Advanced Bash−Scripting Guide

Example 30−3. test24, another buggy script
Advanced Bash−Scripting Guide
#!/bin/bash
# This is supposed to delete all filenames in current directory
#+ containing embedded spaces.
# It doesn't work. Why not?
badname=`ls | grep ' '`
# echo "$badname"
rm "$badname"
exit 0
Try to find out what's wrong with Example 30−3 by uncommenting the echo "$badname" line. Echo
statements are useful for seeing whether what you expect is actually what you get.
In this particular case, rm "$badname" will not give the desired results because $badname should not be
quoted. Placing it in quotes ensures that rm has only one argument (it will match only one filename). A partial
fix is to remove to quotes from $badname and to reset $IFS to contain only a newline, IFS=$'\n'.
However, there are simpler ways of going about it.
# Correct methods of deleting filenames containing spaces.
rm *\ *
rm *" "*
rm *' '*
# Thank you. S.C.
Summarizing the symptoms of a buggy script,
1. It bombs with a "syntax error" message, or
2. It runs, but does not work as expected (logic error).
3. It runs, works as expected, but has nasty side effects (logic bomb).
Tools for debugging non−working scripts include
1. echo statements at critical points in the script to trace the variables, and otherwise give a snapshot of
what is going on.
2. using the tee filter to check processes or data flows at critical points.
3. setting option flags −n −v −x
sh −n scriptname checks for syntax errors without actually running the script. This is the
equivalent of inserting set −n or set −o noexec into the script. Note that certain types of
syntax errors can slip past this check.
sh −v scriptname echoes each command before executing it. This is the equivalent of inserting
set −v or set −o verbose in the script.
The −n and −v flags work well together. sh −nv scriptname gives a verbose syntax check.
Chapter 30. Debugging 320