Bash

Special Characters

- # used to declare flags for functions, "cd -" changes to the previous directory
\ # escapes special characters and allows for line break
# comment # starts a comment
. # the current directory
~ # the home directory
command; # ends the command
$variable # used to call variable
command1 && commad2 # command2 will run if the command1 succeeds
[ conditional ] # evaluates the conditional
command1 | command2 # sends the stdout of command1 to the stdin of command2
command > file # overwrites a file with the output of the command
command >> file # appends the output of the command to the end of the file

File Commands

Directory Contents

ls # directory listing
ls -l # formatted directory listing
ls -a # directory listing with hidden files
ls -al # formatted list with hidden files

Navigating

cd dir # change directory to dir
cd # change to home
pwd # print working directory

Deleting

rm file # delete file
rm -r dir # delete directory
rm -f file # force remove file
rm -rf dir # force remove directory

Creating

cp file1 file2 # copy file1 to file2
mv file1 file2 # move file1 to file2
ln -s file link # create symbolic link 'link' to file
touch file # create or update file
mkdir dir # create directory dir

Reading

cat > file # place standard input into file
more file # output the contents of the file
less file # output the contents of the file
head file # output the first 10 lines of file
tail file # output the last 10 lines of file
tail -f file # output contents of file as it grows

Editing with tr

# Converting a tab delimited file into commas
cat tab_delimited.txt | tr "\\t" "," > comma_delimited.csv
 
# Command chaining
cat README.md | tr "[:punct:][:space:]" "\n" | tr "[:upper:]" "[:lower:]" > output.txt
 
# Converting all upper case letters to lower case with regex
cat filename.csv | tr '[A-Z]' '[a-z]'

tr Built-In Variables

Variable	Definition
[:alnum:]	all letters and digits
[:alpha:]	all letters
[:blank:]	all horizontal whitespace
[:cntrl:]	all control characters
[:digit:]	all digits
[:graph:]	all printable characters, not including space
[:lower:]	all lower case letters
[:print:]	all printable characters, including space
[:punct:]	all punctuation characters
[:space:]	all horizontal or vertical whitespace
[:upper:]	all upper case letters
[:xdigit:]	all hexadecimal digits

SSH

ssh user@host # connect to host as user
ssh -p port user@host # connect using port p
ssh -D port user@host # connect and use bind port
ssh -t user@host "do something" # execute a command on a remote machine

Installation

./configure
make
make install

Network

ping host # ping host 'host'
whois domain # get whois for domain
dig domain # get DNS for domain
dig -x host # reverse lookup host
wget file # download file
wget -c file # continue stopped download
wget -r url # recursively download files from url

System Info

date # show current date/time
cal # show this month's calendar
uptime # show uptime
w # display who is online
whoami # who are you logged in as
uname -a # show kernel config
cat /proc/cpuinfo # cpu info
cat /proc/meminfo # memory info
man command # show manual for command
df # show disk usage
du # show directory space usage
du -sh # human readable size in GB
free # show memory and swap usage
whereis app # show possible locations of app
which app # show which app will be run by default

Searching

grep pattern files # search for pattern in files
grep -r pattern dir # search recursively for pattern in dir
command | grep pattern # search for pattern in the output of command
locate file # find all instances of file

Process Management

ps # display currently active process
ps aux # ps with a lot of detail
kill pid # kill process with pid 'pid'
killall proc #  kill all processes named proc
bg # lists stopped/background jobs, resume stopped job in background
fg # bring most recent job to foreground
fg n # brings job n to foreground

File Permissions

Add up the numbers corresponding to all desired permissions for each user/group, then concat the results.

chmod octal file # change permissions of file
 
  4 - read (r)
  2 - write (w)
  1 - execute (x)
 
  order: owner/group/world
 
  eg:
  chmod 777 file # everyone has rwx permissions for file
  chmod 755 file # rw for owner, rx for group/world

Compression

tar cf file.tar files # tar files into file.tar
tar xf file.tar # untar into current directory
tar tf file.tar # show contents of archive

tar flags

Flag	Use
c	create archive
t	table of contents
x	extract
f	specific filename
z	use zip/gzip
j	bzip2 compression
k	do not overwrite
T	files from file
w	ask for confirmation
v	verbose

gzip file # compress file and rename to file.gz
gzip -d file.gz # decompress file.gz

Shortcuts

ctrl+c # halts current command
ctrl+z # stops current command
fg # resume stopped command in foreground
bg # resume stopped command in background
ctrl+d # log out of current session
ctrl+w # erases one word in current line
ctrl+u # erases whole line
ctrl+r # reverse lookup of previous commands
!! # repeat last command
exit # log out of current session

Variables

export VARIABLE=value # stores value as variable for this session

Aliases

Think of aliases as nicknames. You might have a command that you perform a lot but want to shorten.

alias desktop="cd ~/Desktop" # "desktop" will now execute "cd ~/Desktop"

Control Flow

If… Else…

if conditional; then
  # do something
elif different_conditional; then
  # do something else
else
  # do something else
fi

For Loops

for item in iterable; do
  # do something
done

While Loops

while conditional; do
  # do something
done

Functions

Functions contain logic. In a function, you might make calls to several different programs.

This function executes a git add/commit/pull/push in sequence upon successful completion of the previous command. Correct usage is git_push "commit message"

function git_push() {
  git add -A; \
  git commit -m "$1"; \
  git pull --rebase && \
  git push;
}

Argument Parsing

The while loop within the function loops through all positional arguments, popping the variable in the $1 position off the array, and checking if it’s a custom flag. If it is a custom flag, the next positional argument gets appended to the variable assigned.

Any positional arguments that remain after all flags are handled are appended to the PARAMS variable. Then eval set -- "$PARAMS" assigns PARAMS back as positional arguments, so arguments can be accessed in their original order (minus the arguments that were assigned via flags).

So if you ran do_something -f first -s second third fourth, first would get assigned to FARG, second would get assigned to SARG. third and fourth will be accessible through the $1 and $2 positional arguments, respectively.

function do_something() {
   PARAMS=""
   while (( "$#" )); do
     case "$1" in
       -f|--flag-with-argument)
         FARG=$2
         shift 2
         ;;
       -s|--some-other-flag)
         SARG=$2
         shift 2
         ;;
       --) # end argument parsing
         shift
         break
         ;;
       -*|--*=) # unsupported flags
         echo "Error: Unsupported flag $1" >&2
         exit 1
         ;;
       *) # preserve positional arguments
         PARAMS="$PARAMS $1"
         shift
         ;;
     esac
   done
   # set positional arguments in their proper place
   eval set -- "$PARAMS"
}

---

Sources

• Unix Power Tools
• Conquering The Command Line
• Rock the Command Line
• Command Line Tricks For Data Scientists
• Ten Things I Wish I’d Known About bash
• Bash: Argument Parsing
• How to get arguments with flags in Bash