CODEWITHSUNDEEP
bashecho
firetiger443
firetiger443

Reputation: 107

Writing to a file multiple times with Bash

I am creating a bash script to automate some commands and I am having some trouble 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 being to have a file I can open and see that all the commands worked or failed, currently the file looks like this

-e OK Created all IP Addresses

When I would like it to look like this

OK Protocol Hierarchy Created
OK Created all IP Addresses

I am really new to bash scripting so any tips would be greatly appreciated!

Upvotes: 4

Views: 2933

Answers (1)

Charles Duffy
Charles Duffy

Reputation: 295472

Open it once, and write to that file descriptor multiple 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>&-

The other common idiom is to open in append mode using >>, but doing that once per line is considerably 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

Putting this practice to work in your script (and making some other best-practice improvements) looks like the following:

#!/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

Upvotes: 5

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