Linux Shell Script Logic

(Where's the Logic in That?)

Conditional Operations

Let's look at the if/then/else construct in a Bash shell script and see how to control the flow of a script with conditional logic. The general form of if/then/else is shown here, with the actual syntax shown in boldface and the parts you must supply in normal type:

if [ condition is true ]
then
execute these commands
else
execute those commands
fi

The else clause is optional, but you must end the construct with the fi command. You can also have nested if clauses by using the elif command like this:

if [ condition1 is true ]
then
execute these commands
elif [ condition2 is true ]
then
execute these commands
else
execute those commands
fi

So what kind of conditions can we test for? If you're dealing with numbers, here are the conditional expressions you can use. In other words, any of these expressions can go inside the brackets on the if or elif statement:

num1 -eq num2 True if num1 equals num2.

num1 -ne num2 True if num1 is not equal to num2.

num1 -lt num2 True if num1 is less than num2.

num1 -gt num2 True if num1 is greater than num2.

num1 -le num2 True if num1 is less than or equal to num2.

num1 -ge num2 True if num1 is greater than or equal to num2.

If you're comparing character strings, these are the valid conditional expressions:

str1 = str2 True if str1 and str2 are identical.

str1 != str2 True if str1 and str2 are not identical.

-n str1 True if str1 is not null (length is greater than zero).

-z str1 True if str1 is null (length is zero).

You can also test certain file conditions, such as whether or not files exist, the type of file, and so on. Here are the conditional expressions for files:

-f somefile True if somefile exists and is an ordinary file.

-d somefile True if somefile exists and is a directory.

-s somefile True if somefile contains data (the size is not zero).

-r somefile True if somefile is readable.

-w somefile True if somefile is writable.

-x somefile True if somefile is executable.

And finally, here are the logical operators, for performing tests that involve and, or, and not conditions.

cond1 -a cond2 True if both cond1 and cond2 are true.

cond1 -o cond2 True if either cond1 or cond2 is true.

! cond1 True if cond1 is false.

Some if/then/else Examples

Here are some examples using the conditional expressions just listed. Note that the spaces on either side of the square brackets are not optional!

if [ $carprice -gt 20000 ]
then
echo 'Too rich for my blood.'
else
echo 'Can you get that model in blue?'
fi

if [ $maker = 'Buick' ]
then
echo 'Have you driven a Ford lately?'
fi

if [ -r $1 -a -s $1 ]
then
echo "The $1 file is readable and contains data."
fi

The case Statement

Bash provides a case statement that lets you compare a string with several possible values and execute a block of code when it finds a match. Here's an example of the case command, with the syntax shown in boldface and the parts you would supply in normal type:

case $1 in
-a)
commands;;
-f)
commands;;
*)
commands;;
esac

In this example, if the value of $1 was -a, the first block of commands would execute. If the value of $1 was -f, the second block of commands would execute. Otherwise, the third block of commands, following the asterisk clause, would execute. (Think of the asterisk as meaning "match anything.")

You can put as many commands as you need in place of commands in the sample, but be sure to end the block with a double semicolon. Only the first matching block of commands will execute in a case statement, and you must signal the end of the construct with the esac command.

