Chapter 19 Tips and Traps: Common Goofs for Novices

1.1 Forgetting to Set Execute Permissions
 

First, you could invoke bash and give it the name of the script as a parameter:

Or second (and better still), you could set the execute permission on the script so that you can run it directly:

[[email protected] test]$ ./fun_calc.sh
-bash: ./fun_calc.sh: Permission denied
[[email protected] test]$ bash fun_calc.sh
[[email protected] test]$ chmod a+x fun_calc.sh
[[email protected] test]$ ./fun_calc.sh
[[email protected] test]$

1.2 Fixing "No such file or directory" Errors

[[email protected] Day0801]$ cat busted
#!/bin/bash -
echo "Hello World!"
[[email protected] Day0801]$ # This works
[[email protected] Day0801]$ ./busted
Hello World!
[[email protected] Day0801]$ # But if the file gets DOS line endings,we get:
[[email protected] Day0801]$ ./busted1
/bin/bash: will: No such file or directory
Hello World!
[[email protected] Day0801]$ 

1.3 Forgetting That the Current Directory Is Not in the $PATH

Either add the current directory to the $PATH variable, which we do not recommend, or reference the script via the current directory with a leading ./ before the script name, as in: $ ./busted

[[email protected] Day0801]$ busted
bash: busted: command not found
[[email protected] Day0801]$ ./busted
Hello World!
[[email protected] Day0801]$

1.4 Naming Your Script Test

[[email protected] Day0801]$ type test
test is a shell builtin
[[email protected] Day0801]$ 

1.5 Expecting to Change Exported Variables

[[email protected]ellDBA Day0801]$ ./first.sh
VAL=5
in second
initially VAL=5
changed so VAL=12
back in first
VAL=5
[[email protected] Day0801]$ cat first.sh
#
# a simple example of a common mistake
#
# set the value:
export VAL=5
printf "VAL=%d\n" $VAL
#invoke our other script:
./second.sh
#
# now see what changed (hint:nothing!)
printf "%b" "back in first\n"
printf "VAL=%d\n" $VAL
[[email protected] Day0801]$ cat second.sh
#!/bin/bash
printf "%b" "in second\n"
printf "initially VAL=%d\n" $VAL
VAL=12
printf "changed so VAL=%d\n" $VAL
[[email protected] Day0801]$

 and the second script has to echo the final value (and only the final value) to STDOUT (it could redirect its other messages to STDERR):

[[email protected] Day0801]$ ./first.sh
VAL=5
in second
initially VAL=5
changed so VAL=12
back in first
VAL=5
[[email protected] Day0801]$ cat first.sh
#
# a simple example of a common mistake
#
# set the value:
export VAL=5
printf "VAL=%d\n" $VAL
#invoke our other script:
./second.sh
#
# now see what changed (hint:nothing!)
printf "%b" "back in first\n"
printf "VAL=%d\n" $VAL
[[email protected] Day0801]$ cat second.sh
#!/bin/bash
printf "%b" "in second\n"             >&2
printf "initially VAL=%d\n" $VAL      >&2
VAL=12
printf "changed so VAL=%d\n" $VAL     >&2
[[email protected] Day0801]$ 

1.6 Forgetting Quotes Leads to "command not found" on Assignments

You need quotes around the righthand side of the assignment to $ALLOPT. What is written above as: ALLOPT=$OPT1 $OPT2 really should be: ALLOPT="$OPT1 $OPT2"

[[email protected] Day0801]$ cat goof1.sh
#!/bin/bash -
# common goof:
# X=$Y $Z
# isn't the same as
# X="$Y $Z"
#
OPT1=-l
OPT2=-h
ALLOPT=$OPT1 $OPT2
ls $ALLOPT .
[[email protected] Day0801]$ chmod a+x goof1.sh
[[email protected] Day0801]$ ./goof1.sh
./goof1.sh: line 9: -h: command not found
busted  busted1  dash.sh  default_date.sh  first.sh  goof1.sh  load_mp3.sh  oodiff.sh  second.sh

