12.9. Miscellaneous Commands
Command that fit in no special category
- jot, seq
These utilities emit a sequence of integers, with a user-selected increment.
The normal separator character between each integer is a newline, but this can be changed with the -s option.
bash$ seq 5 1 2 3 4 5 bash$ seq -s : 5 1:2:3:4:5
Both jot and seq come in handy in a for loop.
Example 12-39. Using seq to generate loop arguments
1 #!/bin/bash 2 # Using "seq" 3 4 echo 5 6 for a in `seq 80` # or for a in $( seq 80 ) 7 # Same as for a in 1 2 3 4 5 ... 80 (saves much typing!). 8 # May also use 'jot' (if present on system). 9 do 10 echo -n "$a " 11 done # 1 2 3 4 5 ... 80 12 # Example of using the output of a command to generate 13 # the [list] in a "for" loop. 14 15 echo; echo 16 17 18 COUNT=80 # Yes, 'seq' may also take a replaceable parameter. 19 20 for a in `seq $COUNT` # or for a in $( seq $COUNT ) 21 do 22 echo -n "$a " 23 done # 1 2 3 4 5 ... 80 24 25 echo; echo 26 27 BEGIN=75 28 END=80 29 30 for a in `seq $BEGIN $END` 31 # Giving "seq" two arguments starts the count at the first one, 32 #+ and continues until it reaches the second. 33 do 34 echo -n "$a " 35 done # 75 76 77 78 79 80 36 37 echo; echo 38 39 BEGIN=45 40 INTERVAL=5 41 END=80 42 43 for a in `seq $BEGIN $INTERVAL $END` 44 # Giving "seq" three arguments starts the count at the first one, 45 #+ uses the second for a step interval, 46 #+ and continues until it reaches the third. 47 do 48 echo -n "$a " 49 done # 45 50 55 60 65 70 75 80 50 51 echo; echo 52 53 exit 0
- getopt
The getopt command parses command-line options preceded by a dash. This external command corresponds to the getopts Bash builtin, but it is not nearly as versatile.
Example 12-40. Using getopt to parse command-line options
1 #!/bin/bash 2 3 # Try the following when invoking this script. 4 # sh ex33a -a 5 # sh ex33a -abc 6 # sh ex33a -a -b -c 7 # sh ex33a -d 8 # sh ex33a -dXYZ 9 # sh ex33a -d XYZ 10 # sh ex33a -abcd 11 # sh ex33a -abcdZ 12 # sh ex33a -z 13 # sh ex33a a 14 # Explain the results of each of the above. 15 16 E_OPTERR=65 17 18 if [ "$#" -eq 0 ] 19 then # Script needs at least one command-line argument. 20 echo "Usage $0 -[options a,b,c]" 21 exit $E_OPTERR 22 fi 23 24 set -- `getopt "abcd:" "$@"` 25 # Sets positional parameters to command-line arguments. 26 # What happens if you use "$*" instead of "$@"? 27 28 while [ ! -z "$1" ] 29 do 30 case "$1" in 31 -a) echo "Option \"a\"";; 32 -b) echo "Option \"b\"";; 33 -c) echo "Option \"c\"";; 34 -d) echo "Option \"d\" $2";; 35 *) break;; 36 esac 37 38 shift 39 done 40 41 # It is better to use the 'getopts' builtin in a script, 42 #+ rather than 'getopt'. 43 # See "ex33.sh". 44 45 exit 0
- run-parts
The run-parts command [1] executes all the scripts in a target directory, sequentially in ASCII-sorted filename order. Of course, the scripts need to have execute permission.
The crond daemon invokes run-parts to run the scripts in the /etc/cron.* directories.
- yes
In its default behavior the yes command feeds a continuous string of the character y followed by a line feed to stdout. A control-c terminates the run. A different output string may be specified, as in yes different string, which would continually output different string to stdout. One might well ask the purpose of this. From the command line or in a script, the output of yes can be redirected or piped into a program expecting user input. In effect, this becomes a sort of poor man's version of expect.
yes | fsck /dev/hda1 runs fsck non-interactively (careful!).
yes | rm -r dirname has same effect as rm -rf dirname (careful!).
