Be aware that, busybox tries to imitate popular feature extensions from GNU's implementation of programs and utilities, though busybox often differs from GNU's implementation in subtle ways.
> denotes command input, and a line without the prefix denotes the output from that command.
> docker run -it --rm alpine:latest
# File tests
> [ -e /etc/passwd ] && echo good
> [ -e /etc/passwd -a -f /etc/passwd -a -r /etc/passwd ] && echo good
# Similar file tests
> [[ -e /etc/passwd -a -f /etc/passwd -a -r /etc/passwd ]] && echo good
# More versatile syntax
> [[ true && true ]] && echo good
> [[ true || false ]] && echo good
> [[ -e /etc/passwd && -f /etc/passwd && -r /etc/passwd ]] && echo good
# Try to launch acpid
> mkdir -p /etc/acpi && acpid -d -f
acpid: /dev/input/event0: No such file or directory
# Print out kernel clock tuning adjustments
> adjtimex
...
time.tv_sec: 1603782690
time.tv_usec: 522862482
...
# Print out system architecture
> arch
x86_64
# Print out ARP table entries (GNU calls this "BSD style")
> arp -a
? (10.0.78.1) at 02:98:0a:0c:68:4f [ether] on eth0
# Ask an IP address to respond with its MAC address
> arping 10.0.78.1
ARPING 10.0.78.1 from 10.0.78.238 eth0
Unicast reply from 10.0.78.1 [02:98:0a:0c:68:4f] 0.060ms
Unicast reply from 10.0.78.1 [02:98:0a:0c:68:4f] 0.082ms
...
# Shell safety options similar to those used in bash, busybox ash does not support set -E.
> set -euxo pipefail
# Capture program output
> echo $(cat /etc/os-release)
NAME="Alpine Linux" ID=alpine VERSION_ID=3.12.1 PRETTY_NAME="Alpine Linux v3.12" HOME_URL="https://alpinelinux.org/" BUG_REPORT_URL="https://bugs.alpinelinux.org/"
# Read lines of a file
> while read -r line; do echo "$line"; done < /etc/os-release
...
PRETTY_NAME="Alpine Linux v3.12"
HOME_URL="https://alpinelinux.org/"
...
# Command group and subshell
> { echo a; echo b; } > out.txt && cat out.txt
a
b
> { a=1; ( a=2; echo "$a" ); echo "$a"; }
2
1
# Integer arithmetic
> echo $((123+345)) $((123<456)) $((123>456))
468 1 0
# For loop, busybox ash does not support bash shortcut "for i in {0..2}"
> for i in 1 2 3; do echo $i; done
1
2
3
> for i in $(seq 0 2); do echo $i; done
0
1
2
# Function
> myfun() { local p1="$1"; local p2="$2"; echo $(($p1+$p2)); }
> myfun 1 2
3
# Join lines, busybox ash does not support "printf -v myvar"
> printf "%s\n" line1 line2 line3
line1
line2
line3
# String substitution
> str='### good 123 stuff ###'
> echo "${str#####}" # Remove the longest ### from prefix
good 123 stuff ###
> echo "${str%%###}" # Remove the longest ### from suffix
### good 123 stuff
> echo "${str//good/ok}" # Replace all occurrences of "good" by "ok"
### ok 123 stuff ###
# Parameter substitution
> echo "${non_existing_var:-default123}"
default123
> echo "${PWD:+gotpwd}"
gotpwd
# Use IFS for the field separator used by "read"
> echo 'field 1, field 2, field 3' | while IFS=', ' read -r a b c; do echo "<$a> <$b> <$c>"; done
<field> <1> <field 2, field 3>
# Use IFS for trimming spaces and expanding parameters
> str='field 1 + field 2 + field 3'
> IFS=' +'
> printf "<%s>\n" $str
<field>
<1>
<field>
<2>
<field>
<3>
# Customise field separator and feed awk script as a program argument
> awk -F ':' -e '{print $1}' /etc/passwd
...
ftp
sshd
...
# Slightly more advanced scripting
> awk -v OFS=' --- ' -F ':' -e '{ sh_cnt[$7]++ } END { for (sh in sh_cnt) { print sh, sh_cnt[sh] } }' /etc/passwd
...
/sbin/halt --- 1
/sbin/nologin --- 23
...
# Determine the name excluding extension name of a file identified by its path
> basename /etc/a/b/myfile.txt .txt
myfile
# Floating point math
> echo '1.1 + 2.2 * 3.3 / 4.4' | bc
2.1
# The classic puzzle in C
> echo 'i = 1; i++ + ++i; i' | bc
4
3
# Print the attributes of all block devices
> blkid
/dev/root: LABEL="cloudimg-rootfs" UUID="f387d281-b162-4d60-84b5-e7e94687e6b8" TYPE="ext4"
/dev/nvme0n1p1: LABEL="cloudimg-rootfs" UUID="f387d281-b162-4d60-84b5-e7e94687e6b8" TYPE="ext4"
...
# Print the attributes of a specified block device
> blkid /dev/loop7
/dev/loop7: TYPE="squashfs"
# Discard (trim) all blocks on an SSD
> blkdiscard /dev/root # will not run in that docker container
# Flush block device buffers
> blockdev --flushbufs /dev/root # will not run in that docker container
# The much newer busybox (version >1.13) can list network bridges in addition to creating/deleting them
> brctl show
...
# Decompress data
> echo 'abcdefghijklmnopqrstuvwxyz' | bzip2 -9 | bunzip2
abcdefghijklmnopqrstuvwxyz
# Print decompressed data
> echo 'abcdefghijklmnopqrstuvwxyz' | bzip2 -9 | bzcat
abcdefghijklmnopqrstuvwxyz
# Compress data and inspect the compressed data using hexdump
> echo 'abcdefghijklmnopqrstuvwxyz' | bzip2 -9 | hexdump -C
00000000 42 5a 68 39 31 41 59 26 53 59 df 1d 4d 7f 00 00 |BZh91AY&SY..M...|
....
# Print the entire year's calendar
> cal -y
2020
January February March
...
# Print input data, including non-printable characters (dispalyed as ^x or M-x), and number the lines in the output
> echo 'abcdefghijklmnopqrstuvwxyz' | bzip2 -9 | cat -v -n
# 1 BZh91AY&SYM-_^]M^?^@^@^@M-AM-^@^@^P?M-^?M-^?M-p ^@1M^Z^@M-P^@^D ^@^@^FM-^
# Recursively change group to "nobody" for everything underneath /var
> chgrp -R nobody /var && ls -lR /var
drwxr-xr-x 1 root nobody 4096 Oct 21 09:23 cache
dr-xr-xr-x 1 root nobody 4096 Oct 21 09:23 empty
...