Previous Lesson: Shell Script Variables
Next Lesson: Shell Script Looping

Comments - most recent first (Please feel free to answer questions posted by others!) Dennis Korbinsky     (06 Nov 2012, 21:19) I like your website. Very good information. Thank you please. Sam     (24 Mar 2012, 01:04) I need help with this.Can any one help? Write a script (Linux or Windows) that takes two arguments, names of source and destination directories, and copies all files (including hidden) from source to destination, preserving the directory structure of the source directory. If a user does not supply two arguments, a short help is displayed and the program quits. If the source directory does not exist, an error is displayed on a screen, date, time, and nature of an error are added to a log file (ScriptFileName.log), and the program quits. If the destination directory does not exist, it is created first. If a file cannot be copied, your program should skip it and continue copying. If a newer file exists in the destination directory, your program should skip it and continue copying. If a newer file exists in the source directory, your program should silently overwrite an older file in the destination directory and continue copying. Date, time, and full paths of the files copied are added to a log file, ScriptFileName.log upon completion. Displayed help format: Command syntax: ScriptFileName[src] [dst] jayant     (21 Feb 2012, 03:14) Help me to get execute below commands daily zcat b*20120220* | awk -F "\t" '$40 ~ /3:/' | cut -f 3,5,25,35,40 | grep -w D | grep -w M | cut -f 1 | grep 0:91 | cut -c 3-8 | sort | uniq -c | sort -r I need o/p at path /home/smscsite/trace/bills_backup.d Kerman Mistry     (15 Feb 2012, 23:57) Hi Bob, Your tutorial is a no non-sense; to the point tutorial. Thanks for a great job. AMR     (06 Feb 2012, 10:58) hardworker, look into cron PS     (31 Jan 2012, 04:52) A unix script(ftp files) is running on unix server properly and produced correct output but one workflow which contains same script is not working while running on informatica power center server manually its showing status as waiting but not succeded, could please tell me the cause of the problem and solution for this nordik_14     (28 Jan 2012, 04:48) Very good tutorial. Im fortran programer and for my needs initialization in bash is very useful. Thanks from mexico Raghunath Divekar     (16 Jan 2012, 20:50) Dear BOB, U have done a great job. Really good notes which helped me revise all the commands I had used around 15 years back. You have made Linux look like simple and easy............. kashyap     (16 Nov 2011, 02:37) Hi, I would like to have a script which finds files less then 10MB and if there are no files then echo no files found using if loop. thanks in advance Terry     (18 Sep 2011, 05:19) can someone please help me with a code for writing a script that shows multiple files in the directory that exist and those that do not. hardworker     (30 Jun 2011, 03:45) hi i need to write a perl script that checks for a file for 8 hrs..once 5 hrs over, it should send a mail that the file not came for every half hour,at the same time the check for file should continue. how can i do that?i should not use more loops for this!! mahesh     (11 Mar 2011, 00:26) Write a pgm which test the command from commandline argument then report whether it is directory or file. it is readable or writeable or executable file in linux. Brahma     (28 Feb 2011, 02:11) can any body telll me this.... q) Write a script that takes a numeric input from user and tell him if the value is positive or negative or zero. Russel     (22 Feb 2011, 01:49) Need Help: 1. Write a shell program that takes one command line argument. The program will do one of (a), (b), or (c) below: (a) If the argument is missing or invalid: print "XX: invalid or missing argument" on stderr and exit with 1 (where XX is the name of the shell program NOT hardcoded) (b) If the argument is "more": print the userids of all users who have a process running idle for 2 days or more. Exits with 0 (c) If the argument is "less": print the userids of all users who have a process running idle for less than 2 days. Exits with 0 Someone     (10 Feb 2011, 23:15) @Player: at 0 seconds, test interprets negative 1 (-1) as an option, and so it needs a number to test. If you quote it, like this: #!/bin/bash count=$1 while [ "$count" -gt 0 ] do echo "$count seconds till supper time!" count=$(expr $count - 1 ) sleep 1 done echo "Supper time, YEAH!!" exit 0 It should work. SSS     (08 Feb 2011, 22:54) u r really helpful Rajiv     (24 Jan 2011, 00:59) Hi Gurus, I have a simple shell script as below, sqlplus UN/pw@SID <<EOL @Long_running_requests.sql / exit EOL I'm executing this shell script from command prompt for mailx -s to send the output of the .sql script. some times the SQL does not give any output depending on the where conditions which is an expected behaviour. But the mail is still sent with the SQL*plus connection details like below ********************* SQL*Plus: Release 8.0.6.0.0 - Production on Mon Jan 24 14:18:55 2011 (c) Copyright 1999 Oracle Corporation. All rights reserved. Connected to: Oracle Database 10g Enterprise Edition Release 10.2.0.3.0 - 64bit Production With the Partitioning, OLAP and Data Mining options SQL> 32 SQL> Disconnected from Oracle Database 10g Enterprise Edition SQL> Release 10.2.0.3.0 - 64bit Production With the Partitioning, OLAP and Data Mining options ************************* The requirement is to send a mail only when there is an o/p from the .sql. and no action required when the o/p is null. Please suggest. Ren     (13 Dec 2010, 12:29) I need to test whether some string, (variable name of $string1) is found as text inside a text file. If I read the file and find the string, then I want to backup the