- banner
Prints arguments as a large vertical banner to stdout, using an ASCII character (default '#'). This may be redirected to a printer for hardcopy.
- printenv
Show all the environmental variables set for a particular user.
bash$ printenv | grep HOME HOME=/home/bozo
- lp
The lp and lpr commands send file(s) to the print queue, to be printed as hard copy. [2] These commands trace the origin of their names to the line printers of another era.
bash$ lp file1.txt or bash lp <file1.txt
It is often useful to pipe the formatted output from pr to lp.
bash$ pr -options file1.txt | lp
Formatting packages, such as groff and Ghostscript may send their output directly to lp.
bash$ groff -Tascii file.tr | lp
bash$ gs -options | lp file.ps
Related commands are lpq, for viewing the print queue, and lprm, for removing jobs from the print queue.
- tee
[UNIX borrows an idea here from the plumbing trade.]
This is a redirection operator, but with a difference. Like the plumber's "tee," it permits "siponing off" to a file the output of a command or commands within a pipe, but without affecting the result. This is useful for printing an ongoing process to a file or paper, perhaps to keep track of it for debugging purposes.
tee |------> to file | ===============|=============== command--->----|-operator-->---> result of command(s) ===============================
(The file check.file contains the concatenated sorted "listfiles", before the duplicate lines are removed by uniq.)1 cat listfile* | sort | tee check.file | uniq > result.file
- mkfifo
This obscure command creates a named pipe, a temporary first-in-first-out buffer for transferring data between processes. [3] Typically, one process writes to the FIFO, and the other reads from it. See Example A-16.
- pathchk
This command checks the validity of a filename. If the filename exceeds the maximum allowable length (255 characters) or one or more of the directories in its path is not searchable, then an error message results.
Unfortunately, pathchk does not return a recognizable error code, and it is therefore pretty much useless in a script. Consider instead the file test operators.
- dd
This is the somewhat obscure and much feared "data duplicator" command. Originally a utility for exchanging data on magnetic tapes between UNIX minicomputers and IBM mainframes, this command still has its uses. The dd command simply copies a file (or stdin/stdout), but with conversions. Possible conversions are ASCII/EBCDIC, [4] upper/lower case, swapping of byte pairs between input and output, and skipping and/or truncating the head or tail of the input file. A dd --help lists the conversion and other options that this powerful utility takes.
1 # Exercising 'dd'. 2 3 n=3 4 p=5 5 input_file=project.txt 6 output_file=log.txt 7 8 dd if=$input_file of=$output_file bs=1 skip=$((n-1)) count=$((p-n+1)) 2> /dev/null 9 # Extracts characters n to p from file $input_file. 10 11 12 13 14 echo -n "hello world" | dd cbs=1 conv=unblock 2> /dev/null 15 # Echoes "hello world" vertically. 16 17 18 # Thanks, S.C.
To demonstrate just how versatile dd is, let's use it to capture keystrokes.
Example 12-41. Capturing Keystrokes
1 #!/bin/bash 2 # Capture keystrokes without needing to press ENTER. 3 4 5 keypresses=4 # Number of keypresses to capture. 6 7 8 old_tty_setting=$(stty -g) # Save old terminal settings. 9 10 echo "Press $keypresses keys." 11 stty -icanon -echo # Disable canonical mode. 12 # Disable local echo. 13 keys=$(dd bs=1 count=$keypresses 2> /dev/null) 14 # 'dd' uses stdin, if "if" not specified. 15 16 stty "$old_tty_setting" # Restore old terminal settings. 17 18 echo "You pressed the \"$keys\" keys." 19 20 # Thanks, S.C. for showing the way. 21 exit 0
The dd command can do random access on a data stream.
1 echo -n . | dd bs=1 seek=4 of=file conv=notrunc 2 # The "conv=notrunc" option means that the output file will not be truncated. 3 4 # Thanks, S.C.
The dd command can copy raw data and disk images to and from devices, such as floppies and tape drives (Example A-6). A common use is creating boot floppies.
dd if=kernel-image of=/dev/fd0H1440
Similarly, dd can copy the entire contents of a floppy, even one formatted with a "foreign" OS, to the hard drive as an image file.
dd if=/dev/fd0 of=/home/bozo/projects/floppy.img
Other applications of dd include initializing temporary swap files (Example 29-2) and ramdisks (Example 29-3). It can even do a low-level copy of an entire hard drive partition, although this is not necessarily recommended.
