Linux Bible. Christopher Negus

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Is either of the two expressions true? file1 -nt file2 Is the first file newer than the second file (using the modification time stamp)? file1 -ot file2 Is the first file older than the second file (using the modification time stamp)? file1 -ef file2 Are the two files associated by a link (a hard link or a symbolic link)? var1 = var2 Is the first variable equal to the second variable? var1 -eq var2 Is the first variable equal to the second variable? var1 -ge var2 Is the first variable greater than or equal to the second variable? var1 -gt var2 Is the first variable greater than the second variable? var1 -le var2 Is the first variable less than or equal to the second variable? var1 -lt var2 Is the first variable less than the second variable? var1 != var2 Is the first variable not equal to the second variable? var1 -ne var2 Is the first variable not equal to the second variable?

There is also a special shorthand method of performing tests that can be useful for simple one-command actions. In the following example, the two pipes (||) indicate that if the directory being tested for doesn't exist (-d dirname), then make the directory (mkdir $dirname):

       # [ test ] || action # Perform simple single command if test is false dirname="/tmp/testdir" [ -d "$dirname" ] || mkdir "$dirname"

      Instead of pipes, you can use two ampersands to test if something is true. In the following example, a command is being tested to see if it includes at least three command-line arguments:

       # [ test ] && {action} # Perform simple single action if test is true [ $# -ge 3 ] && echo "There are at least 3 command line arguments."

      You can combine the && and || operators to make a quick, one-line if…then…else. The following example tests that the directory represented by $dirname already exists. If it does, a message says the directory already exists. If it doesn't, the statement creates the directory:

       # dirname=mydirectory [ -e $dirname ] && echo $dirname already exists || mkdir $dirname

      The case command

      Another frequently used construct is the case command. Similar to a switch statement in programming languages, this can take the place of several nested if statements. The following is the general form of the case statement:

       case "VAR" in Result1) { body };; Result2) { body };; *) { body } ;; esac

      Among other things, you can use the case command to help with your backups. The following case statement tests for the first three letters of the current day (case 'date +%a' in). Then, depending on the day, a particular backup directory (BACKUP) and tape drive (TAPE) are set.

       # Our VAR doesn't have to be a variable, # it can be the output of a command as well # Perform action based on day of week case `date +%a` in "Mon") BACKUP=/home/myproject/data0 TAPE=/dev/rft0 # Note the use of the double semi-colon to end each option ;; # Note the use of the "|" to mean "or" "Tue" | "Thu") BACKUP=/home/myproject/data1 TAPE=/dev/rft1 ;; "Wed" | "Fri") BACKUP=/home/myproject/data2 TAPE=/dev/rft2 ;; # Don't do backups on the weekend. *) BACKUP="none" TAPE=/dev/null ;; esac

      The asterisk (*) is used as a catchall, similar to the default keyword in the C programming language. In this example, if none of the other entries are matched on the way down the loop, the asterisk is matched and the value of BACKUP becomes none. Note the use of esac, or case spelled backwards, to end the case statement.

      The ″for…do″ loop

      Loops are used to perform actions over and over again until a condition is met or until all data has been processed. One of the most commonly used loops is the for…do loop. It iterates through a list of values, executing the body of the loop for each element in the list. The syntax and a few examples are presented here:

       for VAR in LIST do { body } done

The for loop assigns the values in LIST to VAR one at a time. Then, for each value, the body in braces between do and done is executed. VAR can be any variable name, and LIST can be composed of pretty much any list of values or anything that generates a list.

       for NUMBER in 0 1 2 3 4 5 6 7 8 9 do echo The number is $NUMBER done for FILE in `/bin/ls` do echo $FILE done

      You can also write it this way, which is somewhat cleaner:

       for NAME in John Paul Ringo George ; do echo $NAME is my favorite Beatle done

      Each element in the LIST is separated from the next by white space. This can cause trouble if you're not careful because some commands, such as ls -l, output multiple fields per line, each separated by white space. The string done ends the for statement.

      If you're a die-hard C programmer, bash allows you to use C syntax to control your loops:

       LIMIT=10 # Double parentheses, and no $ on LIMIT even though it's a variable! for ((a=1; a <= LIMIT ; a++)) ; do echo "$a" done

      The ″while…do″ and ″until…do″ loops

      Two other possible looping constructs are the while…do loop and the until…do loop. The structure of each is presented here:

       while condition until condition do do { body } { body } done done

      The while statement executes while the condition is true. The until statement executes until the condition is true—in other words, while the condition is false.

      Here is an example of a while loop that outputs the number 0123456789:

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