for loops

for (in)

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

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

During each pass through the loop, arg takes on the value of each successive variable in the list.

1 for arg in "$var1" "$var2" "$var3" ... "$varN" 2 # In pass 1 of the loop, $arg = $var1 3 # In pass 2 of the loop, $arg = $var2 4 # In pass 3 of the loop, $arg = $var3 5 # ... 6 # In pass N of the loop, $arg = $varN 7  8 # Arguments in [list] quoted to prevent possible word splitting.

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

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Example 10-1. Simple for loops

1 #!/bin/bash 2 # List the planets. 3  4 for planet in Mercury Venus Earth Mars Jupiter Saturn Uranus Neptune Pluto 5 do 6  echo $planet 7 done 8  9 echo 10  11 # Entire 'list' enclosed in quotes creates a single variable. 12 for planet in "Mercury Venus Earth Mars Jupiter Saturn Uranus Neptune Pluto" 13 do 14  echo $planet 15 done 16  17 exit 0

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Each [list] element may contain multiple parameters. This is useful when processing parameters in groups. In such cases, use the set command (see Example 11-14) to force parsing of each [list] element and assignment of each component to the positional parameters.

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Example 10-2. for loop with two parameters in each [list] element

1 #!/bin/bash 2 # Planets revisited. 3  4 # Associate the name of each planet with its distance from the sun. 5  6 for planet in "Mercury 36" "Venus 67" "Earth 93" "Mars 142" "Jupiter 483" 7 do 8  set -- $planet # Parses variable "planet" and sets positional parameters. 9  # the "--" prevents nasty surprises if $planet is null or begins with a dash. 10  11  # May need to save original positional parameters, since they get overwritten. 12  # One way of doing this is to use an array, 13  # original_params=("$@") 14  15  echo "$1 $2,000,000 miles from the sun" 16  #-------two tabs---concatenate zeroes onto parameter $2 17 done 18  19 # (Thanks, S.C., for additional clarification.) 20  21 exit 0

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

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Example 10-3. Fileinfo: operating on a file list contained in a variable

1 #!/bin/bash 2 # fileinfo.sh 3  4 FILES="/usr/sbin/privatepw 5 /usr/sbin/pwck 6 /usr/sbin/go500gw 7 /usr/bin/fakefile 8 /sbin/mkreiserfs 9 /sbin/ypbind" # List of files you are curious about. 10  # Threw in a dummy file, /usr/bin/fakefile. 11  12 echo 13  14 for file in $FILES 15 do 16  17  if [ ! -e "$file" ] # Check if file exists. 18  then 19  echo "$file does not exist."; echo 20  continue # On to next. 21  fi 22  23  ls -l $file | awk '{ print $9 " file size: " $5 }' # Print 2 fields. 24  whatis `basename $file` # File info. 25  echo 26 done 27  28 exit 0

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The [list] in a for loop may contain filename globbing, that is, using wildcards for filename expansion.

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Example 10-4. Operating on files with a for loop

1 #!/bin/bash 2 # list-glob.sh: Generating [list] in a for-loop using "globbing". 3  4 echo 5  6 for file in * 7 do 8  ls -l "$file" # Lists all files in $PWD (current directory). 9  # Recall that the wild card character "*" matches every filename, 10  # however, in "globbing", it doesn't match dot-files. 11  12  # If the pattern matches no file, it is expanded to itself. 13  # To prevent this, set the nullglob option 14  # (shopt -s nullglob). 15  # Thanks, S.C. 16 done 17  18 echo; echo 19  20 for file in [jx]* 21 do 22  rm -f $file # Removes only files beginning with "j" or "x" in $PWD. 23  echo "Removed file \"$file\"". 24 done 25  26 echo 27  28 exit 0

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Omitting the in [list] part of a for loop causes the loop to operate on $@, the list of arguments given on the command line to the script. A particularly clever illustration of this is Example A-17.

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Example 10-5. Missing in [list] in a for loop

1 #!/bin/bash 2  3 # Invoke this script both with and without arguments, 4 #+ and see what happens. 5  6 for a 7 do 8  echo -n "$a " 9 done 10  11 # The 'in list' missing, therefore the loop operates on '$@' 12 #+ (command-line argument list, including whitespace). 13  14 echo 15  16 exit 0

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It is possible to use command substitution to generate the [list] in a for loop. See also Example 12-39, Example 10-10 and Example 12-33.

