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35.7. Assorted Tips

35.7.1. Ideas for more powerful scripts

  • You have a problem that you want to solve by writing a Bash script. Unfortunately, you don't know quite where to start. One method is to plunge right in and code those parts of the script that come easily, and write the hard parts as pseudo-code.

    #!/bin/bash
    
    ARGCOUNT=1                     # Need name as argument.
    E_WRONGARGS=65
    
    if [ number-of-arguments is-not-equal-to "$ARGCOUNT" ]
    #    ^^^^^^^^^^^^^^^^^^^ ^^^^^^^^^^^^^^^
    #  Can't figure out how to code this . . .
    #+ . . . so write it in pseudo-code.
    
    then
      echo "Usage: name-of-script name"
      #            ^^^^^^^^^^^^^^     More pseudo-code.
      exit $E_WRONGARGS
    fi 
    
    . . .
    
    exit 0
    
    
    # Later on, substitute working code for the pseudo-code.
    
    # Line 6 becomes:
    if [ $# -ne "$ARGCOUNT" ]
    
    # Line 12 becomes:
      echo "Usage: `basename $0` name"

    For an example of using pseudo-code, see the Square Root exercise.

  • To keep a record of which user scripts have run during a particular session or over a number of sessions, add the following lines to each script you want to keep track of. This will keep a continuing file record of the script names and invocation times.

    # Append (>>) following to end of each script tracked.
    
    whoami>> $SAVE_FILE    # User invoking the script.
    echo $0>> $SAVE_FILE   # Script name.
    date>> $SAVE_FILE      # Date and time.
    echo>> $SAVE_FILE      # Blank line as separator.
    
    #  Of course, SAVE_FILE defined and exported as environmental variable in ~/.bashrc
    #+ (something like ~/.scripts-run)
  • The >> operator appends lines to a file. What if you wish to prepend a line to an existing file, that is, to paste it in at the beginning?

    file=data.txt
    title="***This is the title line of data text file***"
    
    echo $title | cat - $file >$file.new
    # "cat -" concatenates stdout to $file.
    #  End result is
    #+ to write a new file with $title appended at *beginning*.

    This is a simplified variant of the Example 19-13 script given earlier. And, of course, sed can also do this.

  • A shell script may act as an embedded command inside another shell script, a Tcl or wish script, or even a Makefile. It can be invoked as an external shell command in a C program using the system() call, i.e., system("script_name");.

  • Setting a variable to the contents of an embedded sed or awk script increases the readability of the surrounding shell wrapper. See Example A-1 and Example 15-20.

  • Put together files containing your favorite and most useful definitions and functions. As necessary, "include" one or more of these "library files" in scripts with either the dot (.) or source command.

    # SCRIPT LIBRARY
    # ------ -------
    
    # Note:
    # No "#!" here.
    # No "live code" either.
    
    
    # Useful variable definitions
    
    ROOT_UID=0             # Root has $UID 0.
    E_NOTROOT=101          # Not root user error. 
    MAXRETVAL=255          # Maximum (positive) return value of a function.
    SUCCESS=0
    FAILURE=-1
    
    
    
    # Functions
    
    Usage ()               # "Usage:" message.
    {
      if [ -z "$1" ]       # No arg passed.
      then
        msg=filename
      else
        msg=$@
      fi
    
      echo "Usage: `basename $0` "$msg""
    }  
    
    
    Check_if_root ()       # Check if root running script.
    {                      # From "ex39.sh" example.
      if [ "$UID" -ne "$ROOT_UID" ]
      then
        echo "Must be root to run this script."
        exit $E_NOTROOT
      fi
    }  
    
    
    CreateTempfileName ()  # Creates a "unique" temp filename.
    {                      # From "ex51.sh" example.
      prefix=temp
      suffix=`eval date +%s`
      Tempfilename=$prefix.$suffix
    }
    
    
    isalpha2 ()            # Tests whether *entire string* is alphabetic.
    {                      # From "isalpha.sh" example.
      [ $# -eq 1 ] || return $FAILURE
    
      case $1 in
      *[!a-zA-Z]*|"") return $FAILURE;;
      *) return $SUCCESS;;
      esac                 # Thanks, S.C.
    }
    
    
    abs ()                           # Absolute value.
    {                                # Caution: Max return value = 255.
      E_ARGERR=-999999
    
      if [ -z "$1" ]                 # Need arg passed.
      then
        return $E_ARGERR             # Obvious error value returned.
      fi
    
      if [ "$1" -ge 0 ]              # If non-negative,
      then                           #
        absval=$1                    # stays as-is.
      else                           # Otherwise,
        let "absval = (( 0 - $1 ))"  # change sign.
      fi  
    
      return $absval
    }
    
    
    tolower ()             #  Converts string(s) passed as argument(s)
    {                      #+ to lowercase.
    
