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>Advanced Bash-Scripting Guide: An in-depth exploration of the art of shell scripting</TH
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>Chapter 33. Gotchas</H1
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CLASS="FOREIGNPHRASE"
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>Gli enigmi sono tre, la morte
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CLASS="FOREIGNPHRASE"
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>No, no! Gli enigmi sono tre, una la
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>Here are some (non-recommended!) scripting practices that
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will bring excitement into an otherwise dull life.</P
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>Assigning reserved words or characters to variable names.</P
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CLASS="PROGRAMLISTING"
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> 1 case=value0 # Causes problems.
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2 23skidoo=value1 # Also problems.
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3 # Variable names starting with a digit are reserved by the shell.
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4 # Try _23skidoo=value1. Starting variables with an underscore is okay.
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6 # However . . . using just an underscore will not work.
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8 echo $_ # $_ is a special variable set to last arg of last command.
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9 # But . . . _ is a valid function name!
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11 xyz((!*=value2 # Causes severe problems.
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12 # As of version 3 of Bash, periods are not allowed within variable names.</PRE
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>Using a hyphen or other reserved characters in a variable name (or
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CLASS="PROGRAMLISTING"
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2 # Use 'var_1' instead.
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4 function-whatever () # Error
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5 # Use 'function_whatever ()' instead.
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8 # As of version 3 of Bash, periods are not allowed within function names.
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9 function.whatever () # Error
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10 # Use 'functionWhatever ()' instead.</PRE
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>Using the same name for a variable and a function. This can make a
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script difficult to understand.</P
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CLASS="PROGRAMLISTING"
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> 1 do_something ()
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3 echo "This function does something with \"$1\"."
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6 do_something=do_something
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8 do_something do_something
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10 # All this is legal, but highly confusing.</PRE
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HREF="special-chars.html#WHITESPACEREF"
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In contrast to other programming languages, Bash can be quite
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finicky about whitespace.</P
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CLASS="PROGRAMLISTING"
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> 1 var1 = 23 # 'var1=23' is correct.
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2 # On line above, Bash attempts to execute command "var1"
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3 # with the arguments "=" and "23".
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5 let c = $a - $b # Instead: let c=$a-$b or let "c = $a - $b"
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7 if [ $a -le 5] # if [ $a -le 5 ] is correct.
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8 # ^^ if [ "$a" -le 5 ] is even better.
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9 # [[ $a -le 5 ]] also works.</PRE
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>Not terminating with a <A
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HREF="special-chars.html#SEMICOLONREF"
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HREF="special-chars.html#CODEBLOCKREF"
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>code block within curly
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CLASS="PROGRAMLISTING"
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> 1 { ls -l; df; echo "Done." }
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2 # bash: syntax error: unexpected end of file
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4 { ls -l; df; echo "Done."; }
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5 # ^ ### Final command needs semicolon.</PRE
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> Assuming uninitialized variables (variables before a value is
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assigned to them) are <SPAN
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uninitialized variable has a value of <I
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CLASS="PROGRAMLISTING"
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3 echo "uninitialized_var = $uninitialized_var"
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4 # uninitialized_var =</PRE
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> in a test. Remember,
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> is for comparing literal variables
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CLASS="PROGRAMLISTING"
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> 1 if [ "$a" = 273 ] # Is $a an integer or string?
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2 if [ "$a" -eq 273 ] # If $a is an integer.
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4 # Sometimes you can interchange -eq and = without adverse consequences.
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5 # However . . .
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8 a=273.0 # Not an integer.
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10 if [ "$a" = 273 ]
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12 echo "Comparison works."
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14 echo "Comparison does not work."
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15 fi # Comparison does not work.
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17 # Same with a=" 273" and a="0273".
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20 # Likewise, problems trying to use "-eq" with non-integer values.
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22 if [ "$a" -eq 273.0 ]
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24 echo "a = $a"
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25 fi # Aborts with an error message.