People (with presumably nothing better to do with their time) are constantly thinking of interesting applications of dd.
Example 12-42. Securely deleting a file
1 #!/bin/bash 2 # blotout.sh: Erase all traces of a file. 3 4 # This script overwrites a target file alternately 5 #+ with random bytes, then zeros before finally deleting it. 6 # After that, even examining the raw disk sectors 7 #+ will not reveal the original file data. 8 9 PASSES=7 # Number of file-shredding passes. 10 BLOCKSIZE=1 # I/O with /dev/urandom requires unit block size, 11 #+ otherwise you get weird results. 12 E_BADARGS=70 13 E_NOT_FOUND=71 14 E_CHANGED_MIND=72 15 16 if [ -z "$1" ] # No filename specified. 17 then 18 echo "Usage: `basename $0` filename" 19 exit $E_BADARGS 20 fi 21 22 file=$1 23 24 if [ ! -e "$file" ] 25 then 26 echo "File \"$file\" not found." 27 exit $E_NOT_FOUND 28 fi 29 30 echo; echo -n "Are you absolutely sure you want to blot out \"$file\" (y/n)? " 31 read answer 32 case "$answer" in 33 [nN]) echo "Changed your mind, huh?" 34 exit $E_CHANGED_MIND 35 ;; 36 *) echo "Blotting out file \"$file\".";; 37 esac 38 39 40 flength=$(ls -l "$file" | awk '{print $5}') # Field 5 is file length. 41 42 pass_count=1 43 44 echo 45 46 while [ "$pass_count" -le "$PASSES" ] 47 do 48 echo "Pass #$pass_count" 49 sync # Flush buffers. 50 dd if=/dev/urandom of=$file bs=$BLOCKSIZE count=$flength 51 # Fill with random bytes. 52 sync # Flush buffers again. 53 dd if=/dev/zero of=$file bs=$BLOCKSIZE count=$flength 54 # Fill with zeros. 55 sync # Flush buffers yet again. 56 let "pass_count += 1" 57 echo 58 done 59 60 61 rm -f $file # Finally, delete scrambled and shredded file. 62 sync # Flush buffers a final time. 63 64 echo "File \"$file\" blotted out and deleted."; echo 65 66 67 # This is a fairly secure, if inefficient and slow method 68 #+ of thoroughly "shredding" a file. The "shred" command, 69 #+ part of the GNU "fileutils" package, does the same thing, 70 #+ but more efficiently. 71 72 # The file cannot not be "undeleted" or retrieved by normal methods. 73 # However... 74 #+ this simple method will likely *not* withstand forensic analysis. 75 76 77 # Tom Vier's "wipe" file-deletion package does a much more thorough job 78 #+ of file shredding than this simple script. 79 # http://www.ibiblio.org/pub/Linux/utils/file/wipe-2.0.0.tar.bz2 80 81 # For an in-depth analysis on the topic of file deletion and security, 82 #+ see Peter Gutmann's paper, 83 #+ "Secure Deletion of Data From Magnetic and Solid-State Memory". 84 # http://www.cs.auckland.ac.nz/~pgut001/pubs/secure_del.html 85 86 87 exit 0- od
The od, or octal dump filter converts input (or files) to octal (base-8) or other bases. This is useful for viewing or processing binary data files or otherwise unreadable system device files, such as /dev/urandom, and as a filter for binary data. See Example 9-26 and Example 12-10.
- hexdump
Performs a hexadecimal, octal, decimal, or ASCII dump of a binary file. This command is the rough equivalent of od, above, but not nearly as useful.
- objdump
Displays an object file or binary executable in either hexadecimal form or as a disassembled listing (with the -d option).
bash$ objdump -d /bin/ls /bin/ls: file format elf32-i386 Disassembly of section .init: 080490bc <.init>: 80490bc: 55 push %ebp 80490bd: 89 e5 mov %esp,%ebp . . .
