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Chapter 9. Repetitive tasks

Upon completion of this chapter, you will be able to

  • Use for, while and until loops, and decide which loop fits which occasion.

  • Use the break and continue Bash built-ins.

  • Write scripts using the select statement.

  • Write scripts that take a variable number of arguments.

9.1. The for loop

9.1.1. How does it work?

The for loop is the first of the three shell looping constructs. This loop allows for specification of a list of values. A list of commands is executed for each value in the list.

The syntax for this loop is:

for NAME [in LIST ]; do COMMANDS; done

If [in LIST] is not present, it is replaced with in $@ and for executes the COMMANDS once for each positional parameter that is set (see and ).

The return status is the exit status of the last command that executes. If no commands are executed because LIST does not expand to any items, the return status is zero.

NAME can be any variable name, although i is used very often. LIST can be any list of words, strings or numbers, which can be literal or generated by any command. The COMMANDS to execute can also be any operating system commands, script, program or shell statement. The first time through the loop, NAME is set to the first item in LIST. The second time, its value is set to the second item in the list, and so on. The loop terminates when NAME has taken on each of the values from LIST and no items are left in LIST.

9.1.2. Examples Using command substitution for specifying LIST items

The first is a command line example, demonstrating the use of a for loop that makes a backup copy of each .xml file. After issuing the command, it is safe to start working on your sources:

[carol@octarine ~/articles] ls *.xml
file1.xml  file2.xml  file3.xml

[carol@octarine ~/articles] ls *.xml > list

[carol@octarine ~/articles] for i in `cat list`; do cp "$i" "$i".bak ; done

[carol@octarine ~/articles] ls *.xml*
file1.xml  file1.xml.bak  file2.xml  file2.xml.bak  file3.xml  file3.xml.bak

This one lists the files in /sbin that are just plain text files, and possibly scripts:

for i in `ls /sbin`; do file /sbin/$i | grep ASCII; done Using the content of a variable to specify LIST items

The following is a specific application script for converting HTML files, compliant with a certain scheme, to PHP files. The conversion is done by taking out the first 25 and the last 21 lines, replacing these with two PHP tags that provide header and footer lines:

[carol@octarine ~/html] cat html2php.sh
# specific conversion script for my html files to php
LIST="$(ls *.html)"
for i in "$LIST"; do
     NEWNAME=$(ls "$i" | sed -e 's/html/php/')
     cat beginfile > "$NEWNAME"
     cat "$i" | sed -e '1,25d' | tac | sed -e '1,21d'| tac >> "$NEWNAME"
     cat endfile >> "$NEWNAME"

Since we don't do a line count here, there is no way of knowing the line number from which to start deleting lines until reaching the end. The problem is solved using tac, which reverses the lines in a file.

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