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26 # test.sh: [: 273.0: integer expression expected</PRE
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HREF="comparison-ops.html#SCOMPARISON1"
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>string comparison</A
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>Example 33-1. Numerical and string comparison are not equivalent</B
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CLASS="PROGRAMLISTING"
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2 # bad-op.sh: Trying to use a string comparison on integers.
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7 # The following while-loop has two errors:
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8 #+ one blatant, and the other subtle.
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10 while [ "$number" < 5 ] # Wrong! Should be: while [ "$number" -lt 5 ]
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12 echo -n "$number "
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13 let "number += 1"
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15 # Attempt to run this bombs with the error message:
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16 #+ bad-op.sh: line 10: 5: No such file or directory
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17 # Within single brackets, "<" must be escaped,
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18 #+ and even then, it's still wrong for comparing integers.
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20 echo "---------------------"
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22 while [ "$number" \< 5 ] # 1 2 3 4
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24 echo -n "$number " # It *seems* to work, but . . .
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25 let "number += 1" #+ it actually does an ASCII comparison,
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26 done #+ rather than a numerical one.
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28 echo; echo "---------------------"
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30 # This can cause problems. For example:
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35 if [ "$greater" \< "$lesser" ]
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37 echo "$greater is less than $lesser"
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38 fi # 105 is less than 5
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39 # In fact, "105" actually is less than "5"
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40 #+ in a string comparison (ASCII sort order).
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>Attempting to use <A
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HREF="internal.html#LETREF"
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to set string variables.</P
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CLASS="PROGRAMLISTING"
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> 1 let "a = hello, you"
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2 echo "$a" # 0</PRE
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>Sometimes variables within <SPAN
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([ ]) need to be quoted (double quotes). Failure to do so may
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cause unexpected behavior. See <A
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HREF="comparison-ops.html#STRTEST"
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HREF="redircb.html#REDIR2"
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HREF="variables2.html#ARGLIST"
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>Quoting a variable containing whitespace <A
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HREF="quoting.html#WSQUO"
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>prevents splitting</A
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HREF="quoting.html#VARSPLITTING"
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>Commands issued from a script may fail to execute because
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the script owner lacks execute permission for them. If a user
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cannot invoke a command from the command-line, then putting it
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into a script will likewise fail. Try changing the attributes of
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the command in question, perhaps even setting the suid bit
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>Attempting to use <B
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operator (which it is not) will usually result in an unpleasant
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CLASS="PROGRAMLISTING"
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> 1 command1 2> - | command2
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2 # Trying to redirect error output of command1 into a pipe . . .
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3 # . . . will not work.
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5 command1 2>& - | command2 # Also futile.
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7 Thanks, S.C.</PRE
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HREF="bash2.html#BASH2REF"
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functionality may cause a bailout with error messages. Older
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Linux machines may have version 1.XX of Bash as the default
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CLASS="PROGRAMLISTING"
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3 minimum_version=2
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4 # Since Chet Ramey is constantly adding features to Bash,
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5 # you may set $minimum_version to 2.XX, 3.XX, or whatever is appropriate.
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6 E_BAD_VERSION=80
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8 if [ "$BASH_VERSION" \< "$minimum_version" ]
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10 echo "This script works only with Bash, version $minimum or greater."
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11 echo "Upgrade strongly recommended."
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12 exit $E_BAD_VERSION
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>Using Bash-specific functionality in a <A
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HREF="why-shell.html#BASHDEF"
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>) on a non-Linux machine
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HREF="gotchas.html#BINSH"
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>may cause unexpected behavior</A
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A Linux system usually aliases <B
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>, but this does not necessarily hold true
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for a generic UNIX machine.</P
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>Using undocumented features in Bash turns out to be a
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dangerous practice. In previous releases of this
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book there were several scripts that depended on the
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> that, although the maximum value
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HREF="exit-status.html#EXITSTATUSREF"
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HREF="functions.html#RETURNREF"
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> value was 255, that limit
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did not apply to <SPAN
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Unfortunately, in version 2.05b and later, that loophole
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HREF="functions.html#RETURNTEST"
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> A script with DOS-type newlines (<TT
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will fail to execute, since <TT
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the same as the expected <TT
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fix is to convert the script to UNIX-style newlines.</P
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CLASS="PROGRAMLISTING"
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5 unix2dos $0 # Script changes itself to DOS format.