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Example 10-6. Generating the [list] in a for loop with command substitution

1 #!/bin/bash 2 # A for-loop with [list] generated by command substitution. 3  4 NUMBERS="9 7 3 8 37.53" 5  6 for number in `echo $NUMBERS` # for number in 9 7 3 8 37.53 7 do 8  echo -n "$number " 9 done 10  11 echo 12 exit 0

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

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Example 10-7. A grep replacement for binary files

1 #!/bin/bash 2 # bin-grep.sh: Locates matching strings in a binary file. 3  4 # A "grep" replacement for binary files. 5 # Similar effect to "grep -a" 6  7 E_BADARGS=65 8 E_NOFILE=66 9  10 if [ $# -ne 2 ] 11 then 12  echo "Usage: `basename $0` string filename" 13  exit $E_BADARGS 14 fi 15  16 if [ ! -f "$2" ] 17 then 18  echo "File \"$2\" does not exist." 19  exit $E_NOFILE 20 fi 21  22  23 for word in $( strings "$2" | grep "$1" ) 24 # The "strings" command lists strings in binary files. 25 # Output then piped to "grep", which tests for desired string. 26 do 27  echo $word 28 done 29  30 # As S.C. points out, the above for-loop could be replaced with the simpler 31 # strings "$2" | grep "$1" | tr -s "$IFS" '[\n*]' 32  33  34 # Try something like "./bin-grep.sh mem /bin/ls" to exercise this script. 35  36 exit 0

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More of the same.

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Example 10-8. Listing all users on the system

1 #!/bin/bash 2 # userlist.sh 3  4 PASSWORD_FILE=/etc/passwd 5 n=1 # User number 6  7 for name in $(awk 'BEGIN{FS=":"}{print $1}' < "$PASSWORD_FILE" ) 8 # Field separator = : ^^^^^^ 9 # Print first field ^^^^^^^^ 10 # Get input from password file ^^^^^^^^^^^^^^^^^ 11 do 12  echo "USER #$n = $name" 13  let "n += 1" 14 done 15  16  17 # USER #1 = root 18 # USER #2 = bin 19 # USER #3 = daemon 20 # ... 21 # USER #30 = bozo 22  23 exit 0

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A final example of the [list] resulting from command substitution.

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Example 10-9. Checking all the binaries in a directory for authorship

1 #!/bin/bash 2 # findstring.sh: 3 # Find a particular string in binaries in a specified directory. 4  5 directory=/usr/bin/ 6 fstring="Free Software Foundation" # See which files come from the FSF. 7  8 for file in $( find $directory -type f -name '*' | sort ) 9 do 10  strings -f $file | grep "$fstring" | sed -e "s%$directory%%" 11  # In the "sed" expression, 12  #+ it is necessary to substitute for the normal "/" delimiter 13  #+ because "/" happens to be one of the characters filtered out. 14  # Failure to do so gives an error message (try it). 15 done 16  17 exit 0 18  19 # Exercise (easy): 20 # --------------- 21 # Convert this script to taking command-line parameters 22 #+ for $directory and $fstring.

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

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

1 #!/bin/bash 2 # symlinks.sh: Lists symbolic links in a directory. 3  4  5 directory=${1-`pwd`} 6 # Defaults to current working directory, 7 #+ if not otherwise specified. 8 # Equivalent to code block below. 9 # ---------------------------------------------------------- 10 # ARGS=1 # Expect one command-line argument. 11 # 12 # if [ $# -ne "$ARGS" ] # If not 1 arg... 13 # then 14 # directory=`pwd` # current working directory 15 # else 16 # directory=$1 17 # fi 18 # ---------------------------------------------------------- 19  20 echo "symbolic links in directory \"$directory\"" 21  22 for file in "$( find $directory -type l )" # -type l = symbolic links 23 do 24  echo "$file" 25 done | sort # Otherwise file list is unsorted. 26  27 # As Dominik 'Aeneas' Schnitzer points out, 28 #+ failing to quote $( find $directory -type l ) 29 #+ will choke on filenames with embedded whitespace. 30 # Even this will only pick up the first field of each argument. 31  32 exit 0 33  34  35 # Jean Helou proposes the following alternative: 36  37 echo "symbolic links in directory \"$directory\"" 38 # Backup of the current IFS. One can never be too cautious. 39 OLDIFS=$IFS 40 IFS=: 41  42 for file in $(find $directory -type l -printf "%p$IFS") 43 do # ^^^^^^^^^^^^^^^^ 44  echo "$file" 45  done|sort

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

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Example 10-11. Symbolic links in a directory, saved to a file

1 #!/bin/bash 2 # symlinks.sh: Lists symbolic links in a directory. 3  4 OUTFILE=symlinks.list # save file 5  6 directory=${1-`pwd`} 7 # Defaults to current working directory, 8 #+ if not otherwise specified. 9  10  11 echo "symbolic links in directory \"$directory\"" > "$OUTFILE" 12 echo "---------------------------" >> "$OUTFILE" 13  14 for file in "$( find $directory -type l )" # -type l = symbolic links 15 do 16  echo "$file" 17 done | sort >> "$OUTFILE" # stdout of loop 18 # ^^^^^^^^^^^^^ redirected to save file. 19  20 exit 0

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

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Example 10-12. A C-like for loop

