16.2. Complex Commands

Commands for more advanced users

find

-exec COMMAND \;

Carries out COMMAND on each file that find matches. The command sequence terminates with ; (the ";" is escaped to make certain the shell passes it to find literally, without interpreting it as a special character).

bash$ find ~/ -name '*.txt' /home/bozo/.kde/share/apps/karm/karmdata.txt /home/bozo/misc/irmeyc.txt /home/bozo/test-scripts/1.txt

If COMMAND contains {}, then find substitutes the full path name of the selected file for "{}".

1 find ~/ -name 'core*' -exec rm {} \; 2 # Removes all core dump files from user's home directory.

1 find /home/bozo/projects -mtime -1 2 # ^ Note minus sign! 3 # Lists all files in /home/bozo/projects directory tree 4 #+ that were modified within the last day (current_day - 1). 5 # 6 find /home/bozo/projects -mtime 1 7 # Same as above, but modified *exactly* one day ago. 8 # 9 # mtime = last modification time of the target file 10 # ctime = last status change time (via 'chmod' or otherwise) 11 # atime = last access time 12  13 DIR=/home/bozo/junk_files 14 find "$DIR" -type f -atime +5 -exec rm {} \; 15 # ^ ^^ 16 # Curly brackets are placeholder for the path name output by "find." 17 # 18 # Deletes all files in "/home/bozo/junk_files" 19 #+ that have not been accessed in *at least* 5 days (plus sign ... +5). 20 # 21 # "-type filetype", where 22 # f = regular file 23 # d = directory 24 # l = symbolic link, etc. 25 # 26 # (The 'find' manpage and info page have complete option listings.)

1 find /etc -exec grep '[0-9][0-9]*[.][0-9][0-9]*[.][0-9][0-9]*[.][0-9][0-9]*' {} \; 2  3 # Finds all IP addresses (xxx.xxx.xxx.xxx) in /etc directory files. 4 # There a few extraneous hits. Can they be filtered out? 5  6 # Possibly by: 7  8 find /etc -type f -exec cat '{}' \; | tr -c '.[:digit:]' '\n' \ 9 | grep '^[^.][^.]*\.[^.][^.]*\.[^.][^.]*\.[^.][^.]*$' 10 # 11 # [:digit:] is one of the character classes 12 #+ introduced with the POSIX 1003.2 standard. 13  14 # Thanks, Stéphane Chazelas.

The -exec option to find should not be confused with the exec shell builtin.

---

Example 16-3. Badname, eliminate file names in current directory containing bad characters and whitespace.

1 #!/bin/bash 2 # badname.sh 3 # Delete filenames in current directory containing bad characters. 4  5 for filename in * 6 do 7  badname=`echo "$filename" | sed -n /[\+\{\;\"\\\=\?~\(\)\<\>\&\*\|\$]/p` 8 # badname=`echo "$filename" | sed -n '/[+{;"\=?~()<>&*|$]/p'` also works. 9 # Deletes files containing these nasties: + { ; " \ = ? ~ ( ) < > & * | $ 10 # 11  rm $badname 2>/dev/null 12 # ^^^^^^^^^^^ Error messages deep-sixed. 13 done 14  15 # Now, take care of files containing all manner of whitespace. 16 find . -name "* *" -exec rm -f {} \; 17 # The path name of the file that _find_ finds replaces the "{}". 18 # The '\' ensures that the ';' is interpreted literally, as end of command. 19  20 exit 0 21  22 #--------------------------------------------------------------------- 23 # Commands below this line will not execute because of _exit_ command. 24  25 # An alternative to the above script: 26 find . -name '*[+{;"\\=?~()<>&*|$ ]*' -maxdepth 0 \ 27 -exec rm -f '{}' \; 28 # The "-maxdepth 0" option ensures that _find_ will not search 29 #+ subdirectories below $PWD. 30  31 # (Thanks, S.C.)

