Chapter 10. Loops and Branches
What needs this iteration, woman? --Shakespeare, Othello |
Operations on code blocks are the key to structured and organized shell scripts. Looping and branching constructs provide the tools for accomplishing this.
10.1. 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 arg in [list]
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
Example 10-1. Simple for loops
1 #!/bin/bash 2 # Listing the planets. 3 4 for planet in Mercury Venus Earth Mars Jupiter Saturn Uranus Neptune Pluto 5 do 6 echo $planet # Each planet on a separate line. 7 done 8 9 echo 10 11 for planet in "Mercury Venus Earth Mars Jupiter Saturn Uranus Neptune Pluto" 12 # All planets on same line. 13 # Entire 'list' enclosed in quotes creates a single variable. 14 # Why? Whitespace incorporated into the variable. 15 do 16 echo $planet 17 done 18 19 exit 0
Each [list] element may contain multiple parameters. This is useful when processing parameters in groups. In such cases, use the set command (see Example 14-16) to force parsing of each [list] element and assignment of each component to the positional parameters.
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" 9 #+ and sets positional parameters. 10 # The "--" prevents nasty surprises if $planet is null or 11 #+ begins with a dash. 12 13 # May need to save original positional parameters, 14 #+ since they get overwritten. 15 # One way of doing this is to use an array, 16 # original_params=("$@") 17 18 echo "$1 $2,000,000 miles from the sun" 19 #-------two tabs---concatenate zeroes onto parameter $2 20 done 21 22 # (Thanks, S.C., for additional clarification.) 23 24 exit 0A variable may supply the [list] in a for loop.
Example 10-3. Fileinfo: operating on a file list contained in a variable
1 #!/bin/bash 2 # fileinfo.sh 3 4 FILES="/usr/sbin/accept 5 /usr/sbin/pwck 6 /usr/sbin/chroot 7 /usr/bin/fakefile 8 /sbin/badblocks 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 $file` # File info. 25 # Note that the whatis database needs to have been set up for this to work. 26 # To do this, as root run /usr/bin/makewhatis. 27 echo 28 done 29 30 exit 0If the [list] in a for loop contains wildcards (* and ?) used in filename expansion, then globbing takes place.
Example 10-4. Operating on files with a for loop
1 #!/bin/bash 2 # list-glob.sh: Generating [list] in a for-loop, using "globbing" 3 4 echo 5 6 for file in * 7 # ^ Bash performs filename expansion 8 #+ on expressions that globbing recognizes. 9 do 10 ls -l "$file" # Lists all files in $PWD (current directory). 11 # Recall that the wild card character "*" matches every filename, 12 #+ however, in "globbing," it doesn't match dot-files. 13 14 # If the pattern matches no file, it is expanded to itself. 15 # To prevent this, set the nullglob option 16 #+ (shopt -s nullglob). 17 # Thanks, S.C. 18 done 19 20 echo; echo 21 22 for file in [jx]* 23 do 24 rm -f $file # Removes only files beginning with "j" or "x" in $PWD. 25 echo "Removed file \"$file\"". 26 done 27 28 echo 29 30 exit 0
Omitting the in [list] part of a for loop causes the loop to operate on $@ -- the positional parameters. A particularly clever illustration of this is Example A-16. See also Example 14-17.
Example 10-5. Missing in [list] in a for loop
1 #!/bin/bash 2 3 # Invoke this script both with and without arguments, 4 #+ and see what happens. 5 6 for a 7 do 8 echo -n "$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
It is possible to use command substitution to generate the [list] in a for loop. See also Example 15-50, Example 10-10 and Example 15-44.
Example 10-6. Generating the [list] in a for loop with command substitution
1 #!/bin/bash 2 # for-loopcmd.sh: for-loop with [list] 3 #+ generated by command substitution. 4 5 NUMBERS="9 7 3 8 37.53" 6 7 for number in `echo $NUMBERS` # for number in 9 7 3 8 37.53 8 do 9 echo -n "$number " 10 done 11 12 echo 13 exit 0
Here is a somewhat more complex example of using command substitution to create the [list].
