history # bash command history
history -c # clear bash command history
cat # print file (all contents)
less # print file (scrollable)
/keyword # search within less result
tail # print last several lines of a file
watch # watch file contents change in real time
echo [text] # print specified text
printf [text] # print specified text, highly customizable
pwd # print full path of current working directory
Deleting
### WARNING ### - be very careful with this
# Delete directory (includes all files contained within):
rm -rf
# -r = recursive, -f = force
Shell-Safe RM
Environment Variables
env # print all environment variables
export # see all environment variables running
Variables
x=3 # No spaces between equal sign.
Timing
sleep 5 # Delay 5 seconds.
date +%s%3N # Get system time in milliseconds.
Printing
echo
# echo Command:
x=" hello world "
echo $x # This automatically trims whitespaces from variable.
echo "$x" # This prints the exact true string (incl. whitespaces), but no leading spaces.
echo $'\n' # Print newline character.
echo -e "\n" # Print newline character. -e enables interpretation of backslash escapes.
n=$'\n'; echo $n # Store newline character to variable and use later.
printf
# Printing:
echo hello; echo "hello" # Same result.
printf %s "hello"
printf '\r'%s "hello" # Carriage return, goes back to beginning of same line and overwrites.
printf "\033[5A" # Jump up 5 lines in the print screen.
printf "\033[5,4A" # Jump up 5 lines and over 4 columns in the print screen. (I haven't tested this.)
Reading from Standard Input
# Read from standard input (prompt for user input):
read
echo $REPLY; echo ${REPLY} # Equivalent. Print what was last read in.
read -r # -r = ignore backslash escapes
read line # read user input and store to variable $line
read -u 3 # read from file descriptor 2 instead of standard input.
# File descriptor must first be created with the following:
exec 3< file # Open $file to file descriptor 3. Don't include $ in front of 'file'.
exec 3>&- # Close file descriptor 3.
# Read returns 0 until end of file is encountered, it times out, or error occurs.
# IMPORTANT NOTE ABOUT FILE DESCRIPTORS:
# File descriptors 0, 1 and 2 are for stdin, stdout and stderr respectively.
# File descriptors 3, 4, .. 9 are for additional files.
Reading from File
Read Text File Line by Line
Method 1
file=$(cat temp.txt)
for line in $file
do
echo -e "$line\n"
done
Method 2
File descriptor is created when file is opened, and stays open until closed.
IMPORTANT NOTE ABOUT FILE DESCRIPTORS:
File descriptors 0, 1 and 2 are for stdin, stdout and stderr respectively.
File descriptors 3, 4, .. 9 are for additional files.
exec 3< file; # open file file as fd 3 (fd = file descriptor)
echo "hello" > file # write to file
read -ur 3 # Reads 1 line and stores to $REPLY.
Keep running this, and it will read each line sequentially.
-u = reads from file instead of stdin
-r = ignores backslash escapes
read -ur line # Reads 1 line and stores to $line.
Close file and re-open to start at the beginning again.
exec 3>&- # Close file descriptor 3.
Method 3
file="temp.txt"
while read -r line
do
echo -e "$line\n"
done <$file
Writing to a File
# Write to a file; overwrite everything.
echo "hello" > ./file.txt
# Write to a file; append, don't overwrite.
echo "hello" >> ./file.txt
# Capture stdout to a file.
command_to_run > output_file.txt
# Capture stderr to a file.
command_to_run 2> error_file.txt
# Send stdout to the terminal, and both
# stdout and stderr to a file.
command_to_run | tee output_file.txt
# Send stdout to the terminal, and
# only stderr to a file.
command_to_run 2>&1 | tee output_file.txt
Bash Configuration
# Add commands that execute every time Bash shell opens.
echo "[commands]" >> ~/.bashrc
Aliases
Set an Alias
Calling the alias will call another command or series of commands.
If an alias overloads an existing command name, there are ways to override the alias.
# ~/.bashrc
alias ls="ls -la"
\ls # Option 1
command ls # Option 2
/bin/ls # Option 3, use full path to command
/usr/bin/ls
Source Command
# Run file, and all variables created by it will be added to the current context.
source ./script.sh
# Add this path to the current shell's context.
# Allows you to execute files from this directory by name
# without providing the complete path.
source ~/andrew/robotics/scripts/
top # see processes currently running
htop # newer, more advanced version of 'top'
ps faux # simple list of processes currently running
ps faux | grep "xxx" # filter processes; grep itself will always show up as a process in the resulting list
Ctrl+C # sends SIGINT signal to program, shuts down program, only works on foreground processes
Ctrl+Z # sends SIGSTOP signal, suspsneds process, sends foreground process to background, only works on foreground processes. Processes in background won't receive Ctrl+C or 'kill' commands.
bg # resume execution of a suspended/background process, bring it back to foreground. It will immediately receive previous Ctrl+C and 'kill' signals that were sent when the process was in the background.
kill <PID> # kill a process by PID, get from 'ps faux' list. Process must be executing in foreground to kill it
./script.sh & # starts process as a background process
# View of system performance information (like task manager):
glances
# System performance monitors
sysbench
Geekbench
Grep
Search for Invisible Control Characters
Note - control characters might not render at all, or they might render inconsistently.
grep -a $'\xff' # search for character '\xff'
Create a dummy test file with control characters to make sure search works.
