(Linux) shell tips

Command-line (unix shell) tips. Some may be bash- or linux-specific.

Grepping mans ¶

Here’s a piece of man git-log | xxd:

00000a0: 2020 5f08 675f 0869 5f08 7420 5f08 6c5f    _.g_.i_.t _.l_
00000b0: 086f 5f08 6720 5b3c 6f70 7469 6f6e 733e  .o_.g [<options>
00000c0: 5d20 5b3c 7265 7669 7369 6f6e 2072 616e  ] [<revision ran
00000d0: 6765 3e5d 205b 5b2d 2d5d 203c 7061 7468  ge>] [[--] <path
00000e0: 3e2e 2e2e 5d0a 0a0a 4408 4445 0845 5308  >...]...D.DE.ES.

08 is a backspace char — which means terminal will display this, but grep won’t find it: it needs a help for yet another command, col:

$ man git-log | col -b | grep 'git log' | head -n 1
    git log [<options>] [<revision range>] [[--] <path>...]

mosh + tmux + systemd ¶

Systemd-logind kills user processes when the session is over. To overcome this, I’ve created the following one-line script in my ~/bin.

#!/bin/sh

exec urxvt -e mosh "${1:-gd-ws}" \
    --server='systemd-run --scope --user mosh-server new' \
    -- tmux att

Explaining this command would make a nice article ;)^[1]

Entering docker container without docker ¶

docker exec executes command within the container cgroup; if this cgroup has hard CPU and/or RAM limitations, this may affect your production code. If you have root access to the box, you can use nsenter instead:

nsenter --target $PID --mount --uts --ipc --net --pid

$PID here is the process ID of one of the processes running in the container, as seen from the host.

Example:

$ sudo nsenter --target 10372  --mount --uts --ipc --net --pid cat /proc/self/cgroup | head -n3
11:cpu,cpuacct:/
10:pids:/user.slice/user-500.slice/session-1.scope
9:memory:/

$ docker exec 2b341597dbf7 cat /proc/self/cgroup | head -n3
11:cpu,cpuacct:/init.scope/system.slice/docker-2b341597dbf7d223f820c1be8edc7c78c394b0f447547f4679f4926e36d07060.scope
10:pids:/init.scope/system.slice/docker-2b341597dbf7d223f820c1be8edc7c78c394b0f447547f4679f4926e36d07060.scope
9:memory:/init.scope/system.slice/docker-2b341597dbf7d223f820c1be8edc7c78c394b0f447547f4679f4926e36d07060.scope

Environment variables of a running process ¶

You can get them from /proc/$PID/environ. Variables are NULL-separated. For example:

# cat /proc/`pgrep dockerd`/environ | xargs -0L1
LANG=ru_RU.UTF-8
PATH=/usr/local/sbin:/usr/local/bin:/usr/sbin:/usr/bin:/sbin:/bin
NOTIFY_SOCKET=/run/systemd/notify
OPTIONS=
DOCKER_STORAGE_OPTIONS=

Find matching line, and extract the matching part ¶

There is -o switch in grep for that exactly.

$ amixer get Master | egrep -o '[0-9]{1,3}?%'
100%
100%

Using less to follow files ¶

less can serve as a convenient replacement for tail -f, especially when you’re watching somewhat untrusted log that may hack your terminal emulator:

less -R +F <file>

Parsing comma-separated string into a bash array ¶

parse_array () { local IFS=','; read -a "$1" < <(echo "$2"); }

This takes two arguments – name and content, and sets global variable with the given name to an array parsed from comma-separated contents. For example:

keys="host,port,service name,comment"
parse_array the_array "$keys"
for x in "${the_array[@]}"; do
    echo "$x"
done

This prints:

host
port
service name
comment

Extracting a single file from the RPM package ¶

Beware of the traces of nasty hacks in the example below:

rpm2cpio net-tools-1.60-114.el6.x86_64.rpm | cpio -iv --to-stdout  ./sbin/ifconfig > /sbin/ifconfig-compat

When you’re too lazy to explain everything ¶

http://explainshell.com/

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1. it’s classy to render smile in Fira Mono when you can do smth like 😉. ↩︎