- mcookie
This command generates a "magic cookie", a 128-bit (32-character) pseudorandom hexadecimal number, normally used as an authorization "signature" by the X server. This also available for use in a script as a "quick 'n dirty" random number.
1 random000=`mcookie | sed -e '2p'` 2 # Uses 'sed' to strip off extraneous characters.
Of course, a script could use md5 for the same purpose.
1 # Generate md5 checksum on the script itself. 2 random001=`md5sum $0 | awk '{print $1}'` 3 # Uses 'awk' to strip off the filename.The mcookie command gives yet another way to generate a "unique" filename.
Example 12-43. Filename generator
1 #!/bin/bash 2 # tempfile-name.sh: temp filename generator 3 4 BASE_STR=`mcookie` # 32-character magic cookie. 5 POS=11 # Arbitrary position in magic cookie string. 6 LEN=5 # Get $LEN consecutive characters. 7 8 prefix=temp # This is, after all, a "temp" file. 9 # For more "uniqueness," generate the filename prefix 10 #+ using the same method as the suffix, below. 11 12 suffix=${BASE_STR:POS:LEN} 13 # Extract a 5-character string, starting at position 11. 14 15 temp_filename=$prefix.$suffix 16 # Construct the filename. 17 18 echo "Temp filename = "$temp_filename"" 19 20 # sh tempfile-name.sh 21 # Temp filename = temp.e19ea 22 23 exit 0- units
This utility converts between different units of measure. While normally invoked in interactive mode, units may find use in a script.
Example 12-44. Converting meters to miles
1 #!/bin/bash 2 # unit-conversion.sh 3 4 5 convert_units () # Takes as arguments the units to convert. 6 { 7 cf=$(units "$1" "$2" | sed --silent -e '1p' | awk '{print $2}') 8 # Strip off everything except the actual conversion factor. 9 echo "$cf" 10 } 11 12 Unit1=miles 13 Unit2=meters 14 cfactor=`convert_units $Unit1 $Unit2` 15 quantity=3.73 16 17 result=$(echo $quantity*$cfactor | bc) 18 19 echo "There are $result $Unit2 in $quantity $Unit1." 20 21 # What happens if you pass incompatible units, 22 #+ such as "acres" and "miles" to the function? 23 24 exit 0- m4
A hidden treasure, m4 is a powerful macro processing filter, [5] virtually a complete language. Although originally written as a pre-processor for RatFor, m4 turned out to be useful as a stand-alone utility. In fact, m4 combines some of the functionality of eval, tr, and awk, in addition to its extensive macro expansion facilities.
The April, 2002 issue of Linux Journal has a very nice article on m4 and its uses.
Example 12-45. Using m4
1 #!/bin/bash 2 # m4.sh: Using the m4 macro processor 3 4 # Strings 5 string=abcdA01 6 echo "len($string)" | m4 # 7 7 echo "substr($string,4)" | m4 # A01 8 echo "regexp($string,[0-1][0-1],\&Z)" | m4 # 01Z 9 10 # Arithmetic 11 echo "incr(22)" | m4 # 23 12 echo "eval(99 / 3)" | m4 # 33 13 14 exit 0
- doexec
The doexec command enables passing an arbitrary list of arguments to a binary executable. In particular, passing argv[0] (which corresponds to $0 in a script) lets the executable be invoked by various names, and it can then carry out different sets of actions, according to the name by which it was called. What this amounts to is roundabout way of passing options to an executable.
For example, the /usr/local/bin directory might contain a binary called "aaa". Invoking doexec /usr/local/bin/aaa list would list all those files in the current working directory beginning with an "a", while invoking (the same executable with) doexec /usr/local/bin/aaa delete would delete those files.
The various behaviors of the executable must be defined within the code of the executable itself, analogous to something like the following in a shell script:
1 case `basename $0` in 2 "name1" ) do_something;; 3 "name2" ) do_something_else;; 4 "name3" ) do_yet_another_thing;; 5 * ) bail_out;; 6 esac
- dialog
The dialog family of tools provide a method of calling interactive "dialog" boxes from a script. The more elaborate variations of dialog -- gdialog, Xdialog, and kdialog -- actually invoke X-Windows widgets. See Example 34-14.