      if [ -z "$1" ]       #  If no argument(s) passed,
      then                 #+ send error message
        echo "(null)"      #+ (C-style void-pointer error message)
        return             #+ and return from function.
      fi  
    
      echo "$@" | tr A-Z a-z
      # Translate all passed arguments ($@).
    
      return
    
    # Use command substitution to set a variable to function output.
    # For example:
    #    oldvar="A seT of miXed-caSe LEtTerS"
    #    newvar=`tolower "$oldvar"`
    #    echo "$newvar"    # a set of mixed-case letters
    #
    # Exercise: Rewrite this function to change lowercase passed argument(s)
    #           to uppercase ... toupper()  [easy].
    }
  • Use special-purpose comment headers to increase clarity and legibility in scripts.

    ## Caution.
    rm -rf *.zzy   ##  The "-rf" options to "rm" are very dangerous,
                   ##+ especially with wild cards.
    
    #+ Line continuation.
    #  This is line 1
    #+ of a multi-line comment,
    #+ and this is the final line.
    
    #* Note.
    
    #o List item.
    
    #> Another point of view.
    while [ "$var1" != "end" ]    #> while test "$var1" != "end"
  • Dotan Barak contributes template code for a progress bar in a script.

    Example 35-15. A Progress Bar

    #!/bin/bash
    # progress-bar.sh
    
    # Author: Dotan Barak (very minor revisions by ABS Guide author).
    # Used in ABS Guide with permission (thanks!).
    
    
    BAR_WIDTH=50
    BAR_CHAR_START="["
    BAR_CHAR_END="]"
    BAR_CHAR_EMPTY="."
    BAR_CHAR_FULL="="
    BRACKET_CHARS=2
    LIMIT=100
    
    print_progress_bar()
    {
            # Calculate how many characters will be full.
            let "full_limit = ((($1 - $BRACKET_CHARS) * $2) / $LIMIT)"
    
            # Calculate how many characters will be empty.
            let "empty_limit = ($1 - $BRACKET_CHARS) - ${full_limit}"
    
            # Prepare the bar.
            bar_line="${BAR_CHAR_START}"
            for ((j=0; j<full_limit; j++)); do
                    bar_line="${bar_line}${BAR_CHAR_FULL}"
            done
    
            for ((j=0; j<empty_limit; j++)); do
                    bar_line="${bar_line}${BAR_CHAR_EMPTY}"
            done
    
            bar_line="${bar_line}${BAR_CHAR_END}"
    
            printf "%3d%% %s" $2 ${bar_line}
    }
    
    # Here is a sample of code that uses it.
    MAX_PERCENT=100
    for ((i=0; i<=MAX_PERCENT; i++)); do
            #
            usleep 10000
            # ... Or run some other commands ...
            #
            print_progress_bar ${BAR_WIDTH} ${i}
            echo -en "\r"
    done
    
    echo ""
    
    exit
  • A particularly clever use of if-test constructs is for comment blocks.

    #!/bin/bash
    
    COMMENT_BLOCK=
    #  Try setting the above variable to some value
    #+ for an unpleasant surprise.
    
    if [ $COMMENT_BLOCK ]; then
    
    Comment block --
    =================================
    This is a comment line.
    This is another comment line.
    This is yet another comment line.
    =================================
    
    echo "This will not echo."
    
    Comment blocks are error-free! Whee!
    
    fi
    
    echo "No more comments, please."
    
    exit 0

    Compare this with using here documents to comment out code blocks.

  • Using the $? exit status variable, a script may test if a parameter contains only digits, so it can be treated as an integer.

    #!/bin/bash
    
    SUCCESS=0
    E_BADINPUT=85
    
    test "$1" -ne 0 -o "$1" -eq 0 2>/dev/null
    # An integer is either equal to 0 or not equal to 0.
    # 2>/dev/null suppresses error message.
    
    if [ $? -ne "$SUCCESS" ]
    then
      echo "Usage: `basename $0` integer-input"
      exit $E_BADINPUT
    fi
    
    let "sum = $1 + 25"             # Would give error if $1 not integer.
    echo "Sum = $sum"
    
    # Any variable, not just a command-line parameter, can be tested this way.
    
    exit 0
  • The 0 - 255 range for function return values is a severe limitation. Global variables and other workarounds are often problematic. An alternative method for a function to communicate a value back to the main body of the script is to have the function write to stdout (usually with echo) the "return value," and assign this to a variable. This is actually a variant of command substitution.

