Chapter 11. Loops and Branches --11.1 Loops

11.1. Loops

A loop is a block of code that iterates [1] a list of commands as long as the loop control condition is true.

for loops

for arg in [list]

    This is the basic looping construct. It differs significantly from its C counterpart.

    for arg in [list]
    do
     command(s)...
    done

    Note

    The argument list may contain wild cards.

    If do is on same line as for, there needs to be a semicolon after list.

    for arg in [list] ; do

    #!/bin/bash
    # Listing the planets.

    for planet in Mercury Venus Earth Mars Jupiter Saturn Uranus Neptune Pluto
    do
      echo $planet  # Each planet on a separate line.
    done

    echo; echo

    for planet in "Mercury Venus Earth Mars Jupiter Saturn Uranus Neptune Pluto"
        # All planets on same line.
        # Entire 'list' enclosed in quotes creates a single variable.
        # Why? Whitespace incorporated into the variable.
    do
      echo $planet
    done

    echo; echo "Whoops! Pluto is no longer a planet!"

    exit 0

    #!/bin/bash
    # Planets revisited.

    # Associate the name of each planet with its distance from the sun.

    for planet in "Mercury 36" "Venus 67" "Earth 93" "Mars 142" "Jupiter 483"
    do
      set -- $planet  #  Parses variable "planet"
                      #+ and sets positional parameters.
      #  The "--" prevents nasty surprises if $planet is null or
      #+ begins with a dash.

      #  May need to save original positional parameters,
      #+ since they get overwritten.
      #  One way of doing this is to use an array,
      #         original_params=("$@")

      echo "$1        $2,000,000 miles from the sun"
      #-------two  tabs---concatenate zeroes onto parameter $2
    done

    # (Thanks, S.C., for additional clarification.)

    exit 0

    A variable may supply the [list] in a for loop.

    Example 11-3. Fileinfo: operating on a file list contained in a variable

    #!/bin/bash
    # fileinfo.sh

    FILES="/usr/sbin/accept
    /usr/sbin/pwck
    /usr/sbin/chroot
    /usr/bin/fakefile
    /sbin/badblocks
    /sbin/ypbind"     # List of files you are curious about.
                      # Threw in a dummy file, /usr/bin/fakefile.

    echo

    for file in $FILES
    do

      if [ ! -e "$file" ] # Check if file exists.
      then
        echo "$file does not exist."; echo
        continue                # On to next.
       fi

      ls -l $file | awk '{ print $8 "         file size: " $5 }' # Print 2 fields.
      whatis `basename $file` # File info.
      # Note that the whatis database needs to have been set up for this to work.
      # To do this, as root run /usr/bin/makewhatis.
      echo
    done

    exit 0

    If the [list] in a for loop contains wild cards (* and ?) used in filename expansion, then globbing takes place.

    Example 11-4. Operating on files with a for loop

    #!/bin/bash
    # list-glob.sh: Generating [list] in a for-loop, using "globbing" ...
    # Globbing = filename expansion.

    echo

    for file in *
    #           ^  Bash performs filename expansion
    #+ on expressions that globbing recognizes.
    do
      ls -l "$file" # Lists all files in $PWD (current directory).
      #  Recall that the wild card character "*" matches every filename,
      #+ however, in "globbing," it doesn't match dot-files.

      #  If the pattern matches no file, it is expanded to itself.
      #  To prevent this, set the nullglob option
      #+   (shopt -s nullglob).
      #  Thanks, S.C.
    done

    echo; echo

    for file in [jx]*
    do
      rm -f $file    # Removes only files beginning with "j" or "x" in $PWD.
      echo "Removed file \"$file\"".
    done

    echo

    exit 0

    Omitting the in [list] part of a for loop causes the loop to operate on $@ -- the positional parameters. A particularly clever illustration of this is Example A-15. See also Example 15-17.