# Recursively change mode to 0700 for everything underneath /var, print out entries that are changed.
> chmod -c -R 0700 /var
mode of '/var' changed to 0700 (rwx------)
mode of '/var/empty' changed to 0700 (rwx------)
...
# Use plain text to change the password of nobody
> echo 'nobody:passwordofnobody' | chpasswd
chpasswd: password for 'nobody' changed
> grep nobody /etc/shadow
nobody:$6$RS84MTWHARoSMBQZ$9tpbATw.pa6Bk476ysBewpgNhyPaX7LiK.jpQP4z6XpAGWu3mJMyvNY51zOozIm6YQMJ9onifFt2tjbbpu6MT/:18564:0:::::
# Use an already hashed password to change the password of nobody
> echo 'nobody:$6$RS84MTWHARoSMBQZ$9tpbATw.pa6Bk476ysBewpgNhyPaX7LiK.jpQP4z6XpAGWu3mJMyvNY51zOozIm6YQMJ9onifFt2tjbbpu6MT/' | chpasswd -e
chpasswd: password for 'nobody' changed
> grep nobody /etc/shadow
nobody:$6$RS84MTWHARoSMBQZ$9tpbATw.pa6Bk476ysBewpgNhyPaX7LiK.jpQP4z6XpAGWu3mJMyvNY51zOozIm6YQMJ9onifFt2tjbbpu6MT/:18564:0:::::
# Be aware that busybox's getent does not support "shadow" database
# The alpine linux container must have sys_chroot capability, which docker already grants to it by default.
> chroot / /bin/sh
/ #
# Try to switch to /dev/tty1 (does not exist)
> chvt 1
chvt: can't open console
# Clear screen
> clear
# Print the first occurrence of content difference between two files
> cmp /etc/passwd /etc/shadow
/etc/passwd /etc/shadow differ: char 6, line 1
# Print byte number and byte value of all differing bytes in a table output format
> cmp -l /etc/passwd /etc/shadow
6 170 41
...
422 151 12
cmp: EOF on /etc/shadow
# Make a backup of /etc while trying to preserve file modes, ownerships, and timestamps.
> cp -p -R /etc /etc-bakup
# Create an archive for all files underneath /etc
> find /etc -print | cpio -H newc -o -v > archive.cpio
# List archive content
> cat ./archive.cpio | cpio -t
etc
etc/apk
...
etc/resolv.conf
573 blocks
# Extract the archive
> mkdir /restoring-etc
> mv archive.cpio /restoring-etc
> cat ./archive.cpio | cpio -i -v -d
etc
etc/apk
...
etc/resolv.conf
573 blocks
# This exercise is carried out on the container host, instead of the alpine linux in docker container.
# Without cttyhack...
> busybox sh
> stty
speed 38400 baud; line = 0;
eol = M-^?; eol2 = M-^?;
-brkint -imaxbel
> tty
/dev/pts/2
# With cttyhack
> busybox cttyhack sh
# This hack does not provide all of TTY's capabilities, evident from malfunctioning tab completion, malfunctioning arrow keys, etc.
> stty
speed 38400 baud; line = 0;
eol = M-^?; eol2 = M-^?;
-brkint -imaxbel
> tty
/dev/tty
# Print the first column of /etc/passwd
> cut -d ':' -d 1 /etc/passwd
root
bin
...
nobody
# Print the first 5 characters of each line of /etc/passwd
> cut -c 1-5 /etc/passwd
root:
bin:x
...
nobod
# Print UTC date
> date -u '+%Y%m%d %H%M%S'
20201029 152416
# Use reverse polish notation to calculate 1.1 + 2.2
> echo '1.1 2.2 + p' | dc
3.3
# Create an empty file 100MB in size
> dd if=/dev/zero of=./empty bs=1M count=100
# Not quite sure what this is for
> deallocvt
deallocvt: can't open console
# "Generate modules.dep, alias, and symbol files"
> depmod
depmod: can't change directory to 'lib/modules/5.4.0-1028-aws': No such file or directory
# Read 8 bytes (32 bits) from memory location 0, this has to run on container host.
> sudo busybox devmem 0 32
0xF000FF53
# Print file system type and file system usage in a friendly readable format
> df -T -h
Filesystem Type Size Used Available Use% Mounted on
overlay overlay 77.5G 24.3G 53.1G 31% /
tmpfs tmpfs 64.0M 0 64.0M 0% /dev
...
# Print the differences (- belongs to the first file, + belongs to the second file)
> diff -w /etc/passwd /etc/shadow
--- /etc/passwd
+++ /etc/shadow
@@ -1,27 +1,27 @@
-root:x:0:0:root:/root:/bin/ash
-bin:x:1:1:bin:/bin:/sbin/nologin
...
# Get the directory name of a path
> dirname /etc/config.txt
/etc
# Read kernel messages
> dmesg
...
...
[1219562.256341] docker0: port 2(veth3b1b773) entered forwarding state
# Print system's NIS/YP domain name
> dnsdomainname
(empty)
# Convert lines from stadard input to UNIX line ending
> echo abc | dos2unix | hexdump -C
00000000 61 62 63 0a |abc.|
00000004
# Calculate the grand total of storage capacity consumed by /etc and /var
> du -hcs /etc /var # h - readable size output; c - print grand total; s - sum each FILE/DIR
536.0K /etc/
72.0K /var
608.0K total
# "Print a binary keyboard translation table to stdout" not quite sure how to use the output
> dumpkmap
(bunch of binary stuff)
# It probably works best with busybox's own udhcpc client
> dumpleases
dumpleases: can't open '': No such file or directory
# Be aware that systemd stores DHCP leases in /run/systemd/netif/leases/NUMBER
# Systemd's DHCP lease report is not supposed to be stable for machine-parsing
# Print a line, interpret backslash escaped characters.
> echo -e 'a\tb\nc'
a b
c
# Use grep -E instead, grep -E is more universally available.
# Start shell with an empty environment and couple of custom assignments
> env -i VarA=123 VarB=haha sh
> printenv VarA VarB
123
haha
# Expand every tab to 5 spaces
> echo -e 'a\tb' | expand -t 5
a b
# Basic integer path
> expr 1 + 2 - 3 + 4
4
# Integer comparison
> expr '1' '<' '3' && echo good
1
good
# Test regular expression match. Note that expr only understands the regular regex in contrast to extended regex.
> str='### good 123 stuff ###'
> expr match "$str" '.*good.*$' && echo good
22
good
> expr match "$str" '.*doesnotexist.*$' && echo good
0
# Extract regex match from string The regex processing behaviour is also highly unusual with expr, nobody wouldn't wirte regex this way with bash match or grep -E.