    Example 35-16. Return value trickery

    #!/bin/bash
    # multiplication.sh
    
    multiply ()                     # Multiplies params passed.
    {                               # Will accept a variable number of args.
    
      local product=1
    
      until [ -z "$1" ]             # Until uses up arguments passed...
      do
        let "product *= $1"
        shift
      done
    
      echo $product                 #  Will not echo to stdout,
    }                               #+ since this will be assigned to a variable.
    
    mult1=15383; mult2=25211
    val1=`multiply $mult1 $mult2`
    echo "$mult1 X $mult2 = $val1"
                                    # 387820813
    
    mult1=25; mult2=5; mult3=20
    val2=`multiply $mult1 $mult2 $mult3`
    echo "$mult1 X $mult2 X $mult3 = $val2"
                                    # 2500
    
    mult1=188; mult2=37; mult3=25; mult4=47
    val3=`multiply $mult1 $mult2 $mult3 $mult4`
    echo "$mult1 X $mult2 X $mult3 X $mult4 = $val3"
                                    # 8173300
    
    exit 0

    The same technique also works for alphanumeric strings. This means that a function can "return" a non-numeric value.

    capitalize_ichar ()          #  Capitalizes initial character
    {                            #+ of argument string(s) passed.
    
      string0="$@"               # Accepts multiple arguments.
    
      firstchar=${string0:0:1}   # First character.
      string1=${string0:1}       # Rest of string(s).
    
      FirstChar=`echo "$firstchar" | tr a-z A-Z`
                                 # Capitalize first character.
    
      echo "$FirstChar$string1"  # Output to stdout.
    
    }  
    
    newstring=`capitalize_ichar "every sentence should start with a capital letter."`
    echo "$newstring"          # Every sentence should start with a capital letter.

    It is even possible for a function to "return" multiple values with this method.

    Example 35-17. Even more return value trickery

    #!/bin/bash
    # sum-product.sh
    # A function may "return" more than one value.
    
    sum_and_product ()   # Calculates both sum and product of passed args.
    {
      echo $(( $1 + $2 )) $(( $1 * $2 ))
    # Echoes to stdout each calculated value, separated by space.
    }
    
    echo
    echo "Enter first number "
    read first
    
    echo
    echo "Enter second number "
    read second
    echo
    
    retval=`sum_and_product $first $second`      # Assigns output of function.
    sum=`echo "$retval" | awk '{print $1}'`      # Assigns first field.
    product=`echo "$retval" | awk '{print $2}'`  # Assigns second field.
    
    echo "$first + $second = $sum"
    echo "$first * $second = $product"
    echo
    
    exit 0

    Caution

    There can be only one echo statement in the function for this to work. If you alter the previous example:

    sum_and_product ()
    {
      echo "This is the sum_and_product function." # This messes things up!
      echo $(( $1 + $2 )) $(( $1 * $2 ))
    }
    ...
    retval=`sum_and_product $first $second`      # Assigns output of function.
    # Now, this will not work correctly.
  • Next in our bag of tricks are techniques for passing an array to a function, then "returning" an array back to the main body of the script.

    Passing an array involves loading the space-separated elements of the array into a variable with command substitution. Getting an array back as the "return value" from a function uses the previously mentioned strategem of echoing the array in the function, then invoking command substitution and the ( ... ) operator to assign it to an array.

    Example 35-18. Passing and returning arrays

    #!/bin/bash
    # array-function.sh: Passing an array to a function and ...
    #                   "returning" an array from a function
    
    
    Pass_Array ()
    {
      local passed_array   # Local variable!
      passed_array=( `echo "$1"` )
      echo "${passed_array[@]}"
      #  List all the elements of the new array
      #+ declared and set within the function.
    }
    
    
    original_array=( element1 element2 element3 element4 element5 )
    
    echo
    echo "original_array = ${original_array[@]}"
    #                      List all elements of original array.
    
    
    # This is the trick that permits passing an array to a function.
    # **********************************
    argument=`echo ${original_array[@]}`
    # **********************************
    #  Pack a variable
    #+ with all the space-separated elements of the original array.
    #
    # Attempting to just pass the array itself will not work.
    