    Example 11-5. Missing in [list] in a for loop

    #!/bin/bash

    #  Invoke this script both with and without arguments,
    #+ and see what happens.

    for a
    do
     echo -n "$a "
    done

    #  The 'in list' missing, therefore the loop operates on '$@'
    #+ (command-line argument list, including whitespace).

    echo

    exit 0

    It is possible to use command substitution to generate the [list] in a for loop. See also Example 16-54, Example 11-10 and Example 16-48.

    Example 11-6. Generating the [list] in a for loop with command substitution

    #!/bin/bash
    #  for-loopcmd.sh: for-loop with [list]
    #+ generated by command substitution.

    NUMBERS="9 7 3 8 37.53"

    for number in `echo $NUMBERS`  # for number in 9 7 3 8 37.53
    do
      echo -n "$number "
    done

    echo
    exit 0

    Here is a somewhat more complex example of using command substitution to create the [list].

    Example 11-7. A grep replacement for binary files

    #!/bin/bash
    # bin-grep.sh: Locates matching strings in a binary file.

    # A "grep" replacement for binary files.
    # Similar effect to "grep -a"

    E_BADARGS=65
    E_NOFILE=66

    if [ $# -ne 2 ]
    then
      echo "Usage: `basename $0` search_string filename"
      exit $E_BADARGS
    fi

    if [ ! -f "$2" ]
    then
      echo "File \"$2\" does not exist."
      exit $E_NOFILE
    fi


    IFS=$'\012'       # Per suggestion of Anton Filippov.
                      # was:  IFS="\n"
    for word in $( strings "$2" | grep "$1" )
    # The "strings" command lists strings in binary files.
    # Output then piped to "grep", which tests for desired string.
    do
      echo $word
    done

    # As S.C. points out, lines 23 - 30 could be replaced with the simpler
    #    strings "$2" | grep "$1" | tr -s "$IFS" '[\n*]'


    #  Try something like  "./bin-grep.sh mem /bin/ls"
    #+ to exercise this script.

    exit 0

    More of the same.

    Example 11-8. Listing all users on the system

    #!/bin/bash
    # userlist.sh

    PASSWORD_FILE=/etc/passwd
    n=1           # User number

    for name in $(awk 'BEGIN{FS=":"}{print $1}' < "$PASSWORD_FILE" )
    # Field separator = :    ^^^^^^
    # Print first field              ^^^^^^^^
    # Get input from password file  /etc/passwd  ^^^^^^^^^^^^^^^^^
    do
      echo "USER #$n = $name"
      let "n += 1"
    done


    # USER #1 = root
    # USER #2 = bin
    # USER #3 = daemon
    # ...
    # USER #33 = bozo

    exit $?

    #  Discussion:
    #  ----------
    #  How is it that an ordinary user, or a script run by same,
    #+ can read /etc/passwd? (Hint: Check the /etc/passwd file permissions.)
    #  Is this a security hole? Why or why not?

    Yet another example of the [list] resulting from command substitution.

    Example 11-9. Checking all the binaries in a directory for authorship

    #!/bin/bash
    # findstring.sh:
    # Find a particular string in the binaries in a specified directory.

    directory=/usr/bin/
    fstring="Free Software Foundation"  # See which files come from the FSF.

    for file in $( find $directory -type f -name '*' | sort )
    do
      strings -f $file | grep "$fstring" | sed -e "s%$directory%%"
      #  In the "sed" expression,
      #+ it is necessary to substitute for the normal "/" delimiter
      #+ because "/" happens to be one of the characters filtered out.
      #  Failure to do so gives an error message. (Try it.)
    done

    exit $?

    #  Exercise (easy):
    #  ---------------
    #  Convert this script to take command-line parameters
    #+ for $directory and $fstring.

    A final example of [list] / command substitution, but this time the "command" is a function.

    generate_list ()
    {
      echo "one two three"
    }

    for word in $(generate_list)  # Let "word" grab output of function.
    do
      echo "$word"
    done

    # one
    # two
    # three

    The output of a for loop may be piped to a command or commands.