1 #!/bin/bash 2 # Two ways to count up to 10. 3  4 echo 5  6 # Standard syntax. 7 for a in 1 2 3 4 5 6 7 8 9 10 8 do 9  echo -n "$a " 10 done 11  12 echo; echo 13  14 # +==========================================+ 15  16 # Now, let's do the same, using C-like syntax. 17  18 LIMIT=10 19  20 for ((a=1; a <= LIMIT ; a++)) # Double parentheses, and "LIMIT" with no "$". 21 do 22  echo -n "$a " 23 done # A construct borrowed from 'ksh93'. 24  25 echo; echo 26  27 # +=========================================================================+ 28  29 # Let's use the C "comma operator" to increment two variables simultaneously. 30  31 for ((a=1, b=1; a <= LIMIT ; a++, b++)) # The comma chains together operations. 32 do 33  echo -n "$a-$b " 34 done 35  36 echo; echo 37  38 exit 0

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See also Example 26-15, Example 26-16, and Example A-7.

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

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Example 10-13. Using efax in batch mode

1 #!/bin/bash 2  3 EXPECTED_ARGS=2 4 E_BADARGS=65 5  6 if [ $# -ne $EXPECTED_ARGS ] 7 # Check for proper no. of command line args. 8 then 9  echo "Usage: `basename $0` phone# text-file" 10  exit $E_BADARGS 11 fi 12  13  14 if [ ! -f "$2" ] 15 then 16  echo "File $2 is not a text file" 17  exit $E_BADARGS 18 fi 19  20  21 fax make $2 # Create fax formatted files from text files. 22  23 for file in $(ls $2.0*) # Concatenate the converted files. 24  # Uses wild card in variable list. 25 do 26  fil="$fil $file" 27 done 28  29 efax -d /dev/ttyS3 -o1 -t "T$1" $fil # Do the work. 30  31  32 # As S.C. points out, the for-loop can be eliminated with 33 # efax -d /dev/ttyS3 -o1 -t "T$1" $2.0* 34 # but it's not quite as instructive [grin]. 35  36 exit 0

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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...
done

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

while [condition] ; do

Note that certain specialized while loops, as, for example, a getopts construct, deviate somewhat from the standard template given here.

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Example 10-14. Simple while loop

1 #!/bin/bash 2  3 var0=0 4 LIMIT=10 5  6 while [ "$var0" -lt "$LIMIT" ] 7 do 8  echo -n "$var0 " # -n suppresses newline. 9  var0=`expr $var0 + 1` # var0=$(($var0+1)) also works. 10 done 11  12 echo 13  14 exit 0

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Example 10-15. Another while loop

1 #!/bin/bash 2  3 echo 4  5 while [ "$var1" != "end" ] # while test "$var1" != "end" 6 do # also works. 7  echo "Input variable #1 (end to exit) " 8  read var1 # Not 'read $var1' (why?). 9  echo "variable #1 = $var1" # Need quotes because of "#". 10  # If input is 'end', echoes it here. 11  # Does not test for termination condition until top of loop. 12  echo 13 done 14  15 exit 0

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

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Example 10-16. while loop with multiple conditions

1 #!/bin/bash 2  3 var1=unset 4 previous=$var1 5  6 while echo "previous-variable = $previous" 7  echo 8  previous=$var1 9  [ "$var1" != end ] # Keeps track of what $var1 was previously. 10  # Four conditions on "while", but only last one controls loop. 11  # The *last* exit status is the one that counts. 12 do 13 echo "Input variable #1 (end to exit) " 14  read var1 15  echo "variable #1 = $var1" 16 done 17  18 # Try to figure out how this all works. 19 # It's a wee bit tricky. 20  21 exit 0

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As with a for loop, a while loop may employ C-like syntax by using the double parentheses construct (see also Example 9-29).

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Example 10-17. C-like syntax in a while loop

1 #!/bin/bash 2 # wh-loopc.sh: Count to 10 in a "while" loop. 3  4 LIMIT=10 5 a=1 6  7 while [ "$a" -le $LIMIT ] 8 do 9  echo -n "$a " 10  let "a+=1" 11 done # No surprises, so far. 12  13 echo; echo 14  15 # +=================================================================+ 16  17 # Now, repeat with C-like syntax. 18  19 ((a = 1)) # a=1 20 # Double parentheses permit space when setting a variable, as in C. 21  22 while (( a <= LIMIT )) # Double parentheses, and no "$" preceding variables. 23 do 24  echo -n "$a " 25  ((a += 1)) # let "a+=1" 26  # Yes, indeed. 27  # Double parentheses permit incrementing a variable with C-like syntax. 28 done 29  30 echo 31  32 # Now, C programmers can feel right at home in Bash. 33  34 exit 0

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

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...
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/in loops, placing the do on the same line as the condition test requires a semicolon.

until [condition-is-true] ; do

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Example 10-18. until loop

1 #!/bin/bash 2  3 until [ "$var1" = end ] # Tests condition here, at top of loop. 4 do 5  echo "Input variable #1 " 6  echo "(end to exit)" 7  read var1 8  echo "variable #1 = $var1" 9 done 10  11 exit 0

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