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` search_string filename" 13 exit $E_BADARGS 14 fi 15 16 if [ ! -f "$2" ] 17 then 18 echo "File \"$2\" does not exist." 19 exit $E_NOFILE 20 fi 21 22 23 IFS=$'\012' # Per suggestion of Anton Filippov. 24 # was: IFS="\n" 25 for word in $( strings "$2" | grep "$1" ) 26 # The "strings" command lists strings in binary files. 27 # Output then piped to "grep", which tests for desired string. 28 do 29 echo $word 30 done 31 32 # As S.C. points out, lines 23 - 30 could be replaced with the simpler 33 # strings "$2" | grep "$1" | tr -s "$IFS" '[\n*]' 34 35 36 # Try something like "./bin-grep.sh mem /bin/ls" 37 #+ to exercise this script. 38 39 exit 0
More of the same.
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 24 25 # Exercise: 26 # -------- 27 # How is it that an ordinary user (or a script run by same) 28 #+ can read /etc/passwd? 29 # Isn't this a security hole? Why or why not?A final example of the [list] resulting from command substitution.
Example 10-9. Checking all the binaries in a directory for authorship
1 #!/bin/bash 2 # findstring.sh: 3 # Find a particular string in the 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 take command-line parameters 22 #+ for $directory and $fstring.
The output of a for loop may be piped to a command or commands.
Example 10-10. Listing the symbolic links in a directory
1 #!/bin/bash 2 # symlinks.sh: Lists symbolic links in a directory. 3 4 5 directory=${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 # Strictly speaking, a loop isn't really necessary here, 27 #+ since the output of the "find" command is expanded into a single word. 28 # However, it's easy to understand and illustrative this way. 29 30 # As Dominik 'Aeneas' Schnitzer points out, 31 #+ failing to quote $( find $directory -type l ) 32 #+ will choke on filenames with embedded whitespace. 33 # Even this will only pick up the first field of each argument. 34 35 exit 0 36 37 38 # -------------------------------------------------------- 39 # Jean Helou proposes the following alternative: 40 41 echo "symbolic links in directory \"$directory\"" 42 # Backup of the current IFS. One can never be too cautious. 43 OLDIFS=$IFS 44 IFS=: 45 46 for file in $(find $directory -type l -printf "%p$IFS") 47 do # ^^^^^^^^^^^^^^^^ 48 echo "$file" 49 done|sort 50 51 # And, James "Mike" Conley suggests modifying Helou's code thusly: 52 53 OLDIFS=$IFS 54 IFS='' # Null IFS means no word breaks 55 for file in $( find $directory -type l ) 56 do 57 echo $file 58 done | sort 59 60 # This works in the "pathological" case of a directory name having 61 #+ an embedded colon. 62 # "This also fixes the pathological case of the directory name having 63 #+ a colon (or space in earlier example) as well." 64The stdout of a loop may be redirected to a file, as this slight modification to the previous example shows.
Example 10-11. Symbolic links in a directory, saved to a file
1 #!/bin/bash 2 # symlinks.sh: Lists symbolic links in a directory. 3 4 OUTFILE=symlinks.list # save file 5 6 directory=${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 >> "$OUTFILE" # stdout of loop 18 # ^^^^^^^^^^^^^ redirected to save file. 19 20 exit 0There is an alternative syntax to a for loop that will look very familiar to C programmers. This requires double parentheses.
Example 10-12. A C-style for loop
1 #!/bin/bash 2 # Two ways to count up to 10. 3 4 echo 5 6 # Standard syntax. 7 for a in 1 2 3 4 5 6 7 8 9 10 8 do 9 echo -n "$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
See also Example 26-15, Example 26-16, and Example A-6.
---
Now, a for loop used in a "real-life" context.