---

---

Example 16-4. Deleting a file by its inode number

1 #!/bin/bash 2 # idelete.sh: Deleting a file by its inode number. 3  4 # This is useful when a filename starts with an illegal character, 5 #+ such as ? or -. 6  7 ARGCOUNT=1 # Filename arg must be passed to script. 8 E_WRONGARGS=70 9 E_FILE_NOT_EXIST=71 10 E_CHANGED_MIND=72 11  12 if [ $# -ne "$ARGCOUNT" ] 13 then 14  echo "Usage: `basename $0` filename" 15  exit $E_WRONGARGS 16 fi 17  18 if [ ! -e "$1" ] 19 then 20  echo "File \""$1"\" does not exist." 21  exit $E_FILE_NOT_EXIST 22 fi 23  24 inum=`ls -i | grep "$1" | awk '{print $1}'` 25 # inum = inode (index node) number of file 26 # ----------------------------------------------------------------------- 27 # Every file has an inode, a record that holds its physical address info. 28 # ----------------------------------------------------------------------- 29  30 echo; echo -n "Are you absolutely sure you want to delete \"$1\" (y/n)? " 31 # The '-v' option to 'rm' also asks this. 32 read answer 33 case "$answer" in 34 [nN]) echo "Changed your mind, huh?" 35  exit $E_CHANGED_MIND 36  ;; 37 *) echo "Deleting file \"$1\".";; 38 esac 39  40 find . -inum $inum -exec rm {} \; 41 # ^^ 42 # Curly brackets are placeholder 43 #+ for text output by "find." 44 echo "File "\"$1"\" deleted!" 45  46 exit 0

---

The find command also works without the -exec option.

1 #!/bin/bash 2 # Find suid root files. 3 # A strange suid file might indicate a security hole, 4 #+ or even a system intrusion. 5  6 directory="/usr/sbin" 7 # Might also try /sbin, /bin, /usr/bin, /usr/local/bin, etc. 8 permissions="+4000" # suid root (dangerous!) 9  10  11 for file in $( find "$directory" -perm "$permissions" ) 12 do 13  ls -ltF --author "$file" 14 done

See Example 16-30, Example 3-4, and Example 11-10 for scripts using find. Its manpage provides more detail on this complex and powerful command.

xargs

A filter for feeding arguments to a command, and also a tool for assembling the commands themselves. It breaks a data stream into small enough chunks for filters and commands to process. Consider it as a powerful replacement for backquotes. In situations where command substitution fails with a too many arguments error, substituting xargs often works. [1] Normally, xargs reads from stdin or from a pipe, but it can also be given the output of a file.

The default command for xargs is echo. This means that input piped to xargs may have linefeeds and other whitespace characters stripped out.

bash$ ls -l total 0 -rw-rw-r-- 1 bozo bozo 0 Jan 29 23:58 file1 -rw-rw-r-- 1 bozo bozo 0 Jan 29 23:58 file2 bash$ ls -l | xargs total 0 -rw-rw-r-- 1 bozo bozo 0 Jan 29 23:58 file1 -rw-rw-r-- 1 bozo bozo 0 Jan... bash$ find ~/mail -type f | xargs grep "Linux" ./misc:User-Agent: slrn/0.9.8.1 (Linux) ./sent-mail-jul-2005: hosted by the Linux Documentation Project. ./sent-mail-jul-2005: (Linux Documentation Project Site, rtf version) ./sent-mail-jul-2005: Subject: Criticism of Bozo's Windows/Linux article ./sent-mail-jul-2005: while mentioning that the Linux ext2/ext3 filesystem . . .

ls | xargs -p -l gzip gzips every file in current directory, one at a time, prompting before each operation.

Note that xargs processes the arguments passed to it sequentially, one at a time. bash$ find /usr/bin | xargs file /usr/bin: directory /usr/bin/foomatic-ppd-options: perl script text executable . . .

