Shell Script writing output to a file

Question

I have a shell script that extracts certain values from a text file (input to it via the terminal). The script does the extraction as intended except, it doesn't print the output to the file correctly.

The script is:

#!/bin/bash

input_file=$1 
while read -r LINE
do
IFS="=" read -r -a params <<< "$LINE"
if [ -n "${params[2]}" ]
    then
    IFS=" " read -r -a param_opcode <<< "${params[2]}"
    echo "${param_opcode}"
fi
done < "$input_file"

The output on the terminal is as follows:

0xd2800140  
0xd2800061  
0x8b010000  
0x8b000042  
0xd1000821  
0xd28001e5  
0xd28000a6  
0x9ac608a5  
0xe7ff0010  
0xe7ff0010

However, when I try to write this to a text file bu doing:

echo "${param_opcodes}" > log.txt

It prints only this to the file:

0xe7ff0010

I tried >> but I don't want to append to it. I want the file to be overwritten every time, I run the script.

oliv · Accepted Answer · 2017-11-13T15:57:54.777

3

Your redirection to log file isn't right because it's inside the while loop. Use the redirection at the end of the while loop:

while read -r LINE; do
...
done < "$input_file" > "log.txt"

edited Nov 13 '17 at 15:57

answered Nov 13 '17 at 15:54

oliv

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Thanks @oliv It works. However, I'm curious to know why the way I try fails. – the_mamba Nov 13 '17 at 15:55
@the_black_mamba, ...because `>` opens a file with `O_TRUNC`, zeroing out any prior contents. – Charles Duffy Nov 13 '17 at 15:58
@Charles could you elaborate – the_mamba Nov 13 '17 at 15:59
You might see `man 2 open` for documentation on `O_TRUNC` (the flag passed to the operating-system-level `open` syscall). If further elaboration is needed, please be specific about what was unclear. – Charles Duffy Nov 13 '17 at 15:59

Shell Script writing output to a file

1 Answers1