example, the test -f command checks for theexistence of a file:

if test -f ./report.out ; then

printf The report file ./report.out exists!\n

fi

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File ExpressionsThe built-in test command contains a wide variety of

tests for files. It can test the type of file, theaccessibility of the file, compare the ages of files orother file attributes. The following is a complete list of

Bash file tests.

File testing is commonly used in the sanity checks at thebeginning of a script. They can be used to check that

all files are present and readable (and writable, ifnecessary). All commands must be executable.

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File ExpressionsOption Performed Action

-b file True if the file is a block device file

-c file True if the file is a character device file

-d file True if the file is a directory

-e file True if the file exists

-f file True if the file exists and is a regular file

-g file True if the file has the set-group-idpermission set

-h fileor -L file True if the file is a symbolic link

-k file True if the file has the sticky permission bit set-p file True if the file is a pipe

-r file True if the file is readable (by your script)

-s file True if the file exists and is not empty

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File ExpressionsOption Performed Action

-S file True if the file is a socket

-t fd True if the file descriptor is opened on a terminal

-u file True if the file has the set-user-idpermission set

-w file True if the file is writable (by your script)-x file True if the file is executable (by your script)

-O file True if the file is (effectively) owned by you

-G file True if the file is (effectively) owned by your group

-N file True if the file has new content (since the last time it was

read)f1 -nt f2 (newer than) True if file f1 is newer than f2

f1 -ot f2 (older than) True if file f1 is older than f2

f1 -ef f2 (equivalent file) True if file f1 is a hard link to f2

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File Expressionsif test ! -x $who ; then

printf $SCRIPT:$LINENO: the command $who is notavailable aborting >&2

exit 192

fi

This script fragment ensures that the who command isexecutable!

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Multiple TestsSingle tests can be combined together with -a (and) and -o (or)

switches.

if test -f $TMP -a ! -w $TMP ; then

printf $SCRIPT:$LINENO: the temp file $TMP exists and cannot be

overwritten aborting >&2

fi

This is not exactly the same as

if test -f $TMP && test ! -w $TMP ; then

printf $SCRIPT:$LINENO: the temp file $TMP exists and cannot beoverwritten aborting >&2

fi

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Multiple TestsOne situation that tends to confuse shell programmers is

when mixing the not operator with -a or -o switch. In mostcomputer languages, not takes precedence as a unaryoperator and executes first. In Bash, -a and -o take

precedence over the not operator, causing the followingtest to always be false:

if test ! -f $TMP -o -f $TMP ; then

Bash interprets this command as if the file neither exists norexists. This odd behavior is in accordance with the POSIXstandard. To get the expected result, you must useparentheses.

if test $ ! -f $TMP $ -o -f $TMP ; then

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Square BracketsSquare brackets are an alternative form of the test command.

Using square brackets makes your if commands easier toread.

if [ -f $TMP -a ! -w $TMP ] ; thenprintf $SCRIPT:$LINENO: the temp file $TMP exists andcannot\

be overwritten aborting >&2

exit 192

fi

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String ExpressionsThe testcommand can compare a pair of strings.

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Option Performed Action

-z s (zero length) True if the string is empty

-n s(or just s) (not null) True if the string is not empty

s1 = s2 True if string s1 is equal to s2

s1 != s2 True if string s1 is not equal to s2

s1 < s2 True if string s1 is less than s2

s1 > s2 True if string s1 is greater than s2


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String ExpressionsDAY=`date +%a`

if [ $DAY = Mon ] ; then

printf The weekend is over...get to work!\n

fi

The -z (zero length) and -n (not zero length) switches areshort forms of = and != , respectively. Notice that

there are no greater than or equal to, or less than orequal to, operators. You can simulate theseoperators by combining the two tests with the aswitch.

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Arithmetic ExpressionsThe built-in let command performs math calculations.

let expects a string containing a variable, an equalssign, and an expression to calculate. The result isassigned to the variable.

$ let SUM=5+5

$ printf %d $SUM

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Arithmetic ExpressionsYou dont need to use $ to expand a variable name in the

string. let understands that any variable appearing on theright side of the equals sign needs to have its valuesubstituted into the expression.

$ let SUM=SUM+5

$ printf %d $SUM

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$ let SUM=$SUM+5

$ printf %d $SUM

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Arithmetic ExpressionsThe optional dollar sign is a special feature of the let

command and does not apply to other commands.

If a variable is declared as an integer with the -iswitch,

the letcommand is optional.

$ SUM=SUM+5

$ printf %d\n $SUM

25

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Arithmetic ExpressionsThe let command provides the four basic math

operators, plus a remainder operator. Only integerexpressions are allowed (no decimal points).

$ let RESULT=5 + 2

$ let RESULT=5 - 2

$ let RESULT=5 * 2

$ let RESULT=5 / 2

$ let RESULT=5 % 2

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Logical ExpressionsIn let, trueis represented by the value of 1, and falseby 0. Any value other than 1

or 0 is treated as true, but the logical operators themselves only return 1 or 0.

Remember that logical truth (a value greater than zero) is not the same as thesuccess of a command (a status code of zero). In this respect, testand letareopposites of each other.

To use logical negation at the shell prompt, you must disable the shell historyoption or Bash will interpret the ! as a history look-up request.

$ let RESULT=!0

$ let RESULT=!1

$ let RESULT=1 && 0

$ let RESULT=1 || 0

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Relational OperationsUnlike string comparisons, let provides a full complement of

numeric comparisons. These are of limited value becausemost of comparisons are tested with the test command inthe if command, resulting in two sets of tests. In logicalexpressions, 0 is a failure.

