Exercise: Run commands efficiently

Investigate a real file on workstation, /usr/bin/zcat, with the commands from this chapter, and use completion, history and line editing so that you type its long name only once.

zcat prints the contents of compressed files. It is a good subject because it is small, and because its name doesn't tell you what kind of file it is.

Run commands efficiently

Identify a file's type and read parts of it.
Reuse arguments and commands from the history instead of retyping them.
Edit a previous command with the line-editing keys.

Show the time on a 24-hour clock

On workstation, as student:

[student@workstation ~]$ date +%R
09:39

Find out what kind of file zcat is

Type file /usr/bin/zc and press Tab to complete the name, then Enter:

[student@workstation ~]$ file /usr/bin/zcat
/usr/bin/zcat: a /usr/bin/sh script, ASCII text executable

It is a script: a text file of shell commands that runs like a program. Because it is text, you can read it.

Count its lines with Alt+.

Type wc and a space, then press Alt+. to bring back the last argument, /usr/bin/zcat:

[student@workstation ~]$ wc /usr/bin/zcat
  51  299 1988 /usr/bin/zcat

51 lines, 299 words and 1988 bytes.

Show its first and last lines

Use Alt+. again for each command:

[student@workstation ~]$ head /usr/bin/zcat
#!/usr/bin/sh
# Uncompress files to standard output.

# Copyright (C) 2007, 2010-2022 Free Software Foundation, Inc.
...output omitted...
[student@workstation ~]$ tail /usr/bin/zcat
With no FILE, or when FILE is -, read standard input.
...output omitted...
exec gzip -cd "$@"

The first line, #!/usr/bin/sh, names the program that runs the script. The last line shows that zcat is a small wrapper: it hands its work to gzip -cd.

Repeat the last command in two keystrokes

Press ↑ then Enter. Then do the same with !!, which prints the command it expands to:

[student@workstation ~]$ !!
tail /usr/bin/zcat
With no FILE, or when FILE is -, read standard input.
...output omitted...

Edit the last command to show 20 lines

Press ↑ to bring back tail /usr/bin/zcat, Ctrl+A to jump to the start, Ctrl+→ to jump past tail, type a space and -n 20, then press Enter:

[student@workstation ~]$ tail -n 20 /usr/bin/zcat
  -l, --list        list compressed file contents
  -q, --quiet       suppress all warnings
...output omitted...
exec gzip -cd "$@"

Rerun the date command from the history

List the history and find the number of date +%R. Yours will differ from this example:

[student@workstation ~]$ history | tail -n 7
   41  date +%R
   42  file /usr/bin/zcat
   43  wc /usr/bin/zcat
   44  head /usr/bin/zcat
   45  tail /usr/bin/zcat
   46  tail -n 20 /usr/bin/zcat
   47  history | tail -n 7
[student@workstation ~]$ !41
date +%R
09:41

You ran tail /usr/bin/zcat three times in a row, but it appears once: RHEL sets HISTCONTROL=ignoredups in /etc/profile, which tells Bash not to store a command that repeats the one before it.

| tail -n 7 sends the long history list through tail, so you see only the last seven lines. Chapter 5 explains the | (pipe).

Find a command with Ctrl+R

Press Ctrl+R and type wc. The prompt changes to a search, and shows the most recent match:

(reverse-i-search)`wc': wc /usr/bin/zcat

Press Enter to run it, or Esc to put it on the command line for editing.