An interesting xargs option is -n NN, which limits to NN the number of arguments passed. ls | xargs -n 8 echo lists the files in the current directory in 8 columns.

Another useful option is -0, in combination with find -print0 or grep -lZ. This allows handling arguments containing whitespace or quotes. find / -type f -print0 | xargs -0 grep -liwZ GUI | xargs -0 rm -f grep -rliwZ GUI / | xargs -0 rm -f Either of the above will remove any file containing "GUI". (Thanks, S.C.) Or: 1 cat /proc/"$pid"/"$OPTION" | xargs -0 echo 2 # Formats output: ^^^^^^^^^^^^^^^ 3 # From Han Holl's fixup of "get-commandline.sh" 4 #+ script in "/dev and /proc" chapter.

The -P option to xargs permits running processes in parallel. This speeds up execution in a machine with a multicore CPU. 1 #!/bin/bash 2  3 ls *gif | xargs -t -n1 -P2 gif2png 4 # Converts all the gif images in current directory to png. 5  6 # Options: 7 # ======= 8 # -t Print command to stderr. 9 # -n1 At most 1 argument per command line. 10 # -P2 Run up to 2 processes simultaneously. 11  12 # Thank you, Roberto Polli, for the inspiration.

---

Example 16-5. Logfile: Using xargs to monitor system log

1 #!/bin/bash 2  3 # Generates a log file in current directory 4 # from the tail end of /var/log/messages. 5  6 # Note: /var/log/messages must be world readable 7 # if this script invoked by an ordinary user. 8 # #root chmod 644 /var/log/messages 9  10 LINES=5 11  12 ( date; uname -a ) >>logfile 13 # Time and machine name 14 echo ---------------------------------------------------------- >>logfile 15 tail -n $LINES /var/log/messages | xargs | fmt -s >>logfile 16 echo >>logfile 17 echo >>logfile 18  19 exit 0 20  21 # Note: 22 # ---- 23 # As Frank Wang points out, 24 #+ unmatched quotes (either single or double quotes) in the source file 25 #+ may give xargs indigestion. 26 # 27 # He suggests the following substitution for line 15: 28 # tail -n $LINES /var/log/messages | tr -d "\"'" | xargs | fmt -s >>logfile 29  30  31  32 # Exercise: 33 # -------- 34 # Modify this script to track changes in /var/log/messages at intervals 35 #+ of 20 minutes. 36 # Hint: Use the "watch" command.

---

As in find, a curly bracket pair serves as a placeholder for replacement text.

---

Example 16-6. Copying files in current directory to another

1 #!/bin/bash 2 # copydir.sh 3  4 # Copy (verbose) all files in current directory ($PWD) 5 #+ to directory specified on command-line. 6  7 E_NOARGS=85 8  9 if [ -z "$1" ] # Exit if no argument given. 10 then 11  echo "Usage: `basename $0` directory-to-copy-to" 12  exit $E_NOARGS 13 fi 14  15 ls . | xargs -i -t cp ./{} $1 16 # ^^ ^^ ^^ 17 # -t is "verbose" (output command-line to stderr) option. 18 # -i is "replace strings" option. 19 # {} is a placeholder for output text. 20 # This is similar to the use of a curly-bracket pair in "find." 21 # 22 # List the files in current directory (ls .), 23 #+ pass the output of "ls" as arguments to "xargs" (-i -t options), 24 #+ then copy (cp) these arguments ({}) to new directory ($1). 25 # 26 # The net result is the exact equivalent of 27 #+ cp * $1 28 #+ unless any of the filenames has embedded "whitespace" characters. 29  30 exit 0

