Essential Linux Shell Commands: From Killing Processes to File Permissions

Introduction

Linux shell commands are fundamental tools for everyday tasks such as terminating processes, managing files, and inspecting system information. Below is a concise reference covering common commands and their usage.

Process Management

$ ps | grep xxx $ kill -9 xx

Using killall simplifies the operation:

$ killall -9 xxx

When killall is unavailable, combine adb and grep:

$ adb shell ps | grep speech | awk '{print $2}' > 1.txt $ cat 1.txt | xargs -n1 adb shell kill -9

Or in a single line:

$ adb shell ps | grep speech | awk '{print $2}' | xargs -n1 kill -9

Shutdown and Reboot

shutdown -h now          # immediate shutdown
shutdown -h +10          # shutdown after 10 minutes
shutdown -h 12:00:00     # shutdown at 12:00
halt                     # equivalent to immediate shutdown

# Reboot
shutdown -r now
reboot                   # equivalent to immediate reboot

Echo Command

root@ubuntu:~# a="hello,world"
root@ubuntu:~# echo a
a
root@ubuntu:~# echo &a
[1] 3091

a: command not found
[1]+  Done                    echo
root@ubuntu:~# echo $a
hello,world

Vim Text Editor

vi somefile.4
# Modes:
# 1. Normal mode – accepts shortcuts, no editing.
# 2. Press i to enter Insert mode for editing.
# 3. Press Esc to return to Normal mode.
# 4. Press : to enter Command mode, then wq to save and quit.

# Common shortcuts (Normal mode):
a   – insert after cursor
A   – insert at end of line
I   – insert at beginning of line
gg  – go to first line
G   – go to last line
dd  – delete a line
3dd – delete three lines
yy  – yank (copy) a line
3yy – yank three lines
p   – paste
u   – undo
v   – visual character mode (y to copy, p to paste)
Ctrl+v – visual block mode
Shift+v – visual line mode

# Search and replace:
:set nu        # show line numbers
:set nonu      # hide line numbers
:/you          # search for "you"
:s/sad/bbb     # replace first "sad" on current line
:%s/sad/bbb    # replace all "sad" in file

Copy, Delete, Move, Rename

cp somefile.1 /home/hadoop/
rm /home/hadoop/somefile.1
rm -f /home/hadoop/somefile.1
mv /home/hadoop/somefile.1 ../
mv a.txt b.txt   # rename a.txt to b.txt

Compression and Archiving

# gzip
gzip a.txt
# unzip
gunzip a.txt.gz
gzip -d a.txt.gz
# bzip2
bzip2 a
# bunzip2
bunzip2 a.bz2
bzip2 -d a.bz2
# tar
# create archive
tar -cvf bak.tar ./aaa
# append file to archive
tar -rvf bak.tar /etc/passwd
# extract archive
tar -xvf bak.tar
# create and compress
tar -zcvf a.tar.gz aaa/
# extract and decompress
tar -zxvf a.tar.gz
# extract to specific directory
tar -zxvf a.tar.gz -C /usr
# list contents
tar -ztvf a.tar.gz
# tar with bzip2
tar -jcvf a.tar.bz2
# extract bzip2 archive
tar -jxvf a.tar.bz2

Find Command Examples

# locate executable
which ls
# locate command and manual
whereis ls
# find files by name
find / -name "hadooop*"
find / -name "hadooop*" -ls
# find and delete
find / -name "hadooop*" -ok rm {} \;
find / -name "hadooop*" -exec rm {} \;
# find files owned by user hadoop
find /usr -user hadoop -ls
# find directories owned by hadoop
find /home -user hadoop -type d -ls
# find files with permission 777
find / -perm -777 -type d -ls
# case‑insensitive search
find ./sound/ -iname ft56Q.c
# show command history
history

Grep Command Examples

# exact word match with line numbers
grep -wrn weiqifa ./sound/
# search excluding directories
grep -E "http" . -R --exclude-dir=./sound/
grep -E "http" . -R --exclude-dir={.git,res,bin}
# exclude extensions
grep -E "http" . -R --exclude=*.{java,js}
# basic usage
grep hadoop /etc/passwd
grep aaa ./*.txt
# extract 7th field after ':'
grep dsl /etc/passwd | cut -d: -f7
# lines not containing hadoop
grep -v hadoop /etc/passwd
# regex examples
grep 'hadoop' /etc/passwd
grep 'h.*p' /etc/passwd
grep '^hadoop' /etc/passwd
grep 'hadoop$' /etc/passwd
# exclude lines starting with '#'
grep -v '^#' a.txt | grep -v '^$'
# lines starting with h or r
grep '^[hr]' /etc/passwd
# lines not starting with h or r
grep '^[^hr]' /etc/passwd
# lines not starting with h‑r range
grep '^[^h-r]' /etc/passwd

File Permission Format

drwxr-xr-x   # or 755 in numeric form
# d: directory, -: regular file, l: link
# r: read, w: write, x: execute
# Owner permissions: rwx
# Group permissions: r-x
# Others permissions: r-x

Changing Permissions (chmod)

chmod g-rw haha.dat   # remove group read/write
chmod o-rw haha.dat   # remove others read/write
chmod u+x haha.dat    # add execute for owner
chmod a-x haha.dat    # remove execute for all
# numeric mode
chmod 664 haha.dat    # rw-rw-r--
chmod -R 770 aaa/    # recursive change for directory

Changing Ownership (chown)

# only root can execute
chown angela aaa          # change owner
chown :angela aaa         # change group
chown angela:angela aaa/  # change both owner and group

Disk Usage

# directory size
du -sh sound/
# display disk space
df -h