> expr "$str" : '.*\([0-9a-z][0-9a-z\ ]\+\)\+'
good 123 stuff # there is a trailing space
# Print prime factors of an integer
> factor 12
12: 2 2 3
# Preallocate space for a file
> fallocate -l 123 my.txt
> stat my.txt
...
Size: 123 Blocks: 8 IO Block: 4096 regular file
...
# Get a process exit code of 1
> false
> echo $?
1
# Change file attributes on a FAT system... I don't have one
# fgrep is equivalent to grep -F, which interprets input pattern as regular string instead of regular expression.
# Find all files executable by everyone, throw in a -print0 for some additional complication.
> find / -type f -perm -a=x -print0 | tr '\0' '\n'
/lib/libcrypto.so.1.1
/lib/ld-musl-x86_64.so.1
...
# Look for the /dev/xxx device node that belongs to a block device identified by LABEL
> findfs LABEL="cloudimg-rootfs"
/dev/root
# Hold onto an advisory exclusive lock on a file while spawning a shell
> flock -x /etc/passwd -c sh
# Be aware that the locks are strictly advisory, a process with suitable permissions may ignore the lock's presence entirely.
# Print file content, wrap lines at maximum length of 20.
> fold -w 20 /etc/passwd
root:x:0:0:root:/roo
t:/bin/ash
...
# Print memory and swap usage information, round numbers to the nearest megabytes.
> free -m
total used free shared buff/cache available
Mem: 7882 1345 3254 0 3281 6229
Swap: 2047 0 2047
# Discard (trim) unused blocks in the file system identified by mount point, print discard stats. Useful for SSDs.
> fstrim -v /
fstrim: ioctl 0xc0185879 failed: Not a tty # hehe wtf
# Write all buffered blocks of a file to disk
> fsync /etc/os-release
# Find the processes currently using file /etc/os-release
> flock -x /etc/os-release -c sh
> fuser /etc/os-release
6 7
# flock itself, and sh spawned from it.
# TODO: ...come back to this one later
# Spawn a TTY number 1 operating at 38400 bauds/sec
> getty 38400 tty1
getty: setsid: Operation not permitted
# Use extended regex to look for lines of X="Y" which has an upper case letter in the value part. Display line number.
> grep -n -E '".*[A-Z]+' /etc/os-release
1:NAME="Alpine Linux"
4:PRETTY_NAME="Alpine Linux v3.12"
# Print the primary group and secondary groups a user belongs to
> groups nobody
nobody
> groups root
root bin daemon sys adm disk wheel floppy dialout tape video
# Decompress data
> echo 'abcdefghijklmnopqrstuvwxyz' | gzip -9 | gunzip
abcdefghijklmnopqrstuvwxyz
# Compress data and inspect the compressed data using hexdump
> echo 'abcdefghijklmnopqrstuvwxyz' | gzip -9 | hexdump -C
00000000 1f 8b 08 00 00 00 00 00 02 03 4b 4c 4a 4e 49 4d |..........KLJNIM|
....
# Halt the system doesn't really work from a container
> halt -d 3 -f # delay 3 seconds and then force halt without going through initi
halt: Operation not permitted
# hd is an alias of hexdump -C, print the content of input in both hex and ASCII.
> echo 'abcdefghijklmnopqrstuvwxyz' | gzip -9 | hd
00000000 1f 8b 08 00 00 00 00 00 02 03 4b 4c 4a 4e 49 4d |..........KLJNIM|
...
# Check the number of readahead sectors of a hard drive
> hdparm -a /dev/root
/dev/root:
readahead = 256 (on)
# Print the first 3 lines
> head -n 3 /etc/passwd
root:x:0:0:root:/root:/bin/ash
bin:x:1:1:bin:/bin:/sbin/nologin
daemon:x:2:2:daemon:/sbin:/sbin/nologin
# Print all lines except the last 20
> head -n -20 /etc/passwd
root:x:0:0:root:/root:/bin/ash
bin:x:1:1:bin:/bin:/sbin/nologin
daemon:x:2:2:daemon:/sbin:/sbin/nologin
...
shutdown:x:6:0:shutdown:/sbin:/sbin/shutdown
# Print the hex and ASCII representation of the first 20 bytes of /etc/passwd
> hexdump -n 20 -C /etc/passwd
00000000 72 6f 6f 74 3a 78 3a 30 3a 30 3a 72 6f 6f 74 3a |root:x:0:0:root:|
00000010 2f 72 6f 6f |/roo|
00000014
# Print the hex representation of the 32-bit UNIX machine ID
> hostid
00000000
# Print the host FQDN
> hostname -f
f2abc39a5794
# Print the IP address of the host name
> hostname -i
172.17.0.2
# Print the static host name - what kernel believes to be the host name in the UTS namespace.
> hostname
f2abc39a5794
# Related to this, host names managed by systemd are:
# - Pretty host name - more like a computer description, this has little to do with UTS host name.
# - Static host name - what kernel believes to be the host name in that UTS namespace, this is "kernel.hostname", gethostname(2), as well as /etc/hostname.
# - Transient host name - what network configuration beleives to be the host name.
# Display the hardware clock time
> hwclock -r
Sat Oct 31 16:00:17 2020 0.000000 seconds
# Get the effective user ID
> id -u
0
# Get the real user ID
> id -r -u
0
# Tip: real UID is the owner of the process, the owner of /proc/PROC_ID.
# Effective UID is used for OS to decide what the process can do.
# Saved UID is used by kernel to restore effective UID after it has executed a setuid program.
# Print information of all configured network interface
> ifconfig
eth0 Link encap:Ethernet HWaddr 02:42:AC:11:00:02
inet addr:172.17.0.2 Bcast:172.17.255.255 Mask:255.255.0.0
...
# Do a dry-run (print what would be done but otherwise noop) to bring offline ("deconfigure") all network interfaces
> ifdown -n -r
ifdown: can't open '/etc/network/interfaces': No such file or directory # hehehehe
# "Configure network interfaces for parallel routing" not quite sure how to use this
# Do a dry-run (print what would be done but otherwise noop) to bring online ("configure") all network interfaces
> ifup -n -a
ifup: can't open '/etc/network/interfaces': No such file or directory # hehehehe
# Spawn the init (the-first-process) that never exits, the init process spawns initial set of processes specified in /etc/inittab.
