12.2. Complex Commands

Command Listing

find

-exec COMMAND \;

Carries out COMMAND on each file that find scores a hit on. COMMAND terminates with \; (the ; is escaped to make certain the shell passes it to find literally, which concludes the command sequence). If COMMAND contains {}, then find substitutes the full path name of the selected file.

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

find /home/bozo/projects -mtime 1
# Lists all files in /home/bozo/projects directory tree
# that were modified within the last day.

find /etc -exec grep '[0-9][0-9]*[.][0-9][0-9]*[.][0-9][0-9]*[.][0-9][0-9]*' {} \;

# Finds all IP addresses (xxx.xxx.xxx.xxx) in /etc directory files.
# There a few extraneous hits - how can they be filtered out?

# Perhaps by:

find /etc -type f -exec cat '{}' \; | tr -c '.[:digit:]' '\n' \
 | grep '^[^.][^.]*\.[^.][^.]*\.[^.][^.]*\.[^.][^.]*$'
# [:digit:] is one of the character classes
# introduced with the POSIX 1003.2 standard. 

# Thanks, S.C. 

Caution

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

Example 12-2. Badname, eliminate file names in current directory containing bad characters and whitespace.

#!/bin/bash

# Delete filenames in current directory containing bad characters.

for filename in *
do
badname=`echo "$filename" | sed -n /[\+\{\;\"\\\=\?~\(\)\<\>\&\*\|\$]/p`
# Files containing those nasties:     + { ; " \ = ? ~ ( ) < > & * | $
rm $badname 2>/dev/null    # So error messages deep-sixed.
done

# Now, take care of files containing all manner of whitespace.
find . -name "* *" -exec rm -f {} \;
# The path name of the file that "find" finds replaces the "{}".
# The '\' ensures that the ';' is interpreted literally, as end of command.

exit 0

#---------------------------------------------------------------------
# Commands below this line will not execute because of "exit" command.

# An alternative to the above script:
find . -name '*[+{;"\\=?~()<>&*|$ ]*' -exec rm -f '{}' \;
exit 0
# (Thanks, S.C.)

Example 12-3. Deleting a file by its inode number

#!/bin/bash
# idelete.sh: Deleting a file by its inode number.

#  This is useful when a filename starts with an illegal character,
#+ such as ? or -.

ARGCOUNT=1                      # Filename arg must be passed to script.
E_WRONGARGS=70
E_FILE_NOT_EXIST=71
E_CHANGED_MIND=72

if [ $# -ne "$ARGCOUNT" ]
then
  echo "Usage: `basename $0` filename"
  exit $E_WRONGARGS
fi  

if [ ! -e "$1" ]
then
  echo "File \""$1"\" does not exist."
  exit $E_FILE_NOT_EXIST
fi  

inum=`ls -i | grep "$1" | awk '{print $1}'`
# inum = inode (index node) number of file
# Every file has an inode, a record that hold its physical address info.

echo; echo -n "Are you absolutely sure you want to delete \"$1\" (y/n)? "
read answer
case "$answer" in
[nN]) echo "Changed your mind, huh?"
      exit $E_CHANGED_MIND
      ;;
*)    echo "Deleting file \"$1\".";;
esac

find . -inum $inum -exec rm {} \;
echo "File "\"$1"\" deleted!"

exit 0

See Example 12-22, Example 4-3, and Example 10-8 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 backquotes fail with a too many arguments error, substituting xargs often works. 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.

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

Tip

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.

Tip

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.)