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6 chmod 755 $0 # Change back to execute permission.
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7 # The 'unix2dos' command removes execute permission.
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9 ./$0 # Script tries to run itself again.
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10 # But it won't work as a DOS file.
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>A shell script headed by <TT
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will not run in full Bash-compatibility mode. Some Bash-specific
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functions might be disabled. Scripts that need complete
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access to all the Bash-specific extensions should start with
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HREF="here-docs.html#INDENTEDLS"
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>Putting whitespace in front of
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the terminating limit string</A
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HREF="here-docs.html#HEREDOCREF"
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> will cause unexpected
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behavior in a script.</P
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>Putting more than one
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> statement in a function <A
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HREF="assortedtips.html#RVT"
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>whose output is captured</A
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CLASS="PROGRAMLISTING"
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3 echo "Whatever ... " # Delete this line!
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4 let "retval = $1 + $2"
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10 echo "Sum of $num1 and $num2 = $(add2 $num1 $num2)"
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12 # Sum of 12 and 43 = Whatever ...
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15 # The "echoes" concatenate.</PRE
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HREF="assortedtips.html#RVTCAUTION"
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NAME="PARCHILDPROBREF"
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HREF="internal.html#FORKREF"
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or to the environment. Just as we learned in biology, a child
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process can inherit from a parent, but not vice versa.</P
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CLASS="PROGRAMLISTING"
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> 1 WHATEVER=/home/bozo
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2 export WHATEVER
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CLASS="COMPUTEROUTPUT"
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> Sure enough, back at the command prompt, $WHATEVER remains unset.
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>Setting and manipulating variables in a <A
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HREF="subshells.html#SUBSHELLSREF"
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to use those same variables outside the scope of the subshell will
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result an unpleasant surprise.</P
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>Example 33-2. Subshell Pitfalls</B
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CLASS="PROGRAMLISTING"
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2 # Pitfalls of variables in a subshell.
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4 outer_variable=outer
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6 echo "outer_variable = $outer_variable"
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10 # Begin subshell
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12 echo "outer_variable inside subshell = $outer_variable"
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13 inner_variable=inner # Set
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14 echo "inner_variable inside subshell = $inner_variable"
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15 outer_variable=inner # Will value change globally?
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16 echo "outer_variable inside subshell = $outer_variable"
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18 # Will 'exporting' make a difference?
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19 # export inner_variable
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20 # export outer_variable
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21 # Try it and see.
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23 # End subshell
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27 echo "inner_variable outside subshell = $inner_variable" # Unset.
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28 echo "outer_variable outside subshell = $outer_variable" # Unchanged.
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33 # What happens if you uncomment lines 19 and 20?
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34 # Does it make a difference?</PRE
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HREF="special-chars.html#PIPEREF"
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HREF="internal.html#READREF"
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> may produce unexpected
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results. In this scenario, the <B
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acts as if it were running in a subshell. Instead, use
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HREF="internal.html#SETREF"
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HREF="internal.html#SETPOS"
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>Example 33-3. Piping the output of <I
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CLASS="PROGRAMLISTING"
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> 1 #!/bin/bash
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2 # badread.sh:
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3 # Attempting to use 'echo and 'read'
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4 #+ to assign variables non-interactively.
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10 echo "one two three" | read a b c
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11 # Try to reassign a, b, and c.
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14 echo "a = $a" # a = aaa
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15 echo "b = $b" # b = bbb
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16 echo "c = $c" # c = ccc
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17 # Reassignment failed.
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19 # ------------------------------
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21 # Try the following alternative.
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23 var=`echo "one two three"`
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25 a=$1; b=$2; c=$3
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27 echo "-------"
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28 echo "a = $a" # a = one
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29 echo "b = $b" # b = two
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30 echo "c = $c" # c = three
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31 # Reassignment succeeded.