> init
init: must be run as PID 1 # hehehe
# Monitor file system changes for a file, print the changes (read/access/open/delete/move/etc) to stdout.
> inotifyd - /etc/passwd &
> # press enter
r /etc/passwd # r means file is opened
a /etc/passwd # a means file is "accessed"
0 /etc/passwd # 0 means file was not written into, and it is closed.
# insmod - load a kernel module file
> insmod /etc/os-release
insmod: can't insert '/etc/os-release': Operation not permitted # hehe
# Create directory /a/b/c, set owner, group, and mode.
> install -d -o nobody -g nogroup -m 0700 /a/b
> stat /a
...
Access: (0755/drwxr-xr-x) Uid: ( 0/ root) Gid: ( 0/ root)
...
> ls -lR /a
...
drwx------ 2 nobody nogroup 4096 Oct 31 16:33 b
...
# Copy file /etc/os-release to /bak/haha/os.txt, set owner, group, and mode.
> install -D -o nobody -g nogroup -m 0700 /etc/os-release /bak/haha/os.txt
> ls -lR /bak
/bak:
total 4
drwxr-xr-x 2 root root 4096 Oct 31 16:35 haha
/bak/haha:
total 4
-rwx------ 1 nobody nogroup 164 Oct 31 16:35 os.txt
# Start a shell using IO priority class idle (3), a regular user may also use IO class best-effort (2).
> ionice -c 3 sh
# Set IO priority class of a running process (PID 1)
> ionice -c 3 -p 1
# Print the system time, CPU usage, and IO activity stats (prefer MB/s) from all block devices.
> iostat -t -c -d -m
Linux 5.4.0-1029-aws (496a1090587a) 11/02/20 _x86_64_ (2 CPU)
11/02/20 17:46:00
avg-cpu: %user %nice %system %iowait %steal %idle
0.95 0.00 0.42 0.06 0.08 98.49
Device: tps MB_read/s MB_wrtn/s MB_read MB_wrtn
...
nvme0n1p1 7.15 0.03 0.09 4815 15721
...
# Note that busybox iostat does not support "-x" flag for displaying extended stats such as queue size.
# Create a pair of interconnected veth interfaces
> ip link add myeth type veth peer name veth1
> ip addr add 10.1.0.0/16 dev myeth
> ip link set myeth up
> ip addr add 10.2.0.0/16 dev veth1
> ip link set veth1 up
> ping -c 1 10.1.0.0
... 1 packets transmitted, 1 packets received, 0% packet loss
> ping -c 2 10.1.0.0
... 1 packets transmitted, 1 packets received, 0% packet loss
# ipaddr is "ip addr"
# Calculate the net mask, braodcast address, and network address of 192.168.1.0/24
> ipcalc -n -b -m 192.168.1.0/24
NETMASK=255.255.255.0
BROADCAST=192.168.1.255
NETWORK=192.168.1.0
# busybox is missing "ipcmk", do this exercise using the fully featured container host OS.
> ipcmk -M 123
Shared memory id: 2
> ipcmk -S 3
Semaphore id: 0
> ipcmk -Q
Message queue id: 2
> ipcs
------ Message Queues --------
key msqid owner perms used-bytes messages
0x95de7a0f 2 howard 644 0 0
------ Shared Memory Segments --------
key shmid owner perms bytes nattch status
0x199f0173 2 howard 644 123 0
------ Semaphore Arrays --------
key semid owner perms nsems
0x4d05ffec 0 howard 644 3
> ipcrm -Q 0x95de7a0f
> ipcrm -M 0x199f0173
> ipcrm -S 0x4d05ffec
> ipcs
(empty in each category)
# See above
# iplink is "ip link"
# ipneigh is "ip neighbour". Display the entries from ARP table
> ipneigh
172.17.0.1 dev eth0 lladdr 02:42:e5:bb:4e:7c used 0/0/0 probes 4 STALE
# iproute is "ip route". Display the entries from IP routing table.
> iproute
default via 172.17.0.1 dev eth0
172.17.0.0/16 dev eth0 scope link src 172.17.0.2
# iprule is "ip rule"
# iptunnel is "ip tunnel"
# Set VT console keyoard mode to UTF-8
> kbd_mode -u
> kbd_mode
The keyboard is in Unicode (UTF-8) mode
# Start a background job and force-killing it
> sleep 100 &
> jobs -l
[1]+ 34 Running sleep 100
> /bin/kill -s 9 -34 # negative PID finds the process group
[1]+ Killed sleep 100
# Kill all "sleep" processes with signal KILL (9)
> for i in $(seq 0 10); do sleep 100 & done
> killall -s KILL sleep
# Kill all proceses except the ones in caller's own session
> for i in $(seq 0 10); do sleep 100 & done
> killall5 -9
[8]+ Stopped (signal) sleep 100
[10]- Stopped (signal) sleep 100
[11] Stopped (signal) sleep 100
...
# killall5 doesn't quite work
# Run a foreground program that logs kernel messages to syslog
> klogd -n
# Use a pager to view /etc/os-release
> less -I -F -R -S /etc/os-release # I - ignore case in search, F - quit if file already fits in a single screen, R - remove colour escape codes, S - truncate long lines.
# Create a hard link for /etc/passwd (the original) at path /etc/passwd2 (the link)
> link /etc/passwd /etc/passwd2
# No help available
# No help available
# Create a symbol link for /etc/passwd (the original) at path /etc/passwd2 (the link), override existing link/file at the path.
> ln -sf /etc/passwd /etc/passwd2
> stat /etc/passwd2
File: '/etc/passwd2' -> '/etc/passwd'
Size: 11 Blocks: 0 IO Block: 4096 symbolic link
...
Access: (0777/lrwxrwxrwx) Uid: ( 0/ root) Gid: ( 0/ root)
...
# Load a VT console font from stdin
> echo whatever | loadfont
loadfont: input file: bad length or unsupported font type
# Load a VT keyboard binary translation table from stdin
> echo whatever | loadkmap
loadkmap: not a valid binary keymap
# Send a message to syslog and print it out to stderr too
> logger -s this is a message
root: this is a message
# Login as root in the current console
> login -f root
Welcome to Alpine!
...
login[28]: root login on 'pts/0'
cbeef31aa40f:~#
# Follow the latest messages in syslog circular buffer
> logread -f
logread: can't find syslogd buffer: No such file or directory
# Do this exercise on container host
> dd if=/dev/zero of=/diskimg bs=1M count=10
> losetup -P /dev/loop9 /diskimg # -P tells losetup to probe for partitions
> losetup -a
...
