2025/02/16

Topic 715: Basic Unix Skills and Shell Scripting

This topic covers using the shell, managing files, processes, regular expressions, editing with vi, and writing simple shell scripts. Detailed explanations and examples are provided below.

715.1: Use the Shell and Work on the Command Line

Environment Variables:

Bourne shell example:
```
export PATH=/usr/local/bin:$PATH
```
Prepends /usr/local/bin to the current PATH.
csh/tcsh example:
```
setenv PATH /usr/local/bin:$PATH
```
Sets the PATH variable in C shell environments.

Redirection and Pipes:

Output redirection:
```
ls > files.txt
```
Saves the output of ls into files.txt.
Input redirection:
```
cat < files.txt
```
Reads the contents of files.txt.
Pipe example:
```
ls | grep conf
```
Passes the output of ls to grep to filter for lines containing "conf".

Manpages and Documentation:

View a manpage:
```
man ls
```
Search for commands:
```
apropos network
```

History and Aliases (csh/tcsh):

View history:
```
history
```
Create an alias:
```
alias ll 'ls -l'
```

715.2: Perform Basic File Management

Copying Files and Directories:

Copy a file:
```
cp file1 file2
```
Copy a directory recursively:
```
cp -r dir1 dir2
```

Moving Files:

Rename or move:
```
mv oldname newname
```

Removing Files and Directories:

Remove a file:
```
rm file.txt
```
Remove a directory recursively:
```
rm -r directory
```

Identifying File Types:

Determine file type:
```
file something.bin
```

Archiving/Backup:

Create a tar archive:
```
tar cvf myarchive.tar mydir
```
Extract a tar archive:
```
tar xvf myarchive.tar
```

715.3: Create, Monitor and Kill Processes

Monitoring Processes:

Interactive process viewer:
```
top
```
List processes:
```
ps aux | grep ssh
```

Killing Processes:

Graceful termination:
```
kill <pid>
```
Force termination (SIGKILL):
```
kill -9 <pid>
```

Changing Process Priority:

Renice a process:
```
renice +5 <pid>
```

Background and Foreground Jobs:

Run in background:
```
some_command &
```
List background jobs:
```
jobs
```
Bring job to foreground:
```
fg %1
```

715.4: Use Simple Regular Expressions

Using grep:

Basic search:
```
grep "error" /var/log/messages
```
Case-insensitive search:
```
grep -i "warning" /var/log/messages
```
Exclude a pattern:
```
grep -v "notice" /var/log/messages
```

Using egrep:

Extended regex search:
```
egrep "ab{2,3}c" file.txt
```

715.5: Perform Basic File Editing Operations (vi)

Basic vi commands:

Save the file:
```
:w
```
Quit vi:
```
:q
```
Quit without saving:
```
:q!
```
Force save and quit:
```
:wq!
```

Line operations in vi:

Delete a line:
```
dd
```
Yank (copy) a line:
```
yy
```
Paste a line:
```
p
```

Searching in vi:

Search for a pattern:
```
/pattern
```
Repeat the search:
```
n
```

715.6: Customize or Write Simple Scripts

Example script (Bourne shell):

#!/bin/sh
# A simple backup script

BACKUPDIR="/backup"
SRCDIR="/home/user"

tar cvf $BACKUPDIR/home.tar $SRCDIR
echo "Backup completed!"

To make a script executable, use:

chmod +x backup.sh
./backup.sh

In shell scripting, positional parameters allow access to arguments passed to the script:

#!/bin/sh
echo "Script name: $0"
echo "First argument: $1"