Personal shell commands cheat sheet

Unix shell commands

Writing some down, because I’m bad at memorizing them all. A lot of those are standard Linux commands or so-called ›shell builtin‹.

cat
→ stands for ›concatenate‹ and prints the output of the file

cp / copy
→ copy a file (copy-paste)

ls
→ list files and folders

less
→ display only as much of a file’s content as fits the screen

cd
→ change directory
when no arguments are supplied, it will take you to your home directory (so cd, cd ~ and cd $HOME usually do the same thing)

export
→ …displays all exported variables, when no arguments given
exports a variable, which can then be verified with printenv
with the -f it lets you export functions

df
→ show free disk memory

du
→ disk usage

echo
→ print strings or variables (just like printf or stdout in C)

env
→ print all current environment variables

fuser
→ used for monitoring ports and directories currently used by a user

head and tail
→ print the first lines of a file or the last lines of a file respectively, where you can specify the number of lines with -n (so head -4 prints the first four lines)

kill
→ …kills a process

ln
→ create a link (“softlink”/“symlink”)

mkdir
→ create directory

mv
→ move a file (cut-paste)

alias
→ show all aliases (stored in ~/.bashrc)
new aliases can be defined like so: alias name='action'

nl
→ …will number the lines (it’s sometimes useful in pipes)

pwd
→ prints path of the current working directory

rm
→ remove a file

-i flag for timid deletion (you will be promoped to confirm each deletion)

rmdir
→ remove a folder

sort
→ …sorts lines of text into ascending order
-r flag for descending order
-n for numeric instead of alphabetical order

source
→ …a Bash shell built-in command that executes the content of the file passed as an argument in the current shell. It has a synonym in . (period). (cf. here)

time
→ …followed by a command prints the time it took to run the command

uniq
→ removes repeated, adjacent lines from a file (-c flag for showing the number of occurences)

umask
→ show access rights in current folder (in umask octal format); use -S to see a verbal non-octal description in the form of u=rwx,g=rwx,o=rwx where u is the current user, g the group and o all others and rwx would be read, write, execute; note that umask octals are not the same as the octals of chmod

diff
→ compare two files line by line

egrep
→ is the same as grep -E (grep with ›extended regular expression‹)

glimpse
→ ›global implicit search‹ for UNIX; searches the entire file system

history
→ …shows your previously executed commands
-c flag deletes the history
history | tail -n1 will print itself and its index

info
→ …prints documentation in info format

jobs
→ prints the currently running jobs/processes

man
→ print the manual for a program

sudo
→ ›superuser do‹

tr
→ replaces some character with another; e.g. echo "This is a string." | tr " " "\n" replaces all whitespaces in the string with a new line character

wc
→ presumably stands for ›word count‹ and prints the number of lines, words, and characters in a file
-l, -w, and -c flags limit the output

whoami
→ ›Print the user name associated with the current effective user ID.‹

cut

cut lets you print columns from a text (file) input. One way to define what a column is, is by using the -f argument and cut by field. Fields are assumed to be tab-seperated, but you can specify the delimiter with the -d flag: cut -d. -f1 will cut by .. The -f flag can take a number to specify which column to print; for the second, third and fourth column, we’d write cut -f2,3,4 or cut -f2-4. With the -c flag we can cut columns by character count: cut -c1-3.

grep

grep is used for searching and filtering strings based on an input string or regular expression. It actually stands for global search for a regular expression and print out matched lines. For inverse searching (filtering out the elements not containing some string, use the -v flag.)

Wildcards and globbing

In the Linux shell, the wildcard operator is *: ls *.js lists all javascript files. If you want a wildcard for only a single character, use ?. Square brackets are used for matching characters from a list, so ls *.[jt]s would list all Javascript and Typescript files for example.

It’s the shell, not the invoked program, that performs this pattern-matching. Therefore, different shells might treat your commands differently. While echo *.js in Linux shell will echo all files ending with .js, Windows Powershell will literally just echo ›*.js‹.

Environment variables

You can use printenv for printing environment variables: printenv HOME. When using these variables in a command you will have to put a $ before: cd $HOME. The Linux shell will then resolve the variable. System-wide environment variables should usually be stored in /etc/environment.