    Example 11-10. Listing the symbolic links in a directory

    #!/bin/bash
    # symlinks.sh: Lists symbolic links in a directory.


    directory=${1-`pwd`}
    #  Defaults to current working directory,
    #+ if not otherwise specified.
    #  Equivalent to code block below.
    # ----------------------------------------------------------
    # ARGS=1                 # Expect one command-line argument.
    #
    # if [ $# -ne "$ARGS" ]  # If not 1 arg...
    # then
    #   directory=`pwd`      # current working directory
    # else
    #   directory=$1
    # fi
    # ----------------------------------------------------------

    echo "symbolic links in directory \"$directory\""

    for file in "$( find $directory -type l )"   # -type l = symbolic links
    do
      echo "$file"
    done | sort                                  # Otherwise file list is unsorted.
    #  Strictly speaking, a loop isn't really necessary here,
    #+ since the output of the "find" command is expanded into a single word.
    #  However, it's easy to understand and illustrative this way.

    #  As Dominik 'Aeneas' Schnitzer points out,
    #+ failing to quote  $( find $directory -type l )
    #+ will choke on filenames with embedded whitespace.
    #  containing whitespace.

    exit 0


    # --------------------------------------------------------
    # Jean Helou proposes the following alternative:

    echo "symbolic links in directory \"$directory\""
    # Backup of the current IFS. One can never be too cautious.
    OLDIFS=$IFS
    IFS=:

    for file in $(find $directory -type l -printf "%p$IFS")
    do     #                              ^^^^^^^^^^^^^^^^
           echo "$file"
           done|sort

    # And, James "Mike" Conley suggests modifying Helou's code thusly:

    OLDIFS=$IFS
    IFS='' # Null IFS means no word breaks
    for file in $( find $directory -type l )
    do
      echo $file
      done | sort

    #  This works in the "pathological" case of a directory name having
    #+ an embedded colon.
    # "This also fixes the pathological case of the directory name having
    #+  a colon (or space in earlier example) as well."

    The stdout of a loop may be redirected to a file, as this slight modification to the previous example shows.

    Example 11-11. Symbolic links in a directory, saved to a file

    #!/bin/bash
    # symlinks.sh: Lists symbolic links in a directory.

    OUTFILE=symlinks.list                         # save-file

    directory=${1-`pwd`}
    #  Defaults to current working directory,
    #+ if not otherwise specified.


    echo "symbolic links in directory \"$directory\"" > "$OUTFILE"
    echo "---------------------------" >> "$OUTFILE"

    for file in "$( find $directory -type l )" # -type l = symbolic links
    do
      echo "$file"
    done | sort >> "$OUTFILE" # stdout of loop
    #           ^^^^^^^^^^^^^                       redirected to save file.

    # echo "Output file = $OUTFILE"

    exit $?

    There is an alternative syntax to a for loop that will look very familiar to C programmers. This requires double parentheses.

    Example 11-12. A C-style for loop

    #!/bin/bash
    # Multiple ways to count up to 10.

    echo

    # Standard syntax.
    for a in 1 2 3 4 5 6 7 8 9 10
    do
      echo -n "$a "
    done

    echo; echo

    # +==========================================+

    # Using "seq" ...
    for a in `seq 10`
    do
      echo -n "$a "
    done

    echo; echo

    # +==========================================+

    # Using brace expansion ...
    # Bash, version 3+.
    for a in {1..10}
    do
      echo -n "$a "
    done

    echo; echo

    # +==========================================+

    # Now, let's do the same, using C-like syntax.

    LIMIT=10

    for ((a=1; a <= LIMIT ; a++))  # Double parentheses, and naked "LIMIT"
    do
      echo -n "$a "
    done                           # A construct borrowed from ksh93.

    echo; echo

    # +=========================================================================+

    # Let's use the C "comma operator" to increment two variables simultaneously.

    for ((a=1, b=1; a <= LIMIT ; a++, b++))
    do  # The comma concatenates operations.
      echo -n "$a-$b "
    done

    echo; echo

    exit 0

    See also Example 27-16, Example 27-17, and Example A-6.

