Shell Scripting Tips
(revisited. Again)

Nick Holland 11/21/2023

Target Audience

Keep it neat and simple

Classes of scripts

Deal with errors

Minimize errors

-e : Trap all errors!

#!/bin/ksh -e

cd /some/dir
rm -r *   # Could be bad if the 'cd' failed!
  • Big hammer. Often too big.
  • (though...IF you handle "ALL possible" errors, -e can work)

    • Header for Q&D scripts?

    #!/bin/ksh
yell() { echo "$0: $*" >&2; }
die()  { yell "$*" ; exit 111; }
try()  { "$@" || die "cannot $*"; }

try cd /some/dir
rm -r *   # Could be bad if the 'cd' failed!

     
$ ./test.ksh   
./test.ksh[7]: cd: /some/dir - No such file or directory
./test.ksh: cannot cd /some/dir
  • A more selective hammer, but still...a hammer
  • but possibly what you need for Q&D.

    • Better Header for Q&D scripts?

    #!/bin/ksh
MYTMP=$(mktmp -d /tmp/tmpdata.XXXXXXXX) || exit 254
yell() { echo "$0: $*" >&2; }
die()  { yell "$*" ; exit 111; }
try()  { "$@" || die "cannot $*"; }

function cleanexit () {
    if [[ -n $MYTMP ]]; then
	rm $MYTMP/*
	rmdir $MYTMP
    fi
    exit
}
  [ ... your code here ... ]

cleanexit

    Footer line for all reports:

    Build a shell script template!

  • try/yell/die
  • Site specific stuff
  • run-as user code
  • --> and comments about how to use it
  • /tmp file cleanup
  • footer line
  • Comment out the "irregular" stuff. Delete it if you are not going to use it in a script.

    • Validate all input

  • Don't try to remove undesired things. Only accept things you expect.
  • "tr" is your friend -- "-d" (delete) and "-c" (complement).
  • tr -dc "valid-characters"
  • INPUT=$(echo "$INPUT"|tr -dc "a-zA-Z_0-9-")
  • definitely good for user-facing code
  • Never let users enter data directly into a data file!
  • What if they entered "$(reboot)"?

    • Make friends with set, typeset

    set -f -- Turn off globbing!

    $ A="*"
$ echo $A
Desktop Downloads ShellScriptTips.odp ShellScriptTips01.odp
   [...] 
$ set -f  
$ A="*"
$ echo $A
*
$
  • don't forget if you are turning globbing off!

    • set -- $V -- put $V into $*

    $ A="     A    b  c D   e    "
$ echo "$A"
     A    b  c D   e    
$ echo "$*"

$ set -- $A
$ echo "$*" 
A b c D e
$ echo "$3"
c

    Case insensitivity: tr(1)

    $ V="AbCdEf"
$ V=$(echo $V | tr "A-Z" "a-z")
$ echo $V
abcdef
$

    Case insensitivity: the lazy way

    $ typeset -l V
$ V="AbCdEf" ; echo $V
abcdef
$ typeset -u V         
$ V="AbCdEf" ; echo $V 
ABCDEF

    tput(1)

    BOLD=$(tput bold)
NORM=$(tput sgr0)
echo -n "Enter your input ->$BOLD"
read response
echo -n "$NORM"
    Lots of other options, varying degrees of portability, however.

    General Tips(using KSH!)

    ^^^ From Robert Peichaer (rpe@openbsd)

    Commenting complex pipelines

    diff -U10000 $MYTMP/sftp.log $MYTMP/ftp.log |\
    grep -v -e "^--- " -e "^+++ " -e "^@@ " |\
    sed -e "s/^+/FTP /" -e "s/^ /SFTP /" -e "s/^-/SFTP /" |\
    sed -e "s/ UNKNOWN / /" -e "s/ +.*/]/" |
    awk -F' ' '{print $3, $2, $1}' |\
    sort | uniq

# diff -U10000 $MYTMP/sftp.log $MYTMP/ftp.log |\
#       -> the U1000 gives a lot of context, so all transfers should be
#          here, assuming at least a few of each type of transfer.  May
#          have to examine process as we move people off FTP.
#    grep -v -e "^--- " -e "^+++ " -e "^@@ " |\
#       -> Eliminate the headers of the diff.  Might be better to just
#          lop off the first few lines of the output, and look for @@s later
#          to indicate a problem.
#    sed -e "s/^+/FTP /" -e "s/^ /SFTP /" -e "s/^-/SFTP /" |\
#       -> explained above.  Look for the first char in the line of the diff
#    sed -e "s/ UNKNOWN / /" -e "s/ +.*/]/" |
#       -> Remove excess data
#    awk -F' ' '{print $3, $2, $1}' |\
#       -> reorder output
#    sort | uniq
#       -> remove redundancies.

    Commonly forgotten tools

    "Here documents"

    Much less picky about formatting characters than many other ways to store data. 
    ## Classic:
cat <<__ENDOFTEXT
this text is to be output.
__ENDOFTEXT
  • really messes with indentation 
    # Starting your "here doc" with "<<-" will 
# strip off leading tab characters. 
    cat <<-__ENDOFTEXT
	this is text to be output.
	__ENDOFTEXT

    • Running as root (or anyone)

    # Check to see who we are running as, if not
#   root, re-invoke with sudo.
if [[ $(whoami) != "root ]]; then
    if ! sudo -n -u root $0 $* 2>/dev/null; then
        echo "Sorry, sudo isn't configured properly
        logger "$0 sudo is broke"
        sleep 2
    fi # sudo failed
    exit 
fi # not yet root
# We are now running as root
  • Hopefully, $0 returns 0.

    • Remote administration with SSH

    Optimizing admin with SSH

    Auto-change control

    ksh "select"

    $ PS3="Pick a motorcycle ->"          
$ select MOTORCYCLE in K100 K1200LT Buell Harley; do
> echo "Riding the $MOTORCYCLE today"
> done
1) K100
2) K1200LT
3) Buell
4) Harley
Pick a motorcycle ->2
Riding the K1200LT today
Pick a motorcycle ->^C
    In modern bash now

    Prompt maddness

  • Traditional Unix Prompt: "#" or "$" (or "%")
  • More info:
    • /usr/src $
    • nick@fluffy3 /usr/src/sys/arch/amd64/compile/GENERIC.MP $
  • Crazy info
    • date time PWD user, machine+domain name, shell name and version, history number ...

    • Prompt maddness

    Shell Scripting Tips

  • Discussion? `
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