/dev/loop9: [66305]:28541 (/file)
...
> losetup -D /dev/loop9
# List files by last modified - latest first
> ls -lt /etc
lrwxrwxrwx 1 root root 11 Nov 3 16:31 passwd2 -> /etc/passwd
...
-rw-r--r-- 1 root root 53 May 30 17:17 sysctl.conf
# List files by size - largest first
> ls -lS /etc
-rw-r--r-- 1 root root 14464 May 30 17:17 services
...
-rw-r--r-- 1 root root 7 Oct 21 09:22 alpine-release
# List loaded kernel modules and their dependencies
# Show all open files
> lsof
1 /bin/busybox /dev/pts/0
...
28 /bin/busybox /dev/tty
# The regular lsof is far more powerful, but the features are not present in busybox lsof.
> lsof -n -i tcp:80 # Look for all TCP sockets whose peer is port 80
laitos.li 933 root 8u IPv4 4944780 0t0 TCP 10.0.78.238:46930->35.207.39.2:http (ESTABLISHED)
laitos.li 933 root 16u IPv6 24602 0t0 TCP *:http (LISTEN)
...
> lsof -n -p 933 # Look for all sockets, pipes, and open files made by PID 933
laitos.li 933 root txt REG 259,1 22324228 6912049 /hg/bin/laitos.linux
...
laitos.li 933 root 6u IPv4 4946704 0t0 TCP 10.0.78.238:32106->149.154.167.220:https (ESTABLISHED)
laitos.li 933 root 16u IPv6 24602 0t0 TCP *:http (LISTEN)
# Print information about devices on PCI buses in a parsable format, along with the driver name.
> lspci -m -k
00:00.0 "Host bridge" "Intel Corporation" "440FX - 82441FX PMC [Natoma]" "Amazon.com, Inc." "440FX - 82441FX PMC [Natoma]"
...
# Print information about devices on USB buses
> lsusb
# Looks a bit empty
# Don't know how to make it work
> echo 'abc' | lzop -5 - | lzcat
lzcat: corrupted data
# Don't know how to make it work
> echo 'abc' | lzop -5 - | lzma -d -c
lzma: corrupted data
# Compress and decompress from pipe
> echo 'abcdefghijklmnopqrstuvwxyz' | lzop -5 - | lzopcat
abcdefghijklmnopqrstuvwxyz
# Compress and decompress from pipe
> echo 'abcdefghijklmnopqrstuvwxyz' | lzop -5 - | lzopcat
abcdefghijklmnopqrstuvwxyz
# Create multipart MIME-encoded message from input file list
> makemime /etc/os-release /etc/alpine-release
Mime-Version: 1.0
Content-Type: multipart/mixed; boundary="1858163020-1256360926-1801041304"
--1858163020-1256360926-1801041304
Content-Type: application/octet-stream; charset=us-ascii
Content-Disposition: inline; filename="os-release"
Content-Transfer-Encoding: base64
...
Content-Disposition: inline; filename="alpine-release"
...
# Calculate the MD5 checksum of input from pipe
> echo -n | md5sum
d41d8cd98f00b204e9800998ecf8427e -
# Create a log file for mdev
> > /dev/mdev.log
# Ask mdev to auto-populate /dev when peripherals are hot-plugged/unplugged
> echo /sbin/mdev >/proc/sys/kernel/hotplug
# Manually execute mdev at time of boot to populate /dev
> mdev -s
# When other users send me a talk/wall message, allow that message to be displayed in my terminal.
> mesg y
# Interactively chat with a serial TTY-capable device
> microcom -s 38400 $(tty)
# Hehe - doesn't really work..
# Make a new directory including any new intermediate parent directories in between.
> mkdir -m 0700 -p /a/b/c
> stat /a/b/c
...
Access: (0700/drwx------) Uid: ( 0/ root) Gid: ( 0/ root)
...
# Make a fat32 file system, same as mkfs.vfat.
# TODO
# Create a named pipe and read from it
> mkfifo /myfifo
> cat /myfifo &
# Write to it
> echo 'aa' >> /myfifo
aa
[1]+ Done cat /myfifo
# Writing to the file blocks when there is no reader
> echo 'aa' >> /myfifo
(Ctrl+C)
/bin/sh: can't create /myfifo: Interrupted system call
# See above mkdosfs
# Create my own "zero" character device
> mknod -m 0400 /myzero c 1 5
# Calculate hashed password conforming to crypt(3)
> echo 'pass' | mkpasswd -m sha512 -S grainofsalt -P 0
$6$grainofsalt$NADWtRPlvlRArLOHrhjpwx0TZ3xkgJGzmF7suF/x7DbEyEA9Yv78POjRHfeA9a/mN3zcLhJKjGFuMFQ18gc8Q.
# Prepare (format) a block device to be used as swap partition
> dd if=/dev/zero of=/myswap bs=1M count=10
10+0 records in
10+0 records out
> mkswap /myswap
Setting up swapspace version 1, size = 10481664 bytes
UUID=4ce4a68b-cd1a-43f6-9f7b-7cd202ed6c52
# Create a temporary file and print out the temporary file path
> mktemp /tmp/helloworld-XXXXXX
> /tmp/helloworld-dpkgbI
# Print kernel module information (do this exercise on container host)
> modinfo drm
filename: /lib/modules/5.8.0-1010-aws/kernel/drivers/gpu/drm/drm.ko
license: GPL and additional rights
description: DRM shared core routines
...
# Add or remove modules from the running kernel
# TODO
# more is a rudimentary pager program
> more /etc/os-release
NAME="Alpine Linux"
...
# mount is able to auto-detect file system present on the device, given that /proc is available.
# Mount all file systems according to /etc/fstab
> mount -a
# Check whether a directory is a mount point
> mountpoint /
/ is a mountpoint
> mountpoint -q /etc/os-release || echo not-mount-point
not-mount-point
# Report interrupt stats, processor stats, and utilisation fromall processors.
> mpstat -I ALL -p ALL -u
Linux 5.8.0-1010-aws (cfdfa48aa979) 11/06/20 _x86_64_ (2 CPU)
...
07:25:22 CPU %usr %nice %sys %iowait %irq %soft %steal %guest %idle
...
07:25:22 0 0.95 0.04 0.44 0.10 0.00 0.02 0.05 0.00 98.40
...
07:25:22 CPU intr/s
...
07:25:22 1 13.53
...