    ---

    Now, a for loop used in a "real-life" context.

    Example 11-13. Using efax in batch mode

    #!/bin/bash
    # Faxing (must have 'efax' package installed).

    EXPECTED_ARGS=2
    E_BADARGS=85
    MODEM_PORT="/dev/ttyS2" # May be different on your machine.
    #                ^^^^^      PCMCIA modem card default port.

    if [ $# -ne $EXPECTED_ARGS ]
    # Check for proper number of command-line args.
    then
       echo "Usage: `basename $0` phone# text-file"
       exit $E_BADARGS
    fi


    if [ ! -f "$2" ]
    then
      echo "File $2 is not a text file."
      # File is not a regular file, or does not exist.
      exit $E_BADARGS
    fi


    fax make $2              #  Create fax-formatted files from text files.

    for file in $(ls $2.0*) #  Concatenate the converted files.
                             #  Uses wild card (filename "globbing")
                 #+ in variable list.
    do
      fil="$fil $file"
    done

    efax -d "$MODEM_PORT" -t "T$1" $fil   # Finally, do the work.
    # Trying adding  -o1  if above line fails.


    #  As S.C. points out, the for-loop can be eliminated with
    #     efax -d /dev/ttyS2 -o1 -t "T$1" $2.0*
    #+ but it's not quite as instructive [grin].

    exit $?   # Also, efax sends diagnostic messages to stdout.

while

    This construct tests for a condition at the top of a loop, and keeps looping as long as that condition is true (returns a 0 exit status). In contrast to a for loop, a while loop finds use in situations where the number of loop repetitions is not known beforehand.

    while [ condition ]
    do
     command(s)...
    done

    The bracket construct in a while loop is nothing more than our old friend, the test brackets used in an if/then test. In fact, a while loop can legally use the more versatile double-brackets construct (while [[ condition ]]).

    As is the case with for loops, placing the do on the same line as the condition test requires a semicolon.

    while [ condition ] ; do

    Note that the test brackets are not mandatory in a while loop. See, for example, the getopts construct.

    Example 11-14. Simple while loop

    #!/bin/bash

    var0=0
    LIMIT=10

    while [ "$var0" -lt "$LIMIT" ]
    #      ^                    ^
    # Spaces, because these are "test-brackets" . . .
    do
      echo -n "$var0 "        # -n suppresses newline.
      #             ^           Space, to separate printed out numbers.

      var0=`expr $var0 + 1`   # var0=$(($var0+1))  also works.
                              # var0=$((var0 + 1)) also works.
                              # let "var0 += 1"    also works.
    done                      # Various other methods also work.

    echo

    exit 0

    Example 11-15. Another while loop

    #!/bin/bash

    echo
                                   # Equivalent to:
    while [ "$var1" != "end" ]     # while test "$var1" != "end"
    do
      echo "Input variable #1 (end to exit) "
      read var1                    # Not 'read $var1' (why?).
      echo "variable #1 = $var1"   # Need quotes because of "#" . . .
      # If input is 'end', echoes it here.
      # Does not test for termination condition until top of loop.
      echo
    done

    exit 0

    A while loop may have multiple conditions. Only the final condition determines when the loop terminates. This necessitates a slightly different loop syntax, however.

    Example 11-16. while loop with multiple conditions

    #!/bin/bash

    var1=unset
    previous=$var1

    while echo "previous-variable = $previous"
          echo
          previous=$var1
          [ "$var1" != end ] # Keeps track of what $var1 was previously.
          # Four conditions on *while*, but only the final one controls loop.
          # The *last* exit status is the one that counts.
    do
    echo "Input variable #1 (end to exit) "
      read var1
      echo "variable #1 = $var1"
    done

    # Try to figure out how this all works.
    # It's a wee bit tricky.

    exit 0

    As with a for loop, a while loop may employ C-style syntax by using the double-parentheses construct (see also Example 8-5).

