Mastering Bash I/O Redirection, Conditionals, and Loops: 13 Essential Questions Explained

Question 11: What’s the difference between > and <?

Understanding I/O redirection starts with file descriptors (FD). The three standard FDs are 0 STDIN (keyboard), 1 STDOUT (monitor), and 2 STDERR (monitor). Commands read from STDIN and write to STDOUT/STDERR unless redirected.

$ mail -s test root
this is a test mail.
please skip.
^d

Here the mail program reads its input from STDIN (keyboard). You can redirect input from a file: $ cat /etc/passwd And you can redirect output to a file using >: $ cat < my.file Redirection symbols can be combined: < changes the input source, > changes the output destination. Using << creates a HERE Document, allowing multi‑line input until a delimiter is reached.

$ cat <<FINISH
first line here
second line there
third line nowhere
FINISH

Redirecting specific file descriptors:

$ ls my.file no.such.file 1>file.out
$ ls my.file no.such.file 2>file.err
$ ls my.file no.such.file 1>file.out 2>file.err

When both stdout and stderr are written to the same file, the later write overwrites the earlier. To merge them, use 2>&1 (stderr to stdout) or 1>&2 (stdout to stderr).

$ ls my.file no.such.file 1>file.both 2>&1

Redirecting output to /dev/null discards it: $ ls my.file no.such.file 2>/dev/null Appending instead of overwriting uses >>: $ echo "3" >> file.out To prevent accidental overwriting, enable noclobber:

$ set -o noclobber
$ echo "4" > file.out   # fails if file exists
$ set +o noclobber
$ echo "5" > file.out   # succeeds

Temporarily overriding noclobber can be done with >|:

$ set -o noclobber
$ echo "6" >| file.out

Question 12: If or case?

Conditional execution can be expressed with && and || or with the more readable if…then…else construct. For multiple branches, elif can be used. When testing string values, case provides a cleaner syntax than multiple if tests.

if comd1
then
    comd2
    comd3
else
    comd4
    comd5
fi

case "$1" in
    start) start;;
    stop) stop;;
    status) rhstatus;;
    restart|reload) restart;;
    *) echo "Usage: $0 {start|stop|status|restart|reload}"; exit 1;;
esac

Question 13: for what? while vs until?

Shell provides three primary loops: for, while, and until. for iterates over a list of values; while repeats while a test command returns true; until repeats until the test returns true (i.e., while it returns false).

for var in one two three; do
    echo "$var"
done

num=1
while [ "$num" -le 10 ]; do
    echo "num is $num"
    num=$((num+1))
done

num=1
until [ "$num" -gt 10 ]; do
    echo "num is $num"
    num=$((num+1))
done

Both break and continue control loop flow: break exits the current (or an outer) loop, while continue skips to the next iteration. They differ from return (exits a function) and exit (terminates the script).