Toggling and Managing Internal Commands

Internal shell commands can be disabled or enabled dynamical. Use enable -n to turn off a built-in and enable to restore it.

[root@localhost opt]# enable -n cd
[root@localhost opt]# cd /mnt/
[root@localhost opt]#

After disabling cd, the command no longer functions, and the prompt remains in the current directory.

Working with the Command Hash Table

The shell maintains a hash table to speed up external command lookups. Entries can be removed individually or cleared entirely.

• Remove a single entry: hash -d <command>
• Clear the entire table: hash -r

[root@localhost mnt]# hash
hits    command
   1    /usr/bin/mv
   2    /usr/bin/ls
   6    /usr/bin/cd

[root@localhost mnt]# hash -d ls
[root@localhost mnt]# hash
hits    command
   1    /usr/bin/mv
   6    /usr/bin/cd

If a hashed binary is moved to a different location, the shell will still reference the old path. Clearing the hash entry allows the shell to locate the new path.

[root@localhost mnt]# mv /usr/bin/ls /usr/local/bin/
[root@localhost mnt]# ls
-bash: /usr/bin/ls: No such file or directory
[root@localhost mnt]# hash -r
[root@localhost mnt]# ls
Desktop  Documents  Downloads

Linux Command Structure

A typical command line consists of three components:

1. Command word – the executable or built-in name.
2. Options – modifiers that adjust behavior. Short options use a single hyphen (-l), while long options use two (--all).
3. Arguments – targets such as files, directories, or users.

# Long format listing
ls -l

# Human-readable sizes
ls -lh

# Multiple commands separated by semicolons
cd /var/log; ls -lt

Options such as -a, -b, -c can usually be combined into -abc, though certain tools enforce order when options require their own arguments (e.g., tar -xzvf archive.tar.gz).

Command-Line Editing Shortcuts

Efficient editing is possible with these key combinations:

Shortcut	Action
Tab	Auto-complete command or path; press twice to list possibilities
Backslash \	Escape character or force a line continuation
Ctrl + a	Move cursor to the begining of the line
Ctrl + e	Move cursor to the end of the line
Ctrl + u	Delete from cursor to the start of the line
Ctrl + k	Delete from cursor to the end of the line
Ctrl + w	Delete the word before the cursor
Alt + d	Delete the word after the cursor
Ctrl + l	Clear the screen
Ctrl + c	Abort the current input or running command
Ctrl + s / Ctrl + q	Freeze / unfreeze terminal output
Up/Down arrows	Navigate through command history

Obtaining Command Help

Internal Command Help

Use help to display usage information for shell built-ins.

[root@localhost ~]# help echo
echo: echo [-neE] [arg ...]
Write arguments to the standard output.

Examples of echo options:

# Suppress trailing newline
echo -n "Processing"

# Interpret backslash escapes
echo -e "Line1\nLine2\nLine3"

# Tab characters
echo -e "Col1\tCol2\tCol3"

# Remove previous character with backspace
echo -e "Error\b\b\bFix"

External Command Help

Most external commands support --help for a quick summary.

ls --help
useradd --help