    Example 11-17. C-style syntax in a while loop

    #!/bin/bash
    # wh-loopc.sh: Count to 10 in a "while" loop.

    LIMIT=10                 # 10 iterations.
    a=1

    while [ "$a" -le $LIMIT ]
    do
      echo -n "$a "
      let "a+=1"
    done                     # No surprises, so far.

    echo; echo

    # +=================================================================+

# Now, we'll repeat with C-like syntax.

    ((a = 1))      # a=1
    # Double parentheses permit space when setting a variable, as in C.

    while (( a <= LIMIT ))   #  Double parentheses,
    do                       #+ and no "$" preceding variables.
      echo -n "$a "
      ((a += 1))             # let "a+=1"
      # Yes, indeed.
      # Double parentheses permit incrementing a variable with C-like syntax.
    done

    echo

    # C and Java programmers can feel right at home in Bash.

    exit 0

    Inside its test brackets, a while loop can call a function.

    t=0

    condition ()
    {
      ((t++))

      if [ $t -lt 5 ]
      then
        return 0  # true
      else
        return 1  # false
      fi
    }

    while condition
    #     ^^^^^^^^^
    #     Function call -- four loop iterations.
    do
      echo "Still going: t = $t"
    done

    # Still going: t = 1
    # Still going: t = 2
    # Still going: t = 3
    # Still going: t = 4

    Similar to the if-test construct, a while loop can omit the test brackets.

    while condition
    do
       command(s) ...
    done

    By coupling the power of the read command with a while loop, we get the handy while read construct, useful for reading and parsing files.

    cat $filename |   # Supply input from a file.
    while read line   # As long as there is another line to read ...
    do
      ...
    done

    # =========== Snippet from "sd.sh" example script ========== #

      while read value   # Read one data point at a time.
      do
        rt=$(echo "scale=$SC; $rt + $value" | bc)
        (( ct++ ))
      done

      am=$(echo "scale=$SC; $rt / $ct" | bc)

      echo $am; return $ct   # This function "returns" TWO values!
      #  Caution: This little trick will not work if $ct > 255!
      #  To handle a larger number of data points,
      #+ simply comment out the "return $ct" above.
    } <"$datafile"   # Feed in data file.

    Note    

    A while loop may have its stdin redirected to a file by a < at its end.

    A while loop may have its stdin supplied by a pipe.
until

    This construct tests for a condition at the top of a loop, and keeps looping as long as that condition is false (opposite of while loop).

    until [ condition-is-true ]
    do
     command(s)...
    done

    Note that an until loop tests for the terminating condition at the top of the loop, differing from a similar construct in some programming languages.

    As is the case with for loops, placing the do on the same line as the condition test requires a semicolon.

    until [ condition-is-true ] ; do

    Example 11-18. until loop

    #!/bin/bash

    END_CONDITION=end

    until [ "$var1" = "$END_CONDITION" ]
    # Tests condition here, at top of loop.
    do
      echo "Input variable #1 "
      echo "($END_CONDITION to exit)"
      read var1
      echo "variable #1 = $var1"
      echo
    done  

    #                     ---                        #

    #  As with "for" and "while" loops,
    #+ an "until" loop permits C-like test constructs.

    LIMIT=10
    var=0

    until (( var > LIMIT ))
    do  # ^^ ^     ^     ^^   No brackets, no $ prefixing variables.
      echo -n "$var "
      (( var++ ))
    done    # 0 1 2 3 4 5 6 7 8 9 10


    exit 0

How to choose between a for loop or a while loop or until loop? In C, you would typically use a for loop when the number of loop iterations is known beforehand. With Bash, however, the situation is fuzzier. The Bash for loop is more loosely structured and more flexible than its equivalent in other languages. Therefore, feel free to use whatever type of loop gets the job done in the simplest way.
Notes
[1]    

Iteration: Repeated execution of a command or group of commands, usually -- but not always, while a given condition holds, or until a given condition is met.