#title Linux Utils Cheatsheet
#author Stefan Hornburg (Racke)
#topics find; linux; rsync; lftp; gpg; sort; date; lsof; GnuPG
#lang en

** Finding files with =find=

*** Remove files by modification date

This example shows how to remove all files with a modification date before 2016:

<example>
find -type f -not -newermt 20160101 | xargs rm
</example>

*** Move old files to another directory

This example shows how to move a large number of files to another directory:

<example>

find /var/www/userpics -name '*.jpg' -mtime +31 -print0 | xargs --null mv --target-directory=/backup/oldpics/

</example>

*** Find files by permission

Exact permission (read,write,execute for owner only):

{{{
find -perm 700
}}}

*** Find large files

{{{
$ du -sh -t 20M $(find /tmp/ -type f)
30M	/tmp/OF0dXjBgX3/0207.zip
56M	/tmp/lYZNO1PD09/0703.zip
29M	/tmp/aF_jPhgvwD/0307.zip
}}}

** Use =cat=

*** Show file contents

{{{
$ cat /etc/os-release
PRETTY_NAME="Debian GNU/Linux 11 (bullseye)"
NAME="Debian GNU/Linux"
VERSION_ID="11"
VERSION="11 (bullseye)"
VERSION_CODENAME=bullseye
ID=debian
HOME_URL="https://www.debian.org/"
SUPPORT_URL="https://www.debian.org/support"
BUG_REPORT_URL="https://bugs.debian.org/"
}}}

This file should be present on any modern Linux distribution.

*** Show whitespace

{{{
$ cat -vet
}}}

** Use =sed=

*** Remove lines from output

Remove single line:

{{{
$ sed '1d' file
}}}

Remove range of lines:

{{{
$ sed '1,5d' file
}}}

*** Extract lines

Extract a single line for a file:

{{{
$ sed '123q;d' file
}}}

The =q= stops processing the file when the line is reached.

If you want to see a couple of lines from a CSV alongside with the header:

{{{
$ sed -n -e '1p' -e '809,820p' my.csv
}}}

** Sort lines with =sort=

Sort by a column:

{{{
~# sort -k 1 myfile
}}}

Sort numeric:

{{{
~# sort -n myfile
}}}

Sort reverse:

{{{
~# sort -r myfile
}}}

** Using =date=

Display date a month ago:

{{{
~# date -d "-1 month" +%Y-%m-%d
}}}

It automatically adjusts the day if the resulting date would be invalid.

Set time:

{{{
~# date +%T -s "12:12:00"
}}}

*** Epoch

Convert epoch to a date:

{{{
$ date +"%s"
1626247585
}}}

Convert date to epoch:

{{{
$ date -d @1626247585
Wed 14 Jul 09:26:25 CEST 2021
}}}

** Using =rsync=

*** Copy files instead of symlinks

{{{
rsync -L /etc/letsencrypt/live/wildcard.linuxia.de/*.pem root@extern.linuxia.de:/etc/letsencrypt/live/wildcard.linuxia.de/.
}}}

*** Sync only files with a certain extension from one directory tree to another

<example>
rsync -av --include="*/" --include='*.status' --exclude='*' /samba/. .
</example>

** Using =ip=

*** Show network interfaces

{{{
$ ip -a
}}}

*** Add IP address

{{{
$ sudo ip a add 192.168.36.11/24 dev eth0
}}}

** Using =lsof=


*** Open files
#lsofopenfiles

Show open files in a directory:

{{{
$ lsof /mnt/backup
...
}}}

*** Network connections
#lsofnetconns

{{{
$ lsof -i :80
COMMAND    PID     USER   FD   TYPE  DEVICE SIZE/OFF NODE NAME
nginx   431729     root    6u  IPv4 7170923      0t0  TCP *:http (LISTEN)
nginx   431729     root    7u  IPv6 7170924      0t0  TCP *:http (LISTEN)
nginx   431730 www-data    6u  IPv4 7170923      0t0  TCP *:http (LISTEN)
nginx   431730 www-data    7u  IPv6 7170924      0t0  TCP *:http (LISTEN)
}}}

It works with service as well, e.g. =lsof -i:ntp=.

*** See also

[[https://danielmiessler.com/study/lsof/][An lsof Primer]]

** Using =lftp=

*** Mirroring

Mirror from remote FTP server to local server

{{{
mirror -c source target
}}}

Please note that doesn't mirror dot files like =.htaccess=.
To include these, adjust the default listing options:

{{{
set ftp:list-options -a
}}}

*** SSL

In case you really need to disable SSL (**not recommended**), use the following command:

{{{
set ftp:ssl-allow false
}}}

** Using =curl=

Display the headers:

{{{
$ curl --head https://example.org
}}}

Skip certificate check:

{{{
# Shortcut: -k
$ curl --insecure https://example.org
}}}

Connect without DNS:

{{{
$ curl https://example.org --connect-to example.org:93.184.216.34
}}}

** Using =wget=

Display the headers:

{{{
$ wget --server-response https://example.org
}}}

** Using =convert=

*** Combine  images into a PDF file

{{{
$ convert 20220518_1450*.jpg photos.pdf
}}}

*** Convert SVG file to PNG with transparency

{{{
$ convert -background none image.svg image.png
}}}

** Using =zip=

*** Create password protected archive

{{{
$ zip -e sccl2.zip sccl2_2.9-4_all.deb sccl2-2.9-4.noarch.rpm 
Enter password: 
Verify password: 
  adding: sccl2_2.9-4_all.deb (deflated 0%)
  adding: sccl2-2.9-4.noarch.rpm (deflated 26%)
}}}

** Encryption with GnuPG

*** Environment variables

**** GNUPGHOME

Select alternative home directory for GPG configuration and keys:

{{{
   export GNUPGHOME=~/.gnupg-old/
}}}