Examples-Advanced Topics

City College of San Francisco - CS160B
Unix/Linux Shell Scripting
Module: Advanced Topics

Examples

Example 1

In Example 1 of Examples-Loops2, we had the following code:

while read file ; do

# remember, the filename does not have the directory component!
# we must add it
if file "$dir/$file" | grep ':.*text' > /dev/null; then

if ! echo "$file" | grep -q '\.txt$'; then

if [ -e "$dir/$file.txt" ]; then

echo "'$dir/file.txt' already exists - skipped." >&2
continue

else

if ! mv "$dir/$file" "$dir/$file.txt" 2>/dev/null; then

echo "cannot rename '$dir/$file' to '$dir/file.txt'" >&2

done < $$tmp
rm -f $$tmp

Change the lines highlighted in bold to use substring operators.

Example 2

In Example 2 of Examples-Loops2, we had the following line of code

members=$(echo "$group" | cut -d: -f4 | tr ',' ' ')

Rewrite the line of code to use substring operators rather than the external commands cut and tr (there are 4 colon-separated fields in $group)

Example 3

You have logged onto a new linux system. You are looking through a shell script you found and encounter the lines

set -o nounset
set -o allexport

Explain what they mean

Example 4

You are writing a shell script and want to use command tracing to trace the execution of your commands to try to find a bug. Explain how you could run your shell script with tracing on.

Example 5

What does the message ok in the following line of code mean?

[ "${file##*.}" = doc ] && echo ok

Answers

Example 1

while read file ; do

# remember, the filename does not have the directory component!
# we must add it
desc=$(file "$dir/$file")
if [ "${desc/:*text/}" != "$desc" ]; then

if [ "${file%.txt}" = "$file" ]; then

if [ -e "$dir/$file.txt" ]; then

echo "'$dir/file.txt' already exists - skipped." >&2
continue

else

if ! mv "$dir/$file" "$dir/$file.txt" 2>/dev/null; then

echo "cannot rename '$dir/$file' to '$dir/file.txt'" >&2

done < $$tmp
rm -f $$tmp

Example 2

Since field 4 is the last colon-delimited field of "$group", this is pretty easy:

members=${group##*:}
members=${members//,/ }

Example 3

Normally, shell scripts want the substitution of a variable that does not exist to simply output the empty string. set -o nounset changes this behavior. When the nounset option is turned on in the shell, the expansion of unset variables results in an error:

$ unset field
$ echo $field

$ set -o nounset
$ echo $field
bash: field: unbound variable
$

A variable that has been set to empty is not considered unset.

$ set -o nounset
$ field=
$ echo $field

$

The allexport option to the shell means that all variables that are created or modified in the future will be marked as envionment variables (i.e., exported). It does not affect existing local variables, unless you modify them.

Example 4

You need to turn on the xtrace (-x) option for the shell script. You could do this in any of the following ways:

add the line set -o xtrace or set -x to the beginning of the shell script. This solution is the most flexible, as you can choose to only trace a portion of your shell script, such as a loop that seems to be malfunctioning. You would do this by enclosing the loop between the command set -x (to turn on xtrace) and set +x (to turn it off)
add the option -x to the #!/bin/bash directive

#!/bin/bash
      -x

run the shell script by starting your own shell for it. If the shell script's name is fixfiles:

$ bash -x fixfiles args

where args is replaced with the normal arguments to fixfiles

Example 5

In the command

[ "${file##*.}" = doc ] && echo ok

The longest string ending in a period is stripped off from the left side of $file. The result is compared to the string doc. This is an attempt to determine whether "$file" ends in .doc . You will probably find it easier to understand than the usual construct

[ "${file%.doc}" != "$file" ] && echo ok

As far as I can tell, there is only one difference: if $file is exactly doc , ok will be output.

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