$ let RESULT=1 > 0

$ printf 1 greater than 0 is %d\n $RESULT1 greater than 0 is 1

$ let RESULT=1 >= 0

$ printf 1 greater than or equal to 0 is %d\n $RESULT1 greater than or equal to 0 is 1

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Bitwise OperationsThere is also a set of bitwise operators, as follows.

$ let RESULT=~5

$ printf bitwise negation of 5 is %d\n $RESULT

bitwise negation of 5 is -6

$ let RESULT=5 >> 2

$ printf 5 left-shifted by 2 is %d\n $RESULT

5 left-shifted by 2 is 1

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Self-Referential OperationsSelf-referential operators are shorthand notations that

combine assignment with one of the other basic operations.The operation is carried out using the assignment variable,and then the result is assigned to the assignment variable.

For example, RESULT+=5 is a short form ofRESULT=RESULT+5.

$ let RESULT=5

$ let RESULT+=5

$ printf The result is %d $RESULT

The result is 10

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Other letFeaturesParentheses are allowed in the expressions.

$ let RESULT=(5+3)*2

$ printf The expression is %d $RESULT

The expression is 16

Assignment in the letcommand is an operator that returns the value beingassigned.

As a result, multiple variables can be assigned at once.

$ let TEST=TEST2=5

$ printf The results are %d and %d\n $TEST $TEST2

The results are 5 and 5

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Other letFeatureslet can evaluate more than one expression at a time. Several, small

assignments can be combined on one line.

$ let SUM=5+5 SUM2=10+5

Excessive numbers of assignments in a single let command will lead toreadability problems in a script. When each let is on a single line, itseasier to look through the script for a specific assignment.

The conditional expression operator (?) is shorthand for an if statementwhen one of two different expression are evaluated based on the leftcondition. Because this is a feature of the letcommand, it works only withnumeric expressions, not strings. The following example constrains atruth value to a 1 or a 0.

$ let RESULT=VALUE > 1 ? 1 : 0

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Other letFeaturesDouble parentheses are an alias for let. They are used to embed let

expressions as parameters in another command. The shellreplaces the double parentheses with the value of the expression.

declare -i X=5;

while (( X-- > 0 )) ; do

printf %d\n $X

Done

This script fragment prints a list of numbers from four to zero. Theembedded let reduces X by one each time through the loop andchecks to see when the value of X reaches zero.

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Arithmetic TestsThe testcommand can compare numeric values, but it

uses different operators than the ones used to comparestring values. Because all shell variables are storedas strings, variables can either be compared as

numbers or strings depending on the choice ofoperator. Perlprogrammers will find this similar to Perl.Bash will not report an error if string operators areused with integer variables.

$ RESULT=`ls -1 | wc -l`

$ printf %d $RESULT

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Arithmetic Tests

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Option Performed Action

n1 -eq n2 (equal) True if n1 is equal to n2

n1 -ne n2 (not equal) True if n1 is not equal to n2

n1 -lt n2 (less than) True if n1 is less than n2

n1 -le n2 (less than or equal) True if n1 is less than or equal to n2

n1 -gt n2 (greater than) True if n1 is greater than n2

n1 -ge n2 (greater than or equal) True if n1 is greater than or equal to n2


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Pattern RecognitionBash pattern recognition is called globbing. Globbing is

used to match filenames given to a command, and itis also used by the Korn shell test command to matchstrings.

$ ls *.txt

notes.txt project_notes.txt

The pattern-recognition feature works by supplying

wildcard symbols that Bash will attempt to match to astring or a filename. The asterisk (*) characterrepresents zero or more characters. The Korn shell testcan be used to match the value of a variable to a stringwith an asterisk.

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Pattern RecognitionCOMPANY=Athabasca

if [[ $COMPANY = A* ]] ; then

printf The company name begins with a letter a A\n

fiif [[ $COMPANY = Z* ]] ; then

printf The company name begins with a letter a Z\n

fi

This behavior doesnt work when quotation marks are used.Quotation marks tell the Korn shell testcommand not to interpretspecial characters. A testfor A* would indicate a file named A*.

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Pattern RecognitionThe question mark (?) character is a wildcard representing any single character.

COMPANY=AIC

if [[ $COMPANY = A?? ]] ; then

printf The company name is 3 characters beginning with A\n

fi

You can specify a set of characters using square brackets. You can list individualcharacters or ranges.

if [[ $COMPANY = [ABC]* ]] ; then

printf The company name begins with a A, B or C\n

fi

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Pattern RecognitionYou can separate lists of patterns by using vertical bar characters

(|).

COMPANY=Champion Ltd

if [[ $COMPANY = Champion*@(Ltd|Corp|Inc) ]] ; then

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Syntax Performed Action

?(pattern-list) Matches zero or one occurrence of the given patterns

*(pattern-list) Matches zero or more occurrences of the given patterns

+(pattern-list) Matches one or more occurrences of the given patterns

@(pattern-list) Matches exactly one of the given patterns

!(pattern-list) Matches anything except one of the given patterns


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Pattern Recognition

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Syntax Performed Action

[:alnum:] Alphanumeric

[:alpha:] Alphabetic

[:ascii:] ASCII characters

[:blank:] Space or tab[:cntrl:] Control characters

[:digit:] Decimal digits

[:graph:] Non-blank characters

[:lower:] Lowercase characters[:space:] Whitespace

[:upper:] Uppercase characters

[:xdigit:] Hexadecimal digits


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Assignment (5 points)Write a shell script in order to convert Fahrenheit to

Celsius.

TempCelsius = 5 * (TempFahrenheit 32) / 9

Deadline: December 11th, 10:00 AM

How to deliver?!

Via email to [email protected]

This assignment is mandatory!

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mailto:[email protected]:[email protected]

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Linux Shell (5)