    
    # This is the trick that allows grabbing an array as a "return value".
    # *****************************************
    returned_array=( `Pass_Array "$argument"` )
    # *****************************************
    # Assign 'echoed' output of function to array variable.
    
    echo "returned_array = ${returned_array[@]}"
    
    echo "============================================================="
    
    #  Now, try it again,
    #+ attempting to access (list) the array from outside the function.
    Pass_Array "$argument"
    
    # The function itself lists the array, but ...
    #+ accessing the array from outside the function is forbidden.
    echo "Passed array (within function) = ${passed_array[@]}"
    # NULL VALUE since the array is a variable local to the function.
    
    echo
    
    ############################################
    
    # And here is an even more explicit example:
    
    ret_array ()
    {
      for element in {11..20}
      do
        echo "$element "   #  Echo individual elements
      done                 #+ of what will be assembled into an array.
    }
    
    arr=( $(ret_array) )   #  Assemble into array.
    
    echo "Capturing array \"arr\" from function ret_array () ..."
    echo "Third element of array \"arr\" is ${arr[2]}."   # 13  (zero-indexed)
    echo -n "Entire array is: "
    echo ${arr[@]}                # 11 12 13 14 15 16 17 18 19 20
    
    echo
    
    exit 0

    For a more elaborate example of passing arrays to functions, see Example A-10.

  • Using the double-parentheses construct, it is possible to use C-style syntax for setting and incrementing/decrementing variables and in for and while loops. See Example 11-12 and Example 11-17.

  • Setting the path and umask at the beginning of a script makes it more "portable" -- more likely to run on a "foreign" machine whose user may have bollixed up the $PATH and umask.
    #!/bin/bash
    PATH=/bin:/usr/bin:/usr/local/bin ; export PATH
    umask 022   # Files that the script creates will have 755 permission.
    
    # Thanks to Ian D. Allen, for this tip.
  • A useful scripting technique is to repeatedly feed the output of a filter (by piping) back to the same filter, but with a different set of arguments and/or options. Especially suitable for this are tr and grep.

    # From "wstrings.sh" example.
    
    wlist=`strings "$1" | tr A-Z a-z | tr '[:space:]' Z | \
    tr -cs '[:alpha:]' Z | tr -s '\173-\377' Z | tr Z ' '`

    Example 35-19. Fun with anagrams

    #!/bin/bash
    # agram.sh: Playing games with anagrams.
    
    # Find anagrams of...
    LETTERSET=etaoinshrdlu
    FILTER='.......'       # How many letters minimum?
    #       1234567
    
    anagram "$LETTERSET" | # Find all anagrams of the letterset...
    grep "$FILTER" |       # With at least 7 letters,
    grep '^is' |           # starting with 'is'
    grep -v 's$' |         # no plurals
    grep -v 'ed$'          # no past tense verbs
    # Possible to add many combinations of conditions and filters.
    
    #  Uses "anagram" utility
    #+ that is part of the author's "yawl" word list package.
    #  http://ibiblio.org/pub/Linux/libs/yawl-0.3.2.tar.gz
    #  http://bash.neuralshortcircuit.com/yawl-0.3.2.tar.gz
    
    exit 0                 # End of code.
    
    
    bash$ sh agram.sh
    islander
    isolate
    isolead
    isotheral
    
    
    
    #  Exercises:
    #  ---------
    #  Modify this script to take the LETTERSET as a command-line parameter.
    #  Parameterize the filters in lines 11 - 13 (as with $FILTER),
    #+ so that they can be specified by passing arguments to a function.
    
    #  For a slightly different approach to anagramming,
    #+ see the agram2.sh script.

    See also Example 29-4, Example 16-25, and Example A-9.

  • Use "anonymous here documents" to comment out blocks of code, to save having to individually comment out each line with a #. See Example 19-11.

  • Running a script on a machine that relies on a command that might not be installed is dangerous. Use whatis to avoid potential problems with this.

    CMD=command1                 # First choice.
    PlanB=command2               # Fallback option.
    
    command_test=$(whatis "$CMD" | grep 'nothing appropriate')
    #  If 'command1' not found on system , 'whatis' will return
    #+ "command1: nothing appropriate."
    #
    #  A safer alternative is:
    #     command_test=$(whereis "$CMD" | grep \/)
    #  But then the sense of the following test would have to be reversed,
    #+ since the $command_test variable holds content only if
    #+ the $CMD exists on the system.
    #     (Thanks, bojster.)
    
    
    if [[ -z "$command_test" ]]  # Check whether command present.
    then
      $CMD option1 option2       #  Run command1 with options.
    else                         #  Otherwise,
      $PlanB                     #+ run command2. 
    fi
  • An if-grep test may not return expected results in an error case, when text is output to stderr, rather that stdout.
    if ls -l nonexistent_filename | grep -q 'No such file or directory'
      then echo "File \"nonexistent_filename\" does not exist."
    fi
    Redirecting stderr to stdout fixes this.
    if ls -l nonexistent_filename 2>&1 | grep -q 'No such file or directory'
    #                             ^^^^
      then echo "File \"nonexistent_filename\" does not exist."
    fi
    
    # Thanks, Chris Martin, for pointing this out.
  • If you absolutely must access a subshell variable outside the subshell, here's a way to do it.
    TMPFILE=tmpfile                  # Create a temp file to store the variable.
    
