showing the name of the hard disk devices using grep - linux

Question

Why does this command:
[jalal#galapagos-20] (42)$ dmesg | egrep '(s|h)d[a-z]'
Shows the following? Can you please show step by step?
ACPI: SSDT 00000000d8ffbbd8 03528 (v01 SaSsdt SaSsdt 00003000 INTL 20091112)
NMI watchdog enabled, takes one hw-pmu counter.
sd 0:0:0:0: [sda] 500118192 512-byte logical blocks: (256 GB/238 GiB)
sd 0:0:0:0: [sda] 4096-byte physical blocks
sd 0:0:0:0: [sda] Write Protect is off
sd 0:0:0:0: [sda] Mode Sense: 00 3a 00 00
sd 0:0:0:0: [sda] Write cache: enabled, read cache: enabled, doesn't support DPO or FUA
sda: sda1 sda2 sda3 sda4 sda5 sda6 sda7
sd 0:0:0:0: [sda] Attached SCSI disk
EXT4-fs (sda3): mounted filesystem with ordered data mode. Opts:
dracut: Mounted root filesystem /dev/sda3
snd_hda_intel 0000:00:03.0: PCI INT A -> GSI 16 (level, low) -> IRQ 16
snd_hda_intel 0000:00:03.0: irq 33 for MSI/MSI-X
snd_hda_intel 0000:00:03.0: setting latency timer to 64
snd_hda_intel 0000:00:1b.0: PCI INT A -> GSI 22 (level, low) -> IRQ 22
snd_hda_intel 0000:00:1b.0: irq 34 for MSI/MSI-X
snd_hda_intel 0000:00:1b.0: setting latency timer to 64
sound hdaudioC1D2: autoconfig for ALC3220: line_outs=1 (0x1b/0x0/0x0/0x0/0x0) type:line
sound hdaudioC1D2: speaker_outs=1 (0x14/0x0/0x0/0x0/0x0)
sound hdaudioC1D2: hp_outs=1 (0x15/0x0/0x0/0x0/0x0)
sound hdaudioC1D2: mono: mono_out=0x0
sound hdaudioC1D2: inputs:
sound hdaudioC1D2: Front Mic=0x1a
sound hdaudioC1D2: Rear Mic=0x18
EXT4-fs (sda1): mounted filesystem with ordered data mode. Opts:
EXT4-fs (sda7): mounted filesystem with ordered data mode. Opts:
EXT4-fs (sda4): mounted filesystem with ordered data mode. Opts:
EXT4-fs (sda6): mounted filesystem with ordered data mode. Opts:
EXT4-fs (sda5): mounted filesystem with ordered data mode. Opts:
Adding 67108860k swap on /dev/sda2. Priority:-1 extents:1 across:67108860k SS
snd_hda_intel 0000:00:1b.0: IRQ timing workaround is activated for card #1. Suggest a bigger bdl_pos_adj.

Answer 1

The command dmesg dumps the kernel log buffer to stdout. This output consists of a bunch of log messages, each on a separate line.
That output is piped to the command egrep with the option (s|h)d[a-z]. egrep checks its stdin (the output of dmesg) one line at a time for a match against the regular expression (s|h)d[a-z]. This regular expression will match a series of characters where:
The first character is s or h
The second character is d
The third character is in the ASCII range (inclusive) between a and z (the lowercase letters a to z).
egrep prints to stdout any lines which match the supplied regular expression.

Answer 2

The egrep command is using the regular expression (s|h)d[a-z] for filtering lines containing the sequences hda to hdz and sda to sdz on the output of dmesg - traditional names for disk devices in Linux.
This regular expression could be switched to [sh]d[a-z], which has the same effect but you may find easier to understand.
There are several false positives if you are looking for disks, like the lines containing "hdaudio".