Example 12-4. Logfile using xargs to monitor system log

#!/bin/bash

# Generates a log file in current directory
# from the tail end of /var/log/messages.

# Note: /var/log/messages must be world readable
# if this script invoked by an ordinary user.
#         #root chmod 644 /var/log/messages

LINES=5

( date; uname -a ) >>logfile
# Time and machine name
echo --------------------------------------------------------------------- >>logfile
tail -$LINES /var/log/messages | xargs |  fmt -s >>logfile
echo >>logfile
echo >>logfile

exit 0

Example 12-5. copydir, copying files in current directory to another, using xargs

#!/bin/bash

# Copy (verbose) all files in current directory
# to directory specified on command line.

if [ -z "$1" ]   # Exit if no argument given.
then
  echo "Usage: `basename $0` directory-to-copy-to"
  exit 65
fi  

ls . | xargs -i -t cp ./{} $1
# This is the exact equivalent of
#    cp * $1
# unless any of the filenames has "whitespace" characters.

exit 0
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

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 12-6. Using expr

#!/bin/bash

# Demonstrating some of the uses of 'expr'
# =======================================

echo

# Arithmetic Operators
# ---------- ---------

echo "Arithmetic Operators"
echo
a=`expr 5 + 3`
echo "5 + 3 = $a"

a=`expr $a + 1`
echo
echo "a + 1 = $a"
echo "(incrementing a variable)"

a=`expr 5 % 3`
# modulo
echo
echo "5 mod 3 = $a"

echo
echo

# Logical Operators
# ------- ---------

#  Returns 1 if true, 0 if false,
#+ opposite of normal Bash convention.

echo "Logical Operators"
echo

x=24
y=25
b=`expr $x = $y`         # Test equality.
echo "b = $b"            # 0  ( $x -ne $y )
echo

a=3
b=`expr $a \> 10`
echo 'b=`expr $a \> 10`, therefore...'
echo "If a > 10, b = 0 (false)"
echo "b = $b"            # 0  ( 3 ! -gt 10 )
echo

b=`expr $a \< 10`
echo "If a < 10, b = 1 (true)"
echo "b = $b"            # 1  ( 3 -lt 10 )
echo
# Note escaping of operators.

b=`expr $a \<= 3`
echo "If a <= 3, b = 1 (true)"
echo "b = $b"            # 1  ( 3 -le 3 )
# There is also a "\>=" operator (greater than or equal to).


echo
echo

# Comparison Operators
# ---------- ---------

echo "Comparison Operators"
echo
a=zipper
echo "a is $a"
if [ `expr $a = snap` ]
# Force re-evaluation of variable 'a'
then
   echo "a is not zipper"
fi   

echo
echo



# String Operators
# ------ ---------

echo "String Operators"
echo

a=1234zipper43231
echo "The string being operated upon is \"$a\"."

# length: length of string
b=`expr length $a`
echo "Length of \"$a\" is $b."

# index: position of first character in substring
#        that matches a character in string
b=`expr index $a 23`
echo "Numerical position of first \"2\" in \"$a\" is \"$b\"."

# substr: extract substring, starting position & length specified
b=`expr substr $a 2 6`
echo "Substring of \"$a\", starting at position 2, and 6 chars long is \"$b\"."


# 'match' operations similarly to 'grep'
#      uses Regular Expressions
b=`expr match "$a" '[0-9]*'`
echo Number of digits at the beginning of \"$a\" is $b.
b=`expr match "$a" '\([0-9]*\)'`                    # Note escaped parentheses.
echo "The digits at the beginning of \"$a\" are \"$b\"."

echo

exit 0

Important

The : 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.

#!/bin/bash

echo
echo "String operations using \"expr $string :\" construct"
echo "-------------------------------------------"
echo

a=1234zipper43231
echo "The string being operated upon is \"`expr "$a" : '\(.*\)'`\"."
#       Escaped parentheses.
#       Regular expression parsing.

echo "Length of \"$a\" is `expr "$a" : '.*'`."   # Length of string

echo "Number of digits at the beginning of \"$a\" is `expr "$a" : '[0-9]*'`."

echo "The digits at the beginning of \"$a\" are `expr "$a" : '\([0-9]*\)'`."

echo

exit 0

Perl and sed have far superior string parsing facilities. A short Perl or sed "subroutine" within a script (see Section 34.2) is an attractive alternative to using expr.

See Section 9.2 for more on string operations.