Shell Programming

Shell programming basics (Detailed)

Shell: Command-line interface between user and OS that interprets and executes commands.
Common shells: sh (Bourne shell), bash (Bourne Again Shell, default in Linux), zsh (Z shell).
Script file extension: .sh

Execution:

bash file.sh        # run using bash
chmod +x file.sh    # make executable
./file.sh           # execute script directly

Shebang (#!): Specifies interpreter for the script, e.g., #!/bin/bash.

Variables

User-defined variables: No spaces around =
Terminal window
```
x=10
name="SEBI"
echo $x
```
$ expands variable value.
Environment variables:
- HOME: Current user’s home directory.
- PATH: Colon-separated directories where shell searches commands.
Exporting variables:
Terminal window
```
export x
```
Makes variable available to child processes/subshells.

Input and Output

Read input:
Terminal window
```
read x
```
Prompted input:
Terminal window
```
read -p "Enter value: " x
```
Print output:
Terminal window
```
echo "Value is $x"
```

Command substitution

Store command output:

d=$(date)      # modern syntax
files=`ls`     # legacy syntax

Arithmetic operations:
Terminal window
```
sum=$((a+b))
mul=`expr $a \* $b`
```

Conditional statements

if syntax:

if [ $a -gt $b ]; then
    echo "a greater"
fi

Comparison operators (integers): -eq, -ne, -gt, -lt, -ge, -le
String operators: =, !=, -z (empty), -n (non-empty)

if-else syntax:

if [ $a -eq 10 ]; then
    echo "Ten"
else
    echo "Not ten"
fi

Case statement

case $x in
  1) echo "One" ;;
  2) echo "Two" ;;
  *) echo "Other" ;;
esac

case: Starts multi-way selection.
in: Begins pattern list.
1): Pattern match.
*): Default case.
;;: Ends a block.
esac: Ends case statement.

Loops

For loop:
Terminal window
```
for i in 1 2 3; do
    echo $i
done
```

While loop:

while [ $x -lt 5 ]; do
    x=$((x+1))
done

Until loop:

until [ $x -gt 5 ]; do
    x=$((x+1))
done

Functions

fun() {
    echo "Hello"
}
fun

Return status via $?.

Command-line arguments

$0   # script name
$1 $2 ...  # arguments
$#   # number of arguments
$@   # all arguments

File test operators

-f: file exists
-d: directory exists
-r: readable, -w: writable, -x: executable

Example:

if [ -f file.txt ]; then echo "Exists"; fi

Redirection

> : overwrite output
>>: append output
< : input from file
2>: redirect errors

ls > out.txt

Pipes

Send output of one command to another:

ps -ef | grep root

ps: list processes, -ef: full format
grep: search text/pattern, root: matches processes owned by root

Important Linux commands (exam-focused)

File operations: ls cd pwd mkdir rmdir rm cp mv
View files: cat less more head tail
Text processing: grep awk sed sort uniq wc cut
Permissions: chmod chown
Process management: ps top kill
Disk & memory: df du free

awk & sed

awk: extract/format text columns
Terminal window
```
awk '{print $1}' file.txt
```
sed: find/replace/modify text
Terminal window
```
sed 's/old/new/' file.txt
```

Exit status

0 = success, non-zero = failure

echo $?

Crontab

Schedule jobs:

crontab -e

-e: edit user’s cron jobs
Format:

min hour day month week command

Extra Tips for SEBI Grade A (IT)

Focus on practical usage of shell commands, loops, conditions, and file operations.
Remember command vs arithmetic substitution: $() vs $(( )).
Know pipes, redirection, cron jobs, awk & sed basics.
Expect questions on environment variables, file tests, and exit status.

If you want, I can create a super-condensed one-page “last-minute revision sheet” for shell scripting for SEBI IT Grade A. It will fit everything exam-relevant in bare minimum lines. Do you want me to do that?