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33 # ------------------------------
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35 # Note also that an echo to a 'read' works within a subshell.
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36 # However, the value of the variable changes *only* within the subshell.
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38 a=aaa # Starting all over again.
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43 echo "one two three" | ( read a b c;
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44 echo "Inside subshell: "; echo "a = $a"; echo "b = $b"; echo "c = $c" )
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48 echo "-----------------"
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49 echo "Outside subshell: "
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50 echo "a = $a" # a = aaa
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51 echo "b = $b" # b = bbb
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52 echo "c = $c" # c = ccc
1117
>In fact, as Anthony Richardson points out, piping to
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> loop can cause a similar problem.</P
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CLASS="PROGRAMLISTING"
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> 1 # Loop piping troubles.
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2 # This example by Anthony Richardson,
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3 #+ with addendum by Wilbert Berendsen.
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6 foundone=false
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7 find $HOME -type f -atime +30 -size 100k |
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11 echo "$f is over 100KB and has not been accessed in over 30 days"
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12 echo "Consider moving the file to archives."
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13 foundone=true
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14 # ------------------------------------
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15 echo "Subshell level = $BASH_SUBSHELL"
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16 # Subshell level = 1
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17 # Yes, we're inside a subshell.
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18 # ------------------------------------
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21 # foundone will always be false here since it is
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22 #+ set to true inside a subshell
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23 if [ $foundone = false ]
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25 echo "No files need archiving."
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28 # =====================Now, here is the correct way:=================
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30 foundone=false
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31 for f in $(find $HOME -type f -atime +30 -size 100k) # No pipe here.
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33 echo "$f is over 100KB and has not been accessed in over 30 days"
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34 echo "Consider moving the file to archives."
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35 foundone=true
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38 if [ $foundone = false ]
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40 echo "No files need archiving."
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43 # ==================And here is another alternative==================
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45 # Places the part of the script that reads the variables
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46 #+ within a code block, so they share the same subshell.
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47 # Thank you, W.B.
1183
49 find $HOME -type f -atime +30 -size 100k | {
1184
50 foundone=false
1185
51 while read f
1187
53 echo "$f is over 100KB and has not been accessed in over 30 days"
1188
54 echo "Consider moving the file to archives."
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55 foundone=true
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58 if ! $foundone
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60 echo "No files need archiving."
1208
> A lookalike problem occurs when trying to write the
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HREF="textproc.html#GREPREF"
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CLASS="PROGRAMLISTING"
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> 1 tail -f /var/log/messages | grep "$ERROR_MSG" >> error.log
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2 # The "error.log" file will not have anything written to it.
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3 # As Samuli Kaipiainen points out, this results from grep
1231
4 #+ buffering its output.
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5 # The fix is to add the "--line-buffered" parameter to grep.</PRE
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> commands within scripts is risky,
1250
as it may compromise system security.
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HREF="#FTN.AEN19504"
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>Using shell scripts for CGI programming may be problematic. Shell
1266
script variables are not <SPAN
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> and this can cause
1270
undesirable behavior as far as CGI is concerned. Moreover, it is
1273
>"cracker-proof"</SPAN
1278
>Bash does not handle the <A
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HREF="internal.html#DOUBLESLASHREF"
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>Bash scripts written for Linux or BSD systems may need
1295
fixups to run on a commercial UNIX (or Apple OSX) machine. Such
1296
scripts often employ the GNU set of commands and filters,
1297
which have greater functionality than their generic UNIX
1298
counterparts. This is particularly true of such text processing
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HREF="textproc.html#TRREF"
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>Danger is near thee --</I
1326
>Beware, beware, beware, beware.</I
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>Many brave hearts are asleep in the deep.</I
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>--A.J. Lamb and H.W. Petrie</I
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HREF="gotchas.html#AEN19504"
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HREF="fto.html#SUIDREF"
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permission on the script itself has no effect in Linux
1376
and most other UNIX flavors.</P
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>Scripting With Style</TD
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