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how to append a file using sed command

i have this file
test

[z_jigneshb.WF:DS_aftab.ST:s_m_EDW_F_SOP_DMND_FCST_DTL_DS]
$$INPUT_FILE_DATE=000000
$$PFFLAG=0

i am writing a script to change the value for the date and flag which it gets from a file in the same directory.
the name of the file is
DS_P_201202

i have the code working but when i redirect it to the test file, it becomes a blank file

Code:

FILE_NAME=$(ls -tr DS_*|head -1)



DATEMONTH=$(echo $FILE_NAME | cut -d "_" -f3)

DATEMONTH=$(echo $DATEMONTH | cut -d "." -f1)

FLAG=$(echo $FILE_NAME | cut -d "_" -f2)

 

sed '/^\$\$INPUT_FILE_DATE=/s/[0-9]\{6\}/'$DATEMONTH'/' test>test

sed '/^\$\$PFFLAG=/s/.$/'$FLAG'/i test>test

Quote:

Code:

sed '/^\$\$INPUT_FILE_DATE=/s/[0-9]\{6\}/'$DATEMONTH'/' test>test

sed '/^\$\$PFFLAG=/s/.$/'$FLAG'/i test>test

You cannot read from a file and redirect output to that same file in a single command.

The shell output redirection operator (>) will truncate/erase the file prior to the command being executed. Because the file has been truncated, there is no data for the sed command to read.

You have two options:

1. Use the sed '-i' option to modify the file in-place. As in:

Code:

sed -i '/^\$\$INPUT_FILE_DATE=/s/[0-9]\{6\}/'$DATEMONTH'/' test

sed -i '/^\$\$PFFLAG=/s/.$/'$FLAG'/i test

2. Redirect the output to a temporary file, then copy the temporary file back on top of the original file.

For option 2 to work, you still need to modify your sed commands. The easiest would be to specify multiple sed commands with the -e option. For instance:

Code:

sed -e '/^\$\$INPUT_FILE_DATE=/s/[0-9]\{6\}/'$DATEMONTH'/' \

    -e '/^\$\$PFFLAG=/s/.$/'$FLAG'/i test > temp_file

mv temp_file test