---

---

Example 16-7. Killing processes by name

1 #!/bin/bash 2 # kill-byname.sh: Killing processes by name. 3 # Compare this script with kill-process.sh. 4  5 # For instance, 6 #+ try "./kill-byname.sh xterm" -- 7 #+ and watch all the xterms on your desktop disappear. 8  9 # Warning: 10 # ------- 11 # This is a fairly dangerous script. 12 # Running it carelessly (especially as root) 13 #+ can cause data loss and other undesirable effects. 14  15 E_BADARGS=66 16  17 if test -z "$1" # No command-line arg supplied? 18 then 19  echo "Usage: `basename $0` Process(es)_to_kill" 20  exit $E_BADARGS 21 fi 22  23  24 PROCESS_NAME="$1" 25 ps ax | grep "$PROCESS_NAME" | awk '{print $1}' | xargs -i kill {} 2&>/dev/null 26 # ^^ ^^ 27  28 # --------------------------------------------------------------- 29 # Notes: 30 # -i is the "replace strings" option to xargs. 31 # The curly brackets are the placeholder for the replacement. 32 # 2&>/dev/null suppresses unwanted error messages. 33 # 34 # Can grep "$PROCESS_NAME" be replaced by pidof "$PROCESS_NAME"? 35 # --------------------------------------------------------------- 36  37 exit $? 38  39 # The "killall" command has the same effect as this script, 40 #+ but using it is not quite as educational.

---

---

Example 16-8. Word frequency analysis using xargs

1 #!/bin/bash 2 # wf2.sh: Crude word frequency analysis on a text file. 3  4 # Uses 'xargs' to decompose lines of text into single words. 5 # Compare this example to the "wf.sh" script later on. 6  7  8 # Check for input file on command-line. 9 ARGS=1 10 E_BADARGS=85 11 E_NOFILE=86 12  13 if [ $# -ne "$ARGS" ] 14 # Correct number of arguments passed to script? 15 then 16  echo "Usage: `basename $0` filename" 17  exit $E_BADARGS 18 fi 19  20 if [ ! -f "$1" ] # Does file exist? 21 then 22  echo "File \"$1\" does not exist." 23  exit $E_NOFILE 24 fi 25  26  27  28 ##################################################### 29 cat "$1" | xargs -n1 | \ 30 # List the file, one word per line. 31 tr A-Z a-z | \ 32 # Shift characters to lowercase. 33 sed -e 's/\.//g' -e 's/\,//g' -e 's/ /\ 34 /g' | \ 35 # Filter out periods and commas, and 36 #+ change space between words to linefeed, 37 sort | uniq -c | sort -nr 38 # Finally remove duplicates, prefix occurrence count 39 #+ and sort numerically. 40 ##################################################### 41  42 # This does the same job as the "wf.sh" example, 43 #+ but a bit more ponderously, and it runs more slowly (why?). 44  45 exit $?

---

expr

All-purpose expression evaluator: Concatenates and evaluates the arguments according to the operation given (arguments must be separated by spaces). Operations may be arithmetic, comparison, string, or logical.

expr 3 + 5

returns 8

expr 5 % 3

returns 2

expr 1 / 0

returns the error message, expr: division by zero

Illegal arithmetic operations not allowed.

expr 5 \* 3

returns 15

The multiplication operator must be escaped when used in an arithmetic expression with expr.

y=`expr $y + 1`

Increment a variable, with the same effect as let y=y+1 and y=$(($y+1)). This is an example of arithmetic expansion.

z=`expr substr $string $position $length`

Extract substring of $length characters, starting at $position.

