SYSCTL(8)

💖 SYSCTL(8) — the system manager's manual, but make it cute 💖

okay bestie, sit down, hydrate, and let me tell you about sysctl from the System Manager's Manual (that's section 8, the big-kid admin section 👑). this is the tool that pokes at your kernel's live settings, and honestly? it's giving main character energy.

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✨ NAME ✨

sysctl — configure kernel parameters at runtime 🎛️

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🌸 SYNOPSIS 🌸

sysctl [option ...] variable[=value] ...

sysctl -p file-or-regexp ...

that's it, that's the whole vibe. read stuff, write stuff, or load a whole file of stuff. 🫶

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🦋 DESCRIPTION 🦋

so sysctl is what you use to modify kernel parameters at runtime. the parameters you can touch are the ones listed under /proc/sys/. Procfs is required for sysctl support in Linux, so no procfs = no dice, sorry. 💀

and like, you can do both: use sysctl to read sysctl data and to write it. a little multitasking icon, we love to see it. ✨

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🎀 PARAMETERS 🎀

okay this section is doing a lot of work because it's got the arguments AND the options, so buckle up.

• variable — the name of a key to read from. an example is kernel.ostype. and ngl, the / separator is also accepted in place of a ., so you've got options. 💅
• variable=value — to set a key, use the form variable=value where variable is the key and value is the value you're setting it to. heads up though: if the value contains quotes or characters which are parsed by the shell, you may need to enclose the value in double quotes. the shell is nosy like that. 🙈
• -n, --values — use this option to disable printing of the key name when printing values. just the value, no name, no small talk.
• -e, --ignore — use this option to ignore errors about unknown keys. she's unbothered. ✨
• -N, --names — use this option to only print the names. it may be useful with shells that have programmable completion. iykyk. 🐚
• -q, --quiet — use this option to not display the values set to stdout. silent but deadly (in a cute way).
• -w, --write — force all arguments to be write arguments and print an error if they cannot be parsed this way. no reading today, we're writing only. ✍️
• -p[FILE], --load[=FILE] — load in sysctl settings from the file specified, or /etc/sysctl.conf if none given. specifying - as filename means reading data from standard input. using this option will mean arguments to sysctl are files, which are read in the order they are specified. and plot twist: the file argument may be specified as regular expression. 👑
• -a, --all — display all values currently available except deprecated and verboten parameters. (yes, "verboten." they really said that. iconic.)
• --deprecated — include deprecated parameters to --all values listing. peek at the ghosts if you must. 👻
• -b, --binary — print value without new line.
• --system — load settings from all system configuration files. see the SYSTEM FILE PRECEDENCE section below. 👇
• -r, --pattern pattern — only apply settings that match pattern. the pattern uses extended regular expression syntax. regex girlies rise. 🌈
• -A — alias of -a
• -d — alias of -h
• -f — alias of -p
• -X — alias of -a
• -o — does nothing, exists for BSD compatibility. she's just here for the vibes. 🫥
• -x — does nothing, exists for BSD compatibility. also just vibing. 🫥
• -h, --help — display help text and exit.
• -V, --version — display version information and exit.

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🧚 FILES 🧚

these are the spots on the map bestie:

• /proc/sys
• /etc/sysctl.d/*.conf
• /run/sysctl.d/*.conf
• /usr/local/lib/sysctl.d/*.conf
• /usr/lib/sysctl.d/*.conf
• /lib/sysctl.d/*.conf
• /etc/sysctl.conf

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⭐ NOTES ⭐

🌸 System file precedence

when you use the --system option, sysctl will read files from directories in the following list in given order from top to bottom. once a file of a given filename is loaded, any file of the same name in subsequent directories is ignored. first come, first served, no cap. 🏁

• /etc/sysctl.d/*.conf
• /run/sysctl.d/*.conf
• /usr/local/lib/sysctl.d/*.conf
• /usr/lib/sysctl.d/*.conf
• /lib/sysctl.d/*.conf

all configuration files are sorted in lexicographic order, regardless of the directory they reside in. configuration files can either be completely replaced (by having a new configuration file with the same name in a directory of higher priority) or partially replaced (by having a configuration file that is ordered later).

the file /etc/sysctl.conf is read last, which means it can replace/override any parameters previously set in files in the listed directories. she gets the final word. 💅

🎀 Deprecated parameters

base_reachable_time and retrans_time are deprecated. the sysctl command does not allow changing values of these parameters. users who insist to use deprecated kernel interfaces should push values to /proc file system by other means. for example:

echo 256 > /proc/sys/net/ipv6/neigh/eth0/base_reachable_time

ok so lowkey if you're insisting on this, you're on your own, bestie. do not come for me. 😭

💜 Verboten parameters

certain kernel parameters should not be read due to side-effects and will be skipped by sysctl using the --all option.

• stat_refresh — this parameter returns no value but flushes per-cpu vm statistics.

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🔥 EXAMPLES 🔥

here's the group chat showing receipts:

/sbin/sysctl -a
/sbin/sysctl -n kernel.hostname
/sbin/sysctl -w kernel.domainname="example.com"
/sbin/sysctl -p/etc/sysctl.conf
/sbin/sysctl -a --pattern forward
/sbin/sysctl -a --pattern forward$
/sbin/sysctl -a --pattern 'net.ipv4.conf.(eth|wlan)0.arp'
/sbin/sysctl --pattern '^net.ipv6' --system

look at her go. ate and left no crumbs. 👑

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👀 SEE ALSO 👀

proc(5), sysctl.conf(5), regex(7)

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💀 REPORTING BUGS 💀

please send bug reports to procps@freelists.org.

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and that's the whole page, bestie!! this beauty is from procps-ng, dated 2024-07-19, and it lives in section 8 of the manual. go forth and configure your kernel, you absolute legend 🫶✨🎀💖