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trap                         Command                         trap




Execute command on receipt of signal

ttrraapp [_c_o_m_m_a_n_d] [_n ...]

trap instructs the shell sh to execute the given command when the
shell receives signal n or any other signal in the optional list.
If the  command is omitted, trap resets traps  for the given sig-
nals to  the original  values.  If the  command is a  null string
(i.e., a  string that consists  only of one  null character), the
shell ignores  the given  signals.  If n  is zero, the  shell ex-
ecutes the  specified command when it exits.   When it is invoked
with no arguments, trap  prints the signal number and command for
each signal on which a trap is set.

The shell executes trap directly.

***** Example *****

The  following example  takes two  files  and outputs  only those
lines which are the same.


# If input only one file-name then simply "cat".
if [ $# = 1 ]; then
        cat $1
        exit 0



# If input two file-names - Ok, else "Usage".
else
        if [ $# != 2 ]; then
                echo "Usage: cmn file1 [file2]"
                exit 1
        fi
fi



# TMP is original name of temporary file (/tmp/temp_(pid)
TMP=/tmp/temp_$$



# Temporary file has to be removed
trap 'rm $TMP; exit 1' 1 2 9



# Difference between "file1" and "difference between file1 and file2"
# is the common strings "file1" and "file2"
# The strings that are in "file1" and absent in "file2" print in TMP.
diff $1 $2 | sed -n -e "s/^< //p" > $TMP



COHERENT Lexicon                                           Page 1




trap                         Command                         trap





# The strings that are in "file1" and absent in TMP print in stdout.
diff $1 $TMP | sed -n -e "s/^< //p"



# Remove temporary file
rm $TMP


***** See Also *****

commands, sh, signal











































COHERENT Lexicon                                           Page 2

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