---

Example 16-9. Using expr

1 #!/bin/bash 2  3 # Demonstrating some of the uses of 'expr' 4 # ======================================= 5  6 echo 7  8 # Arithmetic Operators 9 # ---------- --------- 10  11 echo "Arithmetic Operators" 12 echo 13 a=`expr 5 + 3` 14 echo "5 + 3 = $a" 15  16 a=`expr $a + 1` 17 echo 18 echo "a + 1 = $a" 19 echo "(incrementing a variable)" 20  21 a=`expr 5 % 3` 22 # modulo 23 echo 24 echo "5 mod 3 = $a" 25  26 echo 27 echo 28  29 # Logical Operators 30 # ------- --------- 31  32 # Returns 1 if true, 0 if false, 33 #+ opposite of normal Bash convention. 34  35 echo "Logical Operators" 36 echo 37  38 x=24 39 y=25 40 b=`expr $x = $y` # Test equality. 41 echo "b = $b" # 0 ( $x -ne $y ) 42 echo 43  44 a=3 45 b=`expr $a \> 10` 46 echo 'b=`expr $a \> 10`, therefore...' 47 echo "If a > 10, b = 0 (false)" 48 echo "b = $b" # 0 ( 3 ! -gt 10 ) 49 echo 50  51 b=`expr $a \< 10` 52 echo "If a < 10, b = 1 (true)" 53 echo "b = $b" # 1 ( 3 -lt 10 ) 54 echo 55 # Note escaping of operators. 56  57 b=`expr $a \<= 3` 58 echo "If a <= 3, b = 1 (true)" 59 echo "b = $b" # 1 ( 3 -le 3 ) 60 # There is also a "\>=" operator (greater than or equal to). 61  62  63 echo 64 echo 65  66  67  68 # String Operators 69 # ------ --------- 70  71 echo "String Operators" 72 echo 73  74 a=1234zipper43231 75 echo "The string being operated upon is \"$a\"." 76  77 # length: length of string 78 b=`expr length $a` 79 echo "Length of \"$a\" is $b." 80  81 # index: position of first character in substring 82 # that matches a character in string 83 b=`expr index $a 23` 84 echo "Numerical position of first \"2\" in \"$a\" is \"$b\"." 85  86 # substr: extract substring, starting position & length specified 87 b=`expr substr $a 2 6` 88 echo "Substring of \"$a\", starting at position 2,\ 89 and 6 chars long is \"$b\"." 90  91  92 # The default behavior of the 'match' operations is to 93 #+ search for the specified match at the BEGINNING of the string. 94 # 95 # Using Regular Expressions ... 96 b=`expr match "$a" '[0-9]*'` # Numerical count. 97 echo Number of digits at the beginning of \"$a\" is $b. 98 b=`expr match "$a" '\([0-9]*\)'` # Note that escaped parentheses 99 # == == #+ trigger substring match. 100 echo "The digits at the beginning of \"$a\" are \"$b\"." 101  102 echo 103  104 exit 0

---

The : (null) operator can substitute for match. For example, b=`expr $a : [0-9]*` is the exact equivalent of b=`expr match $a [0-9]*` in the above listing. 1 #!/bin/bash 2  3 echo 4 echo "String operations using \"expr \$string : \" construct" 5 echo "===================================================" 6 echo 7  8 a=1234zipper5FLIPPER43231 9  10 echo "The string being operated upon is \"`expr "$a" : '\(.*\)'`\"." 11 # Escaped parentheses grouping operator. == == 12  13 # *************************** 14 #+ Escaped parentheses 15 #+ match a substring 16 # *************************** 17  18  19 # If no escaped parentheses ... 20 #+ then 'expr' converts the string operand to an integer. 21  22 echo "Length of \"$a\" is `expr "$a" : '.*'`." # Length of string 23  24 echo "Number of digits at the beginning of \"$a\" is `expr "$a" : '[0-9]*'`." 25  26 # ------------------------------------------------------------------------- # 27  28 echo 29  30 echo "The digits at the beginning of \"$a\" are `expr "$a" : '\([0-9]*\)'`." 31 # == == 32 echo "The first 7 characters of \"$a\" are `expr "$a" : '\(.......\)'`." 33 # ===== == == 34 # Again, escaped parentheses force a substring match. 35 # 36 echo "The last 7 characters of \"$a\" are `expr "$a" : '.*\(.......\)'`." 37 # ==== end of string operator ^^ 38 # (In fact, means skip over one or more of any characters until specified 39 #+ substring found.) 40  41 echo 42  43 exit 0