hl - read and highlight regular expression matches

hl pattern

hl [--ansi attribute] [--buffer] pattern

hl [--ansi attribute] [--no-buffer] pattern

The hl utility copies standard input to standard output, highlighting lines that match pattern, which must be a valid regular expression. The optional command line arguments are as follows:

--ansi attribute
Highlights pattern matches using the ANSI code corresponding to attribute.

By default, hl uses ANSI codes to embolden text to highlight match text, and non-bold text for all other text. Other ANSI codes are supported as shown in the below table. Several rows contain multiple attributes, indicating they are intended synonyms.

attribute	ANSI code
-----------------------	---------
bold	ESC[1m
dim|faint	ESC[2m
italic	ESC[3m
underline|underscore	ESC[4m
blinking	ESC[5m
reverse|inverse	ESC[7m
hidden|invisible	ESC[8m
strikethrough	ESC[9m
black	ESC[30m
red	ESC[31m
green	ESC[32m
yellow	ESC[33m
blue	ESC[34m
magenta	ESC[35m
cyan	ESC[36m
white	ESC[37m

NOTE: Not all ANSI codes work on every terminal.

--buffer
Buffers its output even when it detects that it is writing to a character device.

By default when hl detects that its standard output is directed to a character device, like a TTY, it will write each line of output immediately after processing it. However, if hl detects that its standard output is not directed to a character device, for instance when it is being piped into another process, it will use a small memory buffer to reduce the number of system write calls it makes. However, when given the --buffer command line flag, hl will buffer its output even when it detects that it is writing to a character device.

--no-buffer
Does not buffer its output even when its output is not a character device. Use this when piping the output of hl to another process, and each complete line should be flushed immedidately after it is processed.

0
Success

1
Syntax or usage error

2
Error reading input

3
Error writing output

Example 1: Highlighting a text pattern from a stream
This example tails a file and highlights any instances of the word failure using a bold typeface:

# tail -f /var/log/messages | hl failure

Example 2: Highlighting multiple patterns from a stream
hl invocations may be chained together to highlight multiple patterns using different ANSI codes. When doing so, it is common to turn off buffering in every instance of hl that pipes its output to another program. This example will tail a file and highlight BAD with red, and OK with green.

# tail -f /var/log/messages | hl --ansi red --no-buffer BAD | hl --ansi green OK

Karrick McDermott https://linkedin.com/in/karrick