Example 10-13. Using efax in batch mode
1 #!/bin/bash 2 # Faxing (must have 'efax' package installed). 3 4 EXPECTED_ARGS=2 5 E_BADARGS=65 6 MODEM_PORT="/dev/ttyS2" # May be different on your machine. 7 # ^^^^^ PCMCIA modem card default port. 8 9 if [ $# -ne $EXPECTED_ARGS ] 10 # Check for proper number of command line args. 11 then 12 echo "Usage: `basename $0` phone# text-file" 13 exit $E_BADARGS 14 fi 15 16 17 if [ ! -f "$2" ] 18 then 19 echo "File $2 is not a text file." 20 # File is not a regular file, or does not exist. 21 exit $E_BADARGS 22 fi 23 24 25 fax make $2 # Create fax-formatted files from text files. 26 27 for file in $(ls $2.0*) # Concatenate the converted files. 28 # Uses wild card (filename "globbing") 29 #+ in variable list. 30 do 31 fil="$fil $file" 32 done 33 34 efax -d "$MODEM_PORT" -t "T$1" $fil # Finally, do the work. 35 # Trying adding -o1 if above line fails. 36 37 38 # As S.C. points out, the for-loop can be eliminated with 39 # efax -d /dev/ttyS2 -o1 -t "T$1" $2.0* 40 #+ but it's not quite as instructive [grin]. 41 42 exit $? # Also, efax sends diagnostic messages to stdout.
- while
This construct tests for a condition at the top of a loop, and keeps looping as long as that condition is true (returns a 0 exit status). In contrast to a for loop, a while loop finds use in situations where the number of loop repetitions is not known beforehand.
while [ condition ]
do
command(s)...
doneThe bracket construct in a while loop is nothing more than our old friend, the test brackets used in an if/then test. In fact, a while loop can legally use the more versatile double brackets construct (while [[ condition ]]).
As is the case with for loops, placing the do on the same line as the condition test requires a semicolon.
while [ condition ] ; do
Note that the test brackets are not mandatory in a while loop. See, for example, the getopts construct.
Example 10-14. Simple while loop
1 #!/bin/bash 2 3 var0=0 4 LIMIT=10 5 6 while [ "$var0" -lt "$LIMIT" ] 7 # ^ ^ 8 # Spaces, because these are "test-brackets" . . . 9 do 10 echo -n "$var0 " # -n suppresses newline. 11 # ^ Space, to separate printed out numbers. 12 13 var0=`expr $var0 + 1` # var0=$(($var0+1)) also works. 14 # var0=$((var0 + 1)) also works. 15 # let "var0 += 1" also works. 16 done # Various other methods also work. 17 18 echo 19 20 exit 0
Example 10-15. Another while loop
1 #!/bin/bash 2 3 echo 4 # Equivalent to: 5 while [ "$var1" != "end" ] # while test "$var1" != "end" 6 do 7 echo "Input variable #1 (end to exit) " 8 read var1 # Not 'read $var1' (why?). 9 echo "variable #1 = $var1" # Need quotes because of "#" . . . 10 # If input is 'end', echoes it here. 11 # Does not test for termination condition until top of loop. 12 echo 13 done 14 15 exit 0
A while loop may have multiple conditions. Only the final condition determines when the loop terminates. This necessitates a slightly different loop syntax, however.
Example 10-16. while loop with multiple conditions
1 #!/bin/bash 2 3 var1=unset 4 previous=$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 = $var1" 16 done 17 18 # Try to figure out how this all works. 19 # It's a wee bit tricky. 20 21 exit 0
As with a for loop, a while loop may employ C-style syntax by using the double parentheses construct (see also Example 9-32).
Example 10-17. C-style syntax in a while loop
1 #!/bin/bash 2 # wh-loopc.sh: Count to 10 in a "while" loop. 3 4 LIMIT=10 5 a=1 6 7 while [ "$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 # C programmers can feel right at home in Bash. 33 34 exit 0
Inside its test brackets, a while loop can call a function.
1 t=0 2 3 condition () 4 { 5 ((t++)) 6 7 if [ $t -lt 5 ] 8 then 9 return 0 # true 10 else 11 return 1 # false 12 fi 13 } 14 15 while condition 16 # ^^^^^^^^^ 17 # Function call -- four loop iterations. 18 do 19 echo "Still going: t = $t" 20 done 21 22 # Still going: t = 1 23 # Still going: t = 2 24 # Still going: t = 3 25 # Still going: t = 4Similar to the if-test construct, a while loop can omit the test brackets.
1 while condition 2 do 3 ... 4 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.
1 cat $filename | # Supply input from a file. 2 while read line # As long as there is another line to read ... 3 do 4 ... 5 done
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)...
doneNote 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
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.