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Logs, logs, logs!

Posted on 08 Feb 2015

As services become more and more complex, log analysis has become increasingly imperative when trying to diagnose bugs. Services now generate megabytes or even gigabytes of log files on an average day, and it’s more often than not impossible to look through data manually from each file, or even open them in certain text editors (*ahem* Notepad). So how do we work with these files?

Unix Command-Line Utilities

Unix command-line utilities are known for (1) their composability and (2) doing one thing and doing it well. Below are a few of the most useful command-line tools for text processing.


Manpage: http://linux.die.net/man/1/cat

cat is one of the simplest Unix utilities. It concatenates the contents of the filename parameters and prints the result to stdout. Alternatively, if there are no parameters, it echos all input to stdout until it encounters EOF.

cat is most often used to put the contents of a file into a pipe to be read by another program, but can also be handy for forcing weird programs like man to output to stdout in a jiffy.


Printing from stdin.

$ echo "Hello world!" | cat
Hello world!

Count the lines in test.txt.

$ cat test.txt | wc -l


Manpage: http://linux.die.net/man/1/grep

grep is the bread and butter of text search. At the basic level, grep searches file parameters or stdin line-by-line for a specified regex and lists the lines that match.

Some commonly used grep flags include -o, which prints only the text that matches your regex; -c, which prints the number of matches; and -v, which inverts your search (only prints lines that don’t match).

Many programmers are more familar with Perl Compatible Regular Expressions (PCRE) than grep’s default POSIX regexs since the regex implementations in popular languages like Java, Javascript, and Python are based on Perl’s. To use grep in PCRE mode, run it with the -P flag.


List matches of regex.

$ echo "The cat in the hat" | grep -o at

Invert your search.

$ echo "Hello world" > file.txt
$ echo "Foo bar" >> file.txt
$ grep -v "Hello" file.txt
Foo bar


Manpage: http://linux.die.net/man/1/cut

cut is a great tool for working with delimited data. The default delimiter is tab, but can be easily changed with the -d flag. The -f flag then specifies which delimited field(s) to print. The manpage details how to select a range of fields:

Each range is one of:

N'th byte, character or field, counted from 1
from N'th byte, character or field, to end of line
from N'th to M'th (included) byte, character or field
from first to M'th (included) byte, character or field

You can also use -f - (range omitting N and M) to select all fields.


Print the first 3 fields of a CSV line.

$ echo "eggs,bananas,oranges,pears" | cut -d',' -f1-3

Print from character 22 to the end.

$ echo "eggs,bananas,oranges,pears" | cut -c22- 

Convert a CSV line to TSV.

Note: You can insert tab into the commandline with <CTRL> v + <TAB>

$ echo "eggs,bananas,oranges,pears" | cut -d',' -f - --output-delimiter='   '
eggs    bananas oranges pears


Manpage: http://linux.die.net/man/1/find

While not directly involved in text manipulation, find is a handy utility for searching for files based on specific criterion. find is commonly used with the -exec flag, which allows you to run commands (like those discussed in this section) on files matching the search parameters.


Search files in current directory for filenames beginning with string “bacon”.

$ find . -name "bacon*"

List contents of all directories in the current directory.

$ find . -type d -exec ls {} \;


Manpage: http://linux.die.net/man/1/sort

True to its name, sort sorts lines from stdin or a file. It’s important to note that, by default, sort sorts character by character from the beginning on the string. This can be problematic when sorting numbers, where 55 would come before 60 but after 100. Luckily, sort comes with the -n flag which sorts strings by their value.


Sorting without -n flag.

$ echo -e "123\n34\n678" | sort

Sorting with the -n flag.

$ echo -e "123\n34\n678" | sort -n


Manpage: http://linux.die.net/man/1/uniq

uniq is a useful utility for dealing with potentially duplicated lines in data. By default, uniq de-dupes contiguous duplicate lines in the input. When using the -d flag, uniq does the opposite; it prints only lines that have occurred multiple times in a row.

It’s especially important to keep in mind that uniq de-dupes contiguous duplicate lines. Practically, this means that data is usually piped through sort before uniq.


De-duping a list of fruits.

$ echo -e "apples\noranges\npears\napples\noranges\nbananas" | sort | uniq

Printing only strings that are duplicated.

$ echo -e "apples\noranges\npears\napples\noranges\nbananas" | sort | uniq -d

Scripting Languages

For doing more complex work, scripting languages shine. Popular scripting languages for text processing include awk, ruby, perl, and python. These languages can be helpful for doing more powerful data transformations and aggregations. It’s just as important, however, to avoid reinventing the wheel when you can just as easily pipe together command-line tools and make your life much, much easier.


Ruby is a popular scripting language frequently used in conjunction with the web framework Rails. However, ruby is also a powerful language in it’s own right. Here’s how to leverage ruby’s expressive one-liners to do your bidding.


Like perl, ruby has an -e switch which executes the specified argument as ruby code.

$ ruby -e 'puts "Hello world!"'
Hello world!

The -n switch compliments that flag by wrapping the ruby one-liner in a while gets ... end. This means the one-liner is run on each line of input from stdin and stored in the variable $_.

For a file file.txt containing:


We can calculate the average price of all items with the following one-liner:

$ cat file.txt | ruby -ne 'BEGIN { count = 0; price = 0 }; _, p, c = $_.split(","); count += c.to_i; price += p.to_f; END { puts price / count; };'

Thus, the average price of all items in the file is about $0.26.

Further Reading

Here are some extra links for those intrigued and looking to improve their command-line fu:

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