Writing to a file multiple times with Bash
I am creating a bash script to automate some commands and I am having problems writing my error checking to the same file.
#!/bin/bash
touch ErrorLog.txt
bro-cut service < conn.log | sort | uniq -c > ProtocolHierarchy.txt
if [ $? -eq 0 ]; then
echo -e "OK Protocol Hierarchy Created\n" > ErrorLog.txt
else
echo -e "FAILED Creating Protocol Hierarchy\n" > ErrorLog.txt
fi
bro-cut id.orig_h < dns.log | sort | uniq -c > AllIPAddresses.txt
if [ $? -eq 0 ]; then
echo -e "OK Created all IP Addresses\n" > ErrorLog.txt
else
echo -e "FAILED Creating all IP Addresses\n" > ErrorLog.txt
fi
The goal is to have a file that I can open and see if all commands were working or not, currently this file looks like
-e OK Created all IP Addresses
When I want it to look like
OK Protocol Hierarchy Created
OK Created all IP Addresses
I am really new to bash scripting, so any advice would be greatly appreciated!
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1 answer
Open it once and write to this file descriptor several times.
# Open (creating or truncating) the output file (only once!)
exec 3>ErrorLog.txt
# Write a line to that already-open file
echo "something" >&3
# Write a second line to that already-open file
echo "something else" >&3
# Optional: close the output file (can also be implicit when the script exits)
exec 3>&-
Another common idiom is to open in append mode with >>
, but doing this once per line is significantly less efficient.
# Open ErrorLog.txt, truncating if it exist, write one line, and close it
echo "something" >ErrorLog.txt
# Reopen ErrorLog.txt, write an additional line to the end, and close it again
echo "something else" >>ErrorLog.txt
Bringing this practice into your script (and making some better practice improvements) looks like this:
#!/bin/bash
# not related to file output, but to making sure we detect errors
# only works correctly if run with bash, not sh!
set -o pipefail ## set exit status based on whole pipeline, not just last command
# picking 3, since FD numbers 0-2 are reserved for stdin/stdout/stderr
exec 3>ErrorLog.txt
if bro-cut service <conn.log | sort | uniq -c >ProtocolHierarchy.txt; then
echo "OK Protocol Hierarchy Created" >&3
else
echo "FAILED Creating Protocol Hierarchy" >&3
fi
if bro-cut id.orig_h <dns.log | sort | uniq -c >AllIPAddresses.txt; then
echo "OK Created all IP Addresses" >&3
else
echo "FAILED Creating all IP Addresses" >&3
fi
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