Bash tips

This note was created on 3/20/2023
This note was last edited on 5/7/2023

=== Jobs ===
Stop current job: ^Z
Move job #1 to the background:
> bg %1
List jobs:
> jobs
Start job in the background:
> sleep 500 &
Move job #2 to the foreground:
> fg %2

"jobs" options:
~~~
-l - list PIDs in addition to default info
-n - list only processes that have changed since the last notification
-p - list PIDs only
-r - show only running jobs
-s - show only stopped jobs
~~~

"bg" options:
~~~
%n - where n is the job number
%abc - refers to a job started by a command beginning with abc
%?abc - refers to a job started by a command containing abc
%- - specifies the previous job
~~~

=== Backups ===
Create encrypted backup of few directories:
> tar -cz ./Documents ./Keys | gpg -c -o backup.tar.gz.gpg

Decrypt and unpack backup:
> gpg -d backup.tar.gz.gpg | tar xz -C ./Temp

=== Other ===

Delete big file, when rm fails (does not change owner, permissions or descriptors):
> /path/to/file.log

View list of open files:
> lsof

Find what take a lot of storage:
> ncdu

Measure execution time for application:
> time command

Monitor command output (execute command every second):
> watch command

Watch changes in file in real time:
> tail -f file.log

Install or change locale/language:
> sudo dpkg-reconfigure locales

Change "string1" to "string2":
> sed 's/string1/string2/g'

Check version of the package in the repository:
> apt-cache policy package-name

Edit crontab for other user:
> sudo crontab -e -u www-data

Search for a string in files:
> grep -rn '/path/to/somewhere/' -e "pattern"

Run command from other user:
> sudo -u www-data ps

Find *.log files older then 7 days and delete them:
> find /tmp/*.log -mtime +7 -type f -delete

Add prefix for all lines in file:
> awk '$0="prefix"$0' file > newfile

List mounted file system as a table:
> mount | column -t
> column -t /proc/mounts

Set audible alarm when an IP address comes online:
> ping -i 60 -a 1.1.1.1

Display the top ten running processes, sorted by memory usage:
> ps aux --sort -rss | head

Watch network activity in real-time:
> lsof -i 

diff two unsorted files without creating temporary files:
> diff <(sort file1) <(sort file2)

Show file system hierarchy:
> man hier

Kill a process, that is locking a file:
> fuser -k filename 

Replace spaces in filenames with underscores:
> rename 'y/ /_/' *

Invoke an editor for long and complex command:
> fc

Save manpage as pdf:
> man -t diff | ps2pdf - diff.pdf

Create a tree of dirs:
> mkdir -p root/child0/child1/{ab,cd,de,ef}

Remove all but one specific file:
> rm -f !(survivor.txt)

Generate a random password 30 characters long:
> strings /dev/urandom | grep -o '[[:alnum:]]' | head -n 30 | tr -d '\n'; echo

Graph # of connections for each hosts:
> netstat -an | grep ESTABLISHED | awk '{print $5}' | awk -F: '{print $1}' | sort | uniq -c | awk '{ printf("%s\t%s\t",$2,$1) ; for (i = 0; i < $1; i++) {printf("*")}; print "" }'

Show apps that use internet connection:
> ss -p

Recursively remove all empty directories:
> find . -type d -empty -delete

Convert seconds to human-readable format:
> date -d@1234567890

Create a quick backup copy of a file:
> cp file.txt{,.bak}

Make the permissions of file2 the same as file1:
> chmod --reference file1 file2

Send pop-up notifications on Gnome:
> notify-send ["*title*"] "*body*"

Run a long running script and notify when it's finished:
> ./script.sh && notify-send "Script finished"

Find what package a file belongs to (on Debian/Ubuntu):
> dpkg -S /usr/bin/zsh

Mount .iso file in GNU/Linux:
> mount /path/to/file.iso /mnt/cdrom -o loop
"-o loop" lets you use a file as a block device

Print specific line from file:
> sed -n 5p <file>

Remove all files previously extracted from a tar.gz file:
> tar -tf <file.tar.gz> | xargs rm -r

Which program use port 80?
> lsof -i tcp:80

Run a file system check on next boot:
> sudo touch /forcefsck

Find out how much data is waiting to be written to disk:
> grep ^Dirty /proc/meminfo

Generete random number between 1 and X:
> echo $[RANDOM%X+1]

Alternative to if/then/else/fi construct:
> [[ test_condition ]] && if_true_do_this || otherwise_do_that

List installed deb packages by size:
> dpkg-query -Wf '${Installed-Size}\t${Package}\n' | sort -n

Switch 2 characters on a command line:
<ctrl-t>
("sl" -> "ls")

Insert the last argument of the previous command:
<ESC> .

Execute a command with a timeout:
> timeout 10 sleep 11

Fast, built-in pipe-based data sink (faster then "/dev/null"):
<command> |:

Shorter "ls":
ls -hog

Start a command on only one CPU core:
> taskset -c 0 <command>
"0" is a number of CPU core.

Check if your ISP is intercepting DNS queries:
> dig +short which.opendns.com txt @208.67.220.220

Run entire shell script as root:
~~~
#!/usr/bin/sudo /bin/bash
~~~

Start HTTP server in current dir:
> python3 -m http.server

Find USB device when connecting:
> diff <(lsusb) <(sleep 5s && lsusb)

Cleanup Firefox database:
> find ~/.mozilla/firefox/ -type f -name "*.sqlite" -exec sqlite3 {} VACUUM \;