[[email protected] Day0801]$ cat goof1.sh
#!/bin/bash -
# common goof:
# X=$Y $Z
# isn't the same as
# X="$Y $Z"
#
OPT1=-l
OPT2=-h
ALLOPT="$OPT1 $OPT2"
ls $ALLOPT .
[[email protected] Day0801]$ ./goof1.sh
total 36K
-rwxrwxr-x 1 maxwell maxwell   34 Aug  1 19:33 busted
-rwxrwxr-x 1 maxwell maxwell   58 Aug  1 19:39 busted1
-rw-rw-r-- 1 maxwell maxwell  595 Aug  1 13:27 dash.sh
-rw-rw-r-- 1 maxwell maxwell  915 Aug  1 08:11 default_date.sh
-rwxrwxr-x 1 maxwell maxwell  228 Aug  1 19:49 first.sh
-rwxrwxr-x 1 maxwell maxwell  123 Aug  1 19:59 goof1.sh
-rw-rw-r-- 1 maxwell maxwell 1.4K Aug  1 13:49 load_mp3.sh
-rw-rw-r-- 1 maxwell maxwell 1.3K Aug  1 13:58 oodiff.sh
-rwxrwxr-x 1 maxwell maxwell  145 Aug  1 19:54 second.sh
[[email protected] Day0801]$ 

1.10 Deleting Files Using an Empty Variable

Never do:
rm -rf $files_to_delete
Never, ever, ever do:
rm -rf /$files_to_delete
Use this instead:
[ "$files_to_delete" ] && rm -rf $files_to_delete

1.11 Seeing Odd Behavior from printf

[[email protected] Day0801]$ cat oddscript
#!/bin/bash -
badnode=6

printf "good nodes: %d\n" $goodnode
printf "bad nodes: %d\n" $badnode
printf "miss nodes: %d\n" $missnode
printf "GOOD=%d BAD=%d MISS=%d\n" $goodnode $badnode $missnode
[[email protected] Day0801]$ bash oddscript
good nodes: 0
bad nodes: 6
miss nodes: 0
GOOD=6 BAD=0 MISS=0
[[email protected] Day0801]$ 

1.13 Debugging Scripts

Add set -x to the top of the script when you run it. Or use set -x to turn on xtrace before a troublesome spot and set +x to turn it off after.

[[email protected] Day0801]$ cat buggy
#!/usr/bin/env bash
# cookbook filename: buggy
#
set -x 

result=$1

[ $result = 1 ] \
  && { echo "Result is 1; excellent." ; exit 0; } \
  || { echo "Uh-oh, ummm, RUN AWAY! " ; exit 120; }

Now we invoke this script, but first we set and export the value of the PS4 prompt. bash will print out the value of PS4 before each command that it displays during an execution trace (i.e., after a set -x ):

[[email protected] Day0801]$ cat buggy
#!/usr/bin/env bash
# cookbook filename: buggy
#
set -x 

result=$1

[ $result = 1 ] \
  && { echo "Result is 1; excellent." ; exit 0; } \
  || { echo "Uh-oh, ummm, RUN AWAY! " ; exit 120; }
[[email protected] Day0801]$ export PS4='+trace $LINENO:'
[[email protected] Day0801]$ echo $PS4
+trace $LINENO:
[[email protected] Day0801]$ chmod a+x buggy
[[email protected] Day0801]$ ./buggy
+trace 6:result=
+trace 8:'[' = 1 ']'
./buggy: line 8: [: =: unary operator expected
+trace 10:echo 'Uh-oh, ummm, RUN AWAY! '
Uh-oh, ummm, RUN AWAY! 
+trace 10:exit 120
[[email protected] Day0801]$ ./buggy 1
+trace 6:result=1
+trace 8:'[' 1 = 1 ']'
+trace 9:echo 'Result is 1; excellent.'
Result is 1; excellent.
+trace 9:exit 0
[[email protected] Day0801]$ ./buggy 2
+trace 6:result=2
+trace 8:'[' 2 = 1 ']'
+trace 10:echo 'Uh-oh, ummm, RUN AWAY! '
Uh-oh, ummm, RUN AWAY! 
+trace 10:exit 120
[[email protected] Day0801]$