# Rename/move a file
> mv /etc/os-release /haha
# Create an ethernet peer
> ip link add myeth type veth peer name veth1
> ip link
3: myeth@veth0: <BROADCAST,MULTICAST,M-DOWN> mtu 1500 qdisc noop state DOWN qlen 1000
link/ether 0a:60:da:56:ee:9f brd ff:ff:ff:ff:ff:ff
> nameif neweth mac=0a:60:da:56:ee:9f brd ff:ff:ff:ff:ff:ff
> ip link
3: neweth@veth0: <BROADCAST,MULTICAST,M-DOWN> mtu 1500 qdisc noop state DOWN qlen 1000
link/ether 0a:60:da:56:ee:9f brd ff:ff:ff:ff:ff:ff
# Dump an "MTD" device, not sure how to use it.
# Write to an "MTD" device, not sure how to use it.
# nbd-client connects to an nbd-server (not available on busybox) using raw diskspace of the server as a block device on the client.
# Start a disposable single-use TCP server to copy the content of /etc/os-release to TCP client
> nc -l -p 1234 -e cat /etc/os-release &
> nc localhost 1234
NAME="Alpine Linux"
...
BUG_REPORT_URL="https://bugs.alpinelinux.org/"
[1]+ Done nc -l -p 1234 -e cat /etc/os-release
# Print all IP connection servers
> netstat -lptun # listening, get process name, tcp, udp, do not resolve address.
Active Internet connections (only servers)
Proto Recv-Q Send-Q Local Address Foreign Address State PID/Program name
...
# Switch to a less privileged user and try out nice
> su -s /bin/sh nobody
> nice -n -10 sh
nice: setpriority(-10): Permission denied
> nice -n 10 sh
> nice
10
# Display file content with line numbers
> nl /etc/os-release
1 NAME="Alpine Linux"
...
6 BUG_REPORT_URL="https://bugs.alpinelinux.org/"
# At interval of one second, print several key system stats
> nmeter -d 1000 'IRQ rate %i Ctx switch %x Forks %p block IO %b'
IRQ rate 208 Ctx switch 392 Forks 2 block IO 0 24k
IRQ rate 319 Ctx switch 757 Forks 49 block IO 0 0
IRQ rate 177 Ctx switch 390 Forks 0 block IO 0 192k
# Run a program and make it immune to terminal hangup (SIGHUP)
> nohup yes &>/dev/null &
# Inform logged-in user that shell is not available for this user
> nologin
This account is not available
(and hangs there)
# Print the number of available CPUs
> nproc --all
2
# Do this exercise outside of alpine container.
# Create a new user namespace
> unshare -r -p --fork --mount-proc
# In another shell of the logon user, print a list of all namespaces.
> lsns
# Identify the process ID of the newly created user namespace and enter it
> sudo nsenter --all --target 625537
# Look up the A records of a DNS name
> nslookup -type=a hz.gl
Server: 9.9.9.9
Address: 9.9.9.9:53
Non-authoritative answer:
Name: hz.gl
Address: 13.48.0.5
# Look up the TXT records of a DNS name
> nslookup -type txt hz.gl
Server: 9.9.9.9
Address: 9.9.9.9:53
Non-authoritative answer:
hz.gl text = "v=spf1 mx a mx:hz.gl mx:howard.gg mx:houzuo.net mx:ard.how ?all"
...
# Synchronise system clock with an NTP server
# (verbose, do not daemonize, quit after setting clock, peer server name)
> ntpd -d -n -q -p ca.pool.ntp.org
ntpd: 'ca.pool.ntp.org' is 209.115.181.113
ntpd: sending query to 209.115.181.113
ntpd: reply from 209.115.181.113: offset:-0.008284 delay:0.165039 status:0x24 strat:2 refid:0x83006cce rootdelay:0.035508 reach:0x01
# Print printable characters and escape sequences of the standard input
> echo 'haha' od --format c -
0000000 h a h a \n
0000005
# Start a program on a new virtual terminal
> openvt -c 1 -s -w
can't find open VT (hehe)
# Ask kernel to rescan partition table
> partprobe
# Lock and disable a user account
> passwd -l root
# Paste lines from each input file, separated by a comma.
> paste -d ',' /etc/os-release /etc/os-release
NAME="Alpine Linux",NAME="Alpine Linux"
ID=alpine,ID=alpine
...
# Find the newest PID from sleep process
> sleep 1000 &
> pgrep -n sleep
124
# Find the newest PID from a process that mentions "1000" in its command line
> pgrep -n -f 1000
124
# Find exactly one process that runs the sleep program
> sleep 1000 &
> pidof sleep
133
# Send exactly one ping request to hz.gl
> ping -c 1 hz.gl
PING hz.gl (13.48.0.5): 56 data bytes
...
1 packets transmitted, 1 packets received, 0% packet loss
...
# Send exactly one ICMPv6 ping request to localhost
> ping6 -c 1 ::1
PING ::1(::1) 56 data bytes
...
1 packets transmitted, 1 received, 0% packet loss, time 0ms
# Print a dot every second while a program is running
> sh -c "while IFS='' read -d $'\n' line; do echo \$line; sleep 1; done < /etc/os-release" | pipe_progress
NAME="Alpine Linux"
.ID=alpine
.VERSION_ID=3.13.0
.PRETTY_NAME="Alpine Linux v3.13"
(and so on)
# move the current root file system to ... what?!
# Similar to pgrep, kill the sleep program by matching its command line.
> sleep 1000 &
> pkill -KILL -f 1000
[1]+ Killed sleep 1000
# Display the memory mapping of a process
> pmap -x 1
Address Kbytes PSS Dirty Swap Mode Mapping
0000564b38826000 48 0 0 0 r--p /bin/busybox
0000564b38832000 612 214 0 0 r-xp /bin/busybox
...
0000564b39904000 4 0 0 0 ---p [heap]
0000564b39905000 4 4 4 0 rw-p [heap]
...
00007ffdcc1d9000 132 8 8 4 rw-p [stack]
...
