Echo

echo Hello
Hello
echo Hello World
Hello World
echo Hello    World # multiple whilespaces
Hello World
echo 'Hello   World' # quotes
Hello   World
echo ~ # shell will expand ~ and pass to echo

Working with Files and Directories

mkdir

Can create multiple directories at the same time: mkdir dir1 dir2.

touch

If file already exists, updates its timestamp, else creates the file.

mv

Can move both files and directories. Can act as renaming. Move multiple files: mv file1 file2 new-path.

• -i for confirmation.

cp

• -r for directory.

rm

• -r for directory.

• -i for confirmation.

• wildcards: *?. The shell expands the wildcards, not rm. [AB]: A or B.

  • rm *.txt fails when there are too many files. to get the limit: gefconf ARG_MAX

  • fix: find . -type f -name '*.txt'

Navigation

pwd

Print current working directory

ls

List

• -l: long form.

• -a: show all. Include entries start with a dot

• -h: human readable size

• -R: recursively list sub directory

• -t: order by time of last change

• -r: reverse order

missing:~$ ls -l /home
drwxr-xr-x 1 missing  users  4096 Jun 15  2019 missing

d: missing is a directory.

owner: missing

owning group: users

rwx: read, write, execute permissions of owner, owning group, and everyone else.

4096: size.

cd

Change directory.

• No argument: go to home directory.

• -: go to previous directory.

• Absolute path begins with a /.

• ..: go up one level

Text Manipulation

cat

cat a.txt
cat a.txt b.txt
cat < a.txt

head / tail

head a.txt # show first 10 lines of a.txt
head -n 5 a.txt # show first 5 lines
head -n 5 a.txt | tail -n 2 # show line 4-5
tail -f a.txt # watch a.txt

less / more

• /: search.

• n/N: go to next/previous match.

• q: quit.

• man page is shown in less.

wc

word count

• -l: line count only

• -w: words.

• -c: characters

man wc | wc
      80     483    3587
# 80 lines, 483 words, 3587 bytes

Sort

Sort in alphanumerical order. Does not change the file.

• -n: sort numerically.

• -r: reverse order.

Uniq

Removes adjacent duplicated lines. Usually used with sort: sort a.txt | uniq.

• -c: show the count of occurrences of each line.

Cut

cut -d , -f 4 a.txt use comma as the delimiter, output 4th field.

grep

grep searchterm filename

• -i case insensitive

• -w: only match whole-word match. grep -w The a.txt, Thesis won't match.

• -n show the line number

• -v: invert. Show the lines that does NOT contain the search term.

• -E: use regex as search term and tell shell not to expand the regex. grep -E '^.o' a.txt.

grep -i break-in auth.log | awk {'print $12'} Use awk to print the 12th thing on each line.

find

find . # shows all files and directories
find . -type d # only show directories. type f is for files
find . -name '*.txt' # use quotes around txt so that shell does not expand it. one line one file.
wc -l $(find . -name '*.txt') # subshell commands run first. Just like expanding the wildcards.

xargs

find . -type f -name '*.txt' | xargs -n1 -P8 | mawk 'map code here' | mawk 'reduce code here'
# homemade MapReduce
# xargs -n1: every time take (at most) 1 line as args
# -P8: allow 8 commands run at the same time

Other commands

date

prints current date.

whoami

prints the current user

which

shows the location of a command. e.g. which echo

tee

Duplicate output.

echo hello | tee output.txt
# hello goes to 1. output.txt, 2. stdout
​
echo "newline" | sudo tee -a /etc/file.conf
# sudo echo "newline" > /etc/file.conf does not work
# since you need sudo to write to the file.

yes

• no argument: output y, infinite loop.

• with argument: output argument, infinite loop.

yes no # output no
# usage
# skip user confirmation
yes | sudo apt-get install ...

Multiple Commands

com1; com2 # will run com2 regardless of result of com1
com1 && com2 # only run com2 when com1 succeeds
com1 || echo "Failed" # only run echo when com1 fails
com1 | com2 # the commands in the pipe runs in parallel 
# sleep 3 | sleep 5 | echo '8'

Standard Stream

stdin (0), stdout (1), stderr (2). 0, 1, 2 are their file descriptor (fd). & is stdout + stderr.

Redirect

cat < file1 # redirect stdin
echo "hello" > file2 # override
echo "world" >> file3 # append
command2 2>&1 # combine stderr into stdout
cp -v * ../otherfolder 1> ../success.txt 2>../error.txt
ls > /dev/null # discard the output.

Shell Variables

$ set # shows shell variables
# if you don’t give "set" any arguments, it might as well show you things you could set.
​
$ which set
# no output, meaning it's an internal command of bash
​
$ which env
/usr/bin/env
​
echo $PATH # show the value of a variable
a=abc # create a variable
a=def # change a variable
​
# .bashrc
export a=123 # so a is available outside of .bashrc as well

All shell variables’ values are strings, even those (like UID) that look like numbers. It’s up to programs to convert these strings to other types when necessary.

Some variables (like PATH) store lists of values. In this case, the convention is to use a colon : as a separator. If a program wants the individual elements of such a list, it’s the program’s responsibility to split the variable’s string value into pieces.

Environment Variables

env # show all environement variables
export name=David # set
export full_name="David Feng" # use quotes if value contains space

Spaces in Names

Use "$1", "$filename" to when the variable contains spaces.

Job Control

List all processes

ps: list (your active) processes. -f gives more info.

• PID: process ID.

• PPID: parent's ID.

• UID: user ID.

• STIME: start time.

• TIME: how much time the process has used.

• CMD: the program that process is executing.

• TTY: ID of the terminal that the process.

• C: percentage of CPU utilization.

Kill a process

SIGKILL - process may be killed before it releases its resources, causing some unexpected results.

kill -9 pid # when multiple processes share the same name
killall -9 process_name

Suspend a process

Commands

• Run a command in background: command &

• foreground => pause a program and return to shell: Ctrl + Z.

Then bg resumes it as a background job.

background => foreground: fg

• fg %1 to select to bring which process. The ID is the one shown in jobs.

list all jobs: jobs

History

• history | tail -n 5: show the last 5 commands.

• !123: rerun command 123.

• !!: rerun last command. sudo !! is very common.

• C-r: search in history.

• !$: last word of last command.

Permissions

• Unix groups are stored in /etc/group

• chmod u=rw g=r a= a.txt (user, group, all)

• chmod -x

• chmod 000 *_015*

  • 1: x; 2: w; 4:r

  • don't use 777. Use chown/chgrp (change owner, change group)

• search by permission: $ find . -type f -perm -u=x

• -x permission for directory: right to traverse the directory (can see inner directories), but not to look at the content.

root

root user can do almost anything. Usually we don't login as root, but use sudodo something as su (super user).

Aliases

alias up='cd ..'. We can remove a shortcut with unalias. e.g. unalias upupup.

The .bashrc file is executed whenever entering interactive non-login shells whereas .bash_profile is executed for login shells. If the .bash_logout file exists, then it will be run after exiting a shell session.

rc is short for run control.

Need to source ~/.bashrc for it to work in the same session. (or just use dot instead of source)

# .bash_profile
if [ -f $HOME/.bashrc ]; then
    source $HOME/.bashrc
fi

Prompt

PS1

$ means that you are not the root user.

Misc

• unzip abc.zip.

• Move to start/end of line: C-a/C-e.