    (   # Inside the subshell ...
    inner_variable=Inner
    echo $inner_variable
    echo $inner_variable >>$TMPFILE  # Append to temp file.
    )
    
        # Outside the subshell ...
    
    echo; echo "-----"; echo
    echo $inner_variable             # Null, as expected.
    echo "-----"; echo
    
    # Now ...
    read inner_variable <$TMPFILE    # Read back shell variable.
    rm -f "$TMPFILE"                 # Get rid of temp file.
    echo "$inner_variable"           # It's an ugly kludge, but it works.
  • The run-parts command is handy for running a set of command scripts in a particular sequence, especially in combination with cron or at.

  • For doing multiple revisions on a complex script, use the rcs Revision Control System package.

    Among other benefits of this is automatically updated ID header tags. The co command in rcs does a parameter replacement of certain reserved key words, for example, replacing # $Id$ in a script with something like:
    # $Id: hello-world.sh,v 1.1 2004/10/16 02:43:05 bozo Exp $

35.7.2. Widgets

It would be nice to be able to invoke X-Windows widgets from a shell script. There happen to exist several packages that purport to do so, namely Xscript, Xmenu, and widtools. The first two of these no longer seem to be maintained. Fortunately, it is still possible to obtain widtools here.

Caution

The widtools (widget tools) package requires the XForms library to be installed. Additionally, the Makefile needs some judicious editing before the package will build on a typical Linux system. Finally, three of the six widgets offered do not work (and, in fact, segfault).

The dialog family of tools offers a method of calling "dialog" widgets from a shell script. The original dialog utility works in a text console, but its successors, gdialog, Xdialog, and kdialog use X-Windows-based widget sets.

Example 35-20. Widgets invoked from a shell script

#!/bin/bash
# dialog.sh: Using 'gdialog' widgets.

# Must have 'gdialog' installed on your system to run this script.
# Or, you can replace all instance of 'gdialog' below with 'kdialog' ...
# Version 1.1 (corrected 04/05/05)

# This script was inspired by the following article.
#     "Scripting for X Productivity," by Marco Fioretti,
#      LINUX JOURNAL, Issue 113, September 2003, pp. 86-9.
# Thank you, all you good people at LJ.


# Input error in dialog box.
E_INPUT=65
# Dimensions of display, input widgets.
HEIGHT=50
WIDTH=60

# Output file name (constructed out of script name).
OUTFILE=$0.output

# Display this script in a text widget.
gdialog --title "Displaying: $0" --textbox $0 $HEIGHT $WIDTH



# Now, we'll try saving input in a file.
echo -n "VARIABLE=" > $OUTFILE
gdialog --title "User Input" --inputbox "Enter variable, please:" \
$HEIGHT $WIDTH 2>> $OUTFILE


if [ "$?" -eq 0 ]
# It's good practice to check exit status.
then
  echo "Executed \"dialog box\" without errors."
else
  echo "Error(s) in \"dialog box\" execution."
        # Or, clicked on "Cancel", instead of "OK" button.
  rm $OUTFILE
  exit $E_INPUT
fi



# Now, we'll retrieve and display the saved variable.
. $OUTFILE   # 'Source' the saved file.
echo "The variable input in the \"input box\" was: "$VARIABLE""


rm $OUTFILE  # Clean up by removing the temp file.
             # Some applications may need to retain this file.

exit $?

# Exercise: Rewrite this script using the 'zenity' widget set.
The xmessage command is a simple method of popping up a message/query window. For example:
xmessage Fatal error in script! -button exit
The latest entry in the widget sweepstakes is zenity. This utility pops up GTK+ dialog widgets-and-windows, and it works very nicely within a script.
get_info ()
{
  zenity --entry       #  Pops up query window . . .
                       #+ and prints user entry to stdout.

                       #  Also try the --calendar and --scale options.
}

answer=$( get_info )   #  Capture stdout in $answer variable.

echo "User entered: "$answer""

For other methods of scripting with widgets, try Tk or wish (Tcl derivatives), PerlTk (Perl with Tk extensions), tksh (ksh with Tk extensions), XForms4Perl (Perl with XForms extensions), Gtk-Perl (Perl with Gtk extensions), or PyQt (Python with Qt extensions).


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