---------------- ------ ------ ------ ------
total 1700 632 136 20
# Poweroff the computer in two seconds, do not sync, do not go through init.
> poweroff -d 2 -n -f
# Print an environment variable by name
> printenv HOME
/root
# Print right-justified text using minimum field width of 5
> printf '|%5s|%5d|\n' adam 123
| adam| 123|
> printf '|%5s|%5d|\n' adamandeve 12345678
|adamandeve|12345678|
# Print left-justified text using minimum field width of 5
> printf '|%-5s|%-5d|\n' adam 123
|adam |123 |
> printf '|%-5s|%-5d|\n' adamandeve 12345678
|adamandeve|12345678|
# Print with "precision" - max length for a string, min length for an integer.
> printf '|%.5s|%.5d|\n' adam 123
|adam|00123|
> printf '|%.5s|%.5d|\n' adamandeve 12345678
|adama|12345678|
# Print information about processes and threads
> ps -T -o pid,ppid,pgid,sid,user,group,tty,args
PID PPID PGID SID USER GROUP TT COMMAND
1 0 1 1 root root 136,0 /bin/sh
193 1 193 1 root root 136,0 ps -T -o pid,ppid,pgid,sid,user,group,tty,ar
# Scan for open ports between 1 and 100, using a timeout of 100ms, and display closed/blocked ports too.
> pscan -p 1 -P 100 -t 100 -b -c hz.gl
Scanning hz.gl ports 1 to 100
Port Proto State Service
1 tcp open tcpmux
2 tcp blocked unknown
...
7 tcp blocked echo
8 tcp blocked unknown
9 tcp blocked discard
10 tcp blocked unknown
11 tcp open systat
12 tcp blocked unknown
...
22 tcp open ssh
23 tcp open telnet
...
53 tcp open domain
...
80 tcp open http
...
99 tcp blocked unknown
100 tcp blocked unknown
0 closed, 8 open, 92 timed out (or blocked) ports
/ #
# Display process tree along with process PIDs, starting from PID 1
> sleep 1000 & pstree -p 1
sh(1)-+-pstree(202)
`-sleep(201)
# Display the logical path of working directory, i.e. with symlink intact and unresolved.
> pwd
/
# Print the current working directory of a process identified by its PID
> cd /etc/ && sleep 100 &
> pwdx $(pgrep -n sleep)
209: /etc
# Tell the kernel to automatically search and start RAID arrays
# Print or set system clock on a remote computer
# Print the device node associated with the filesystem mounted at /
# Preload a file into memory cache
> dd if=/dev/urandom of=./largefile bs=1M count=400
400+0 records out
> sudo sysctl -w vm.drop_caches=3 # execute on container host
vm.drop_caches = 3
> free -m
total used free shared buff/cache available
Mem: 953 464 186 0 302 366
Swap: 2047 177 1870
> readahead ./largefile
> free -m
total used free shared buff/cache available
Mem: 953 463 185 0 304 367
Swap: 2047 177 1870
# Apparently readahead preloaded the first 2 MB of the file
# Resolve all symlinks to discover the absolute path to a file
> ln -sf /etc/passwd /tmp/hahapass
> readlink -f /tmp/../tmp/hahapass
/etc/passwd
# Resolve all symlinks to discover the absolute path to a file, similar to readlink.
> ln -sf /etc/passwd /tmp/hahapass
> realpath /tmp/../tmp/hahapass
/etc/passwd
# Reboot the computer in two seconds, do not sync, do not go through init.
> poweroff -d 2 -n -f
reboot: (null): Operation not permitted
# Extract content of an MIME section (does not appear to work)
> makemime /etc/os-release | reformime
(empty output)
# Remove a shell from /etc/shells
> remove-shell /bin/ash
> cat /etc/shells
/bin/sh
# Change the scheduling priority of a live process
> sleep 1000 &
> ps -T -o pid,ppid,user,group,tty,nice,args
PID PPID USER GROUP TT NI COMMAND
308 1 root root 136,0 0 sleep 1000
> renice 17 -g 308 # the ceiling of priority number only goes to 19 (lowest priority)
> ps -T -o pid,ppid,user,group,tty,nice,args
PID PPID USER GROUP TT NI COMMAND
308 1 root root 136,0 17 sleep 1000
# Reset the TTY
> reset
(no output)
# Determine the current TTY size and print shell statements that export COLUMNS and LINES
> eval $(resize)
> printenv COLUMNS LINES
94
41
# Reverse the characters on each line of file
> rev /etc/os-release
"xuniL eniplA"=EMAN
enipla=DI
0.31.3=DI_NOISREV
"31.3v xuniL eniplA"=EMAN_YTTERP
"/gro.xunilenipla//:sptth"=LRU_EMOH
"/gro.xunilenipla.sgub//:sptth"=LRU_TROPER_GUB
# Enable/disable wireless devices
> rfkill list all
rfkill: /dev/rfkill: No such file or directory
# Remove (unlink) files and directories (hehe surely I don't need an example here)
# Remove empty directories, including the empty parent directories
> mkdir -p a/b/c
> rmdir -p a/b/c
(parent directory a no longer exists)
# Wait for a kernel module to become unused and then forcibly unload it
> rmmod -w -f drm
rmmod: can't unload module 'drm': Operation not permitted
# Print kernel IP routing table with extra info, and do not resolve names.
> route -e -n
Destination Gateway Genmask Flags MSS Window irtt Iface
0.0.0.0 172.17.0.1 0.0.0.0 UG 0 0 0 eth0
172.17.0.0 0.0.0.0 255.255.0.0 U 0 0 0 eth0
# Glob and run executable scripts underneath a directory, pass argument "haha" to them.
> mkdir -p scripts
> echo -e '#!/bin/sh\necho script 1 arg $1' > scripts/a1
> echo -e '#!/bin/sh\necho script 2 arg $1' > scripts/a2
> chmod -R 755 scripts
> run-parts -a haha script
script 1 arg haha
script 2 arg haha
# Use POSIX basic regex - the original grep flavour
# Basic regex requires all meta characters (except .) to be escaped with a backslash
> echo 'abc123' | sed 's/[1-3]\{1,3\}y\?/Z/'
abcZ
# Use POSIX extended regex
> echo 'abc123' | sed -E 's/[1-3]{1,3}y?/Z/'
abcZ
# Send a mail using SMTP server on the container's host
> echo -e 'From: from@example.com\r\n\r\nThis Is A Message' | sendmail -S 172.17.0.1:25 -w 5 -f from@example.com -t to@example.com -v
sendmail: recv:'220 ard.how houzuo.net howard.gg hz.gl ESMTP'
sendmail: send:'EHLO de06b04310d2'
sendmail: recv:'250-ard.how houzuo.net howard.gg hz.gl'
...
sendmail: send:'MAIL FROM:<from@example.com>'
sendmail: recv:'250 2.1.0 OK'
...
sendmail: send:'RCPT TO:<to@example.com>'
sendmail: recv:'550 Bad address'
...
sendmail: send:'QUIT'
# Generate an integer sequence, useful for shell programming.
> for i in $(seq 1 5); do echo $i; done
1
2
3
4
5
# Ask everything written to /dev/console to be copied to /dev/pts/0
> setconsole /dev/pts/0
setconsole: ioctl 0x541d failed: Operation not permitted
# Load a console font
# Modify kernel's mapping between key scan codes to key codes.
# Ask kernel output to be printed to VT console 2
> setlogcons 2
setlogcons: can't open '/dev/tty2': No such file or directory
# Print privilege of the invoking process
> setpriv -d
uid: 0
...
gid: 0
...