filename.txt to filename.txt.bak, replace $string1 with the value in $string2, create the new file filename.txt with the new text, and leave filename.txt.bak as a backup of the original file. if [ NEED HELP HERE ]; then mv filename.txt filename.txt.bak sed s/string1/yyy/g filename.txt.bak filename.txt endif If the string is not found, then I want to do absolutely nothing. I do not want to backup the file nor create a new copy of the original file as there is no text that will change. brian muchake     (19 Nov 2010, 02:52) Good effort devi05dct@gmail.com     (15 Nov 2010, 08:46) Kindly help to create one script I need to check some particular packages installed in my os or not. (but i need to search for 4 package what shall i do) rpm -qa | grep "htttpd* >file Player     (02 Nov 2010, 09:11) when i execute the below script i get error: script is below the error msg. [root@CentOS_Vmware scripts_dir]# ./script5.sh 4 4 seconds till supper time! expr: syntax error ./script5.sh: line 3: [: -gt: unary operator expected Supper time!!, YEAH!! #!/bin/bash count=$1 while [ $count -gt 0 ] do echo $count seconds till supper time! count=$(expr $count -1 ) sleep 1 done echo Supper time!!, YEAH!! Jay     (25 Oct 2010, 11:31) Hello Dr.Ravisankaar, >I have a problem. Pls. solve this >I have to read set of lines from a file using >shell script. the lines are like this. I have to >read line by line. pls. help me > >chr18:3000-4000 >Chr20:5000-6000 grep -i chr /path/to/file Jay     (25 Oct 2010, 11:29) This page helped me too. VAL=`dig nick.domain.com| egrep -A 1 CNAME| grep -v CNAME| awk '{print $5}'` grep $VAL /etc/mail/access if [ $? = 0 ]; then echo "Fine"; else echo -e "$VAL \t\t\t RELAY" >> /etc/mail/access;/usr/sbin/makemap hash /etc/mail/access.db < /etc/mail/access; fi Renuka     (25 Oct 2010, 03:30) I want shell script for more and less command,for e.g.more act as less and less act as more command.pls its urgent Dr.Ravisankaar     (05 Oct 2010, 22:02) I have a problem. Pls. solve this I have to read set of lines from a file using shell script. the lines are like this. I have to read line by line. pls. help me chr18:3000-4000 Chr20:5000-6000 rics     (24 Sep 2010, 11:20) @Bob thanks but i get this error message-- expr: non-numeric argument here is the script: tt1=23 tt1=`expr $tt1 - 1` echo $tt1 Bob Rankin     (24 Sep 2010, 10:50) @rics - Try this day1=`expr $day1 - 1` rics     (24 Sep 2010, 09:49) can anyone help or point me to a good website. have not done shell scripting for a long time, now find myself doing it again. need to do math in the script ie. day1=24 want to change day1 to be 23 ie. day1=$day1 -1 does not work what am i doing wrong ? thanks Richard     (20 Sep 2010, 15:40) What is the best syntax for comparing one variable to multiple values in an IF statement. For example: X can be equal to "ABC", "XY2", "XY3", "XY4", "XY5", "XY6" OR "ALG" I want to do the SAME thing if x equals "XY2", "XY5", OR "XY6". I know that I can use the following: if [ (("$variable" == "XY2") -o ("$variable" == "XY5") -o ("$variable" == "XY6") ] But, is there a shorthand? For example, in SQL, I can write as follows to see if my variabe equals any of the values in the parentheses: IF variable IN ("XY2", "XY5", "XY6") Is there a neater way of writing this in Linux? I am using #!/bin/sh. I am new to Linux. My thanks in advance. beginner     (17 Sep 2010, 21:37) Hi, i tried this /* in file ss1 */ if[$1 eq $2] then echo 'args are equal' else echo 'args not equal' fi /* i ran it as */ [bigfoot@localhost linux_work]$ ./ss1 2 2 /* the following warnings appears*/ ./ss1: line 1: if[2 eq 2]: command not found ./ss1: line 2: syntax error near unexpected token `then' ./ss1: line 2: `then' Please help. Bob This is a wonderful website. THANKS Bob Rankin     (16 Jul 2010, 15:07) @hello_world - You might want to check the "Homework Helper" website. :-) hello_world     (16 Jul 2010, 00:44) Write a program which will output greetings ( Good Morning, Good Afternoon, Good Evening to user based on the time the script is run. ( Use case Statement in script) Vishnu     (09 Jul 2010, 00:59) How to find the existence of similar files.. like files having a particular date template If [ -e *date* ] is giving error ,,too many arguments ab     (04 Jul 2010, 13:48) why shell reverses the meaning of true or false compared to other languages?? Kumar Piyus     (21 Jun 2010, 11:25) The information is to the point and very easy to understand. Really nice. abhinav narain     (09 Jun 2010, 12:16) [ -z STRING ] option returns true,if the length of the string is zero abhinav narain     (09 Jun 2010, 09:10) What is -z option used for ? i cant find it in the websites i visited George Lazarides     (26 May 2010, 13:14) Nice page! Your site is a great reference that I'll keep bookmarked for future use! But I think you forgot an option: -e somefile True if somefile exists. if [ -e /var/logs/error.log ] then echo 'There is an error log.' else echo 'No errors today.' fi Roberto     (17 Apr 2010, 03:19) VAR=$(COMMAND_LIST) How I can test if the commands in COMMAND_LIST are run correctly? Bob Rankin     (18 Mar 2010, 20:49) You are right! Fixed now. Luis     (18 Mar 2010, 18:32) better if it was if [ -r $1 -a -s $1 ] not if [ -r $1 -a -s $2 ] right? Pedro Oliveira     (26 Feb 2010, 09:16) Like it! It helped me on a shell script. Thanks! I welcome your comments. However... I am puzzled by many people who say "Please send me the Linux tutorial." This website *is* your Linux Tutorial! Read everything here, learn all you can, ask questions if you like. But don't ask me to send what you already have. :-) NO SPAM! If you post garbage, it will be deleted, and you will be banned. *Name: Email: Notify me about new comments on this page Hide my email *Text:

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