Useful tools and combinations

Piping

In order to combine commands, you will use a pipe operator (|). When combinng two commands, the commands are usually not aware that they are part of the pipeline as they are just handling ›standard inputs‹ (stdin) and ›standard outputs‹ (stdout). (There’s excepetions like ls.)

Sending output to a file can be done with > (overwrites file) and >> (appends to file).

Show hidden characters

For debugging purposes it’s sometimes useful to show all characters in a file, even those that are not printable. You can do the with cat -v followed by the filename, which show’s the ASCII representation of non-printable characters.

You can also use od which displays the contents of an input file, too, but lets you specify the character format. So od -c prints the file content in character format (additionally it prints the byte offset at the beginning of each line that it outputs).

History expansion

History expansion is done via excalamation marks (called ›bangs‹). !! executes the previous command again. In order to execute the third most recent command again, you would type !-3. !cd would execute the most recent command starting with cd, while !?cd? would execute the most recent command that had cd somewhere in it. With !123 you can execute a command again based on its index in the history (as you can retrieve via history or by accessing ~/.bash_history). If you only want to print the command from history, but not execute, add :p to the history expansion command: !-5:p. Though echo "!-5" would work as well. !$ will retrieve the final word from your previous command, and !* will retrieve all arguments from your last command (unlike !!, which retrieves the program and its arguments). With ^ you can easily replace a string in your last command and execute it again. Let’s say you mistyped a command: mount /dev/s1 /media/hdd. You can then correct it and execute it, by just typing ^s1^sda1. Alternatively, you could write: !!:s/s1/sda1/

Mounting and unmounting

lstblk
→ This will list all block devices and their partitions (you can also use blkid alternatively). After you have identified, for example, the HDD you just connected, you can mount it to a specific target with:

mount /dev/sda1 /media/myExternalHarddrive

Unmounting it is done via umount /dev/sda1

Syncing two folders

Bringing to folder in sync can be done with the rsync (remote sync) tool. I would usually run something like this:

rsync --stats --progress -aPv ~/media/Music someone@1.2.3.4:~/media/

Finding a running process

ps -ef | grep processname
→ pipe ps into grep, i.e. list the currently running processes, send the list to grep which searches for lines containing the pattern processname (as regular expression)

Check size of folders

To check the size of the folders in the current folder (and excluding subfolders), run:

du -sh --max-depth=1 ./*/
(-s stands for summarize and -h for human-readable)

Directory stack

You can push directories to a stack and remove directories from them with pushd and popd, respectively, followed by the directory name, and you can view your directory stack with dirs. When you type pushd with no directory name it will swap the two top-most directories from your stack and take you to the new top directory. If you want to swap the top-most and the third directory and move to the latter, you can type pushd +2.

What processes are running on what ports

You can easily find out what processes are running on what ports, by using netstat, lsof or ss:

netstat -nlp
 
lsof -i
 
ss -nlp

useful tools

• pv (pipe viewer) lets you monitor the progress of data in a pipeline (cf.)

• nc (netcat) is useful for analyzing network connections

Awk

Awk is a scripting mostly used for data manipulation. It’s syntax is such:

awk options 'selection_criteria { action }' input-file > output-file

Awk reads its input file line by line and each line is treated as a ›record‹.

random snippets

• in all of the current folder’s subfolders and files, print the paths of those files and folders that contain the string ›ktl‹: ls -d ./**/* | grep -i 'ktl'

• find folders with the string ›needle‹ (case-insensitive) in it in all of the current folder’s subfolders: find ./ -type d -iname "*needle*" -print

• find the most common words in all .txt files that are at least two letters long and show their counts: find ./ -name "*.txt" | xargs cat | egrep -o "(\w){2,}" | sort | uniq -c | sort -k1nr | head

• deletes all file ending with “.xyz.jpg” in a folder and all subfolders: find /path/to/folder -type f -name '*.xyz.jpg' -delete

Books

• Daniel J. Barrett: Efficient Linux at the Command Line (O’Reilly Media, 2022)