Inheritable capabilities: chown ...
Ambient capabilities: [none]
Capability bounding set: chown ...
# busybox setpriv is not particularly easy to use, it is missing many of the features (e.g. specifying capability -all,+chown) supported by fully featured setpriv.
# Print or set serial port parameters
# Make sure a program won't get keyboard signals from the controlling terminal
> sleep 1000 &
> setsid sleep 1234
> ps -T -o pid,ppid,sid,user,group,tty,nice,args
PID PPID SID USER GROUP TT NI COMMAND
1 0 1 root root 136,0 0 /bin/sh
6 1 1 root root 136,0 0 sleep 1000
8 1 8 root root ? 0 sleep 1234
# The shell interpreter
> sh -x -e -u -c 'echo $0 $1 $2' a b c
+ echo a b c
a b c
# Calculate SHA-1 checksum
> echo -n '' | sha1sum
da39a3ee5e6b4b0d3255bfef95601890afd80709 -
# Calculate SHA-2 256-bit checksum
> echo -n '' | sha256sum
e3b0c44298fc1c149afbf4c8996fb92427ae41e4649b934ca495991b7852b855 -
# Calculate SHA-3 256-bit checksum
> echo -n '' | sha3sum -a 256
a7ffc6f8bf1ed76651c14756a061d662f580ff4de43b49fa82d80a4b80f8434a -
# Calculate SHA-2 512-bit checksum
echo -n '' | sha512sum
cf83e1357eefb8bdf1542850d66d8007d620e4050b5715dc83f4a921d36ce9ce47d0d13c5d85f2b0ff8318d2877eec2f63b931bd47417a81a538327af927da3e -
# Show the interpreted keycodes for keys pressed in the VT
> showkey
# Shred a file by overwriting it with random data 3 times, then overwriting it with zero, and finally deleting it.
> shred -n 3 -z -u /etc/os-release
# Get three numbers from a shuffled range of 1 to 10 inclusive
> shuf -n 3 -e $(seq 1 10)
# Configure serial line as SLIP networking interface hehehehe
# Sleep for 0.1 minutes (6 seconds)
> sleep 0.1m
# Sort lines in /etc/os-release, ignore leading blank, ignore case, sort numbers too.
> sort -b -f -n /etc/os-release
BUG_REPORT_URL="https://bugs.alpinelinux.org/"
HOME_URL="https://alpinelinux.org/"
ID=alpine
NAME="Alpine Linux"
PRETTY_NAME="Alpine Linux v3.13"
VERSION_ID=3.13.0
# A convuluted way to grab 3 random lines from /etc/os-release, demonstrating sorting by a file by field selection.
> wc -l /etc/os-release
6
> echo "$(shuf -e $(seq 1 6))" | paste -d '#' /etc/os-release - | sort -n -t '#' -k 2 | head -n 3 | sed -E 's/^(.*)#.*$/\1/'
# Split a file into chunks of two lines each
> split -l 2 /etc/os-release split-part-
> ls
split-part-aa split-part-ab split-part-ac
# Print the status of file
> stat /etc/os-release
File: /etc/os-release
Size: 164 Blocks: 8 IO Block: 4096 regular file
...
# Print the status of the file system containing the file
> stat -f /etc/os-release
File: "/etc/os-release"
ID: 6dcd663b9be28281 Namelen: 255 Type: UNKNOWN
Block size: 4096
Blocks: Total: 10148403 Free: 3175797 Available: 3171701
# Print text strings (minimum 4 characters per string), along with file name and offset in hex numbers.
> strings -f -o -t x /etc/os-release
/etc/os-release: 0 NAME="Alpine Linux"
/etc/os-release: 14 ID=alpine
/etc/os-release: 1e VERSION_ID=3.13.0
# Display or set TTY parameters
> stty size
41 94
# Run shell interpreter under user "nobody", give the shell "id" command to run, and then return to original user.
> su -s /bin/sh -l nobody -c 'id'
uid=65534(nobody) gid=65534(nobody) groups=65534(nobody)
(and now back to original user's shell prompt)
# Print the checksum ("BSD sum algorithm", 1K block size) and the number of blocks a file occupies
> sum /etc/os-release
50215 1
# Turn off all swap block devices
> swapoff -a
# Turn on a swap block device, give it the priority 32767 (highest priority)
> swapon -p 32767 /swapfile
# Free initramfs and switch to another root fs
# Sync disk-backed memory buffer to disk for all files
> sync
# Sync disk-backed memory buffer to disk for a specific file
> sync /etc/os-release
# Get kernel parameter by key name
> sysctl -n net.ipv4.ping_group_range
1 0
# Run syslog daemon, keep messages in /messages file.
> syslogd -O /messages
> logger haha
> cat /messages
Jan 29 10:14:32 1f49bcb3aafb syslog.info syslogd started: BusyBox v1.32.1
Jan 29 10:14:36 1f49bcb3aafb user.notice root: haha
# Print a file backwards
> tac /etc/os-release
BUG_REPORT_URL="https://bugs.alpinelinux.org/"
HOME_URL="https://alpinelinux.org/"
PRETTY_NAME="Alpine Linux v3.13"
# Print the last 2 lines of /etc/os-release
> tail -n 2 /etc/os-release
HOME_URL="https://alpinelinux.org/"
BUG_REPORT_URL="https://bugs.alpinelinux.org/"
# Print the last 20 bytes of /etc/os-release
> tail -c 20 /etc/os-release
s.alpinelinux.org/"
# Print the line after the 3rd line
> tail -n 3 /etc/os-release | head -n 1
PRETTY_NAME="Alpine Linux v3.13"
# Create an archive
> tar zcvf etc.tgz /etc/os-release /etc/shells
tar: removing leading '/' from member names
etc/os-release
etc/shells
# Display content of the archive
> tar tf etc.tgz
etc/os-release
etc/shells
# Extract the archive
> mv etc.tgz /tmp
# Copy standard input to standard output and append to file
> cat /etc/os-release | tee -a copy-of-output
NAME="Alpine Linux"
...
> cat copy-of-output
NAME="Alpine Linux"
...
# Test whether a path is a readable file
> test -r /etc/os-release -a -f /etc/os-release && echo good
good