Whenever you travel to a foreign country in which the inhabitants speak an unusual native tongue, you might arm yourself with essential survival phrases, such as "How much does this cost?," "What kind of meat is this?," and "Where is the bathroom?" Memorizing such little quips ensures that you don't get overcharged for that snake sandwich you ordered, and you know where to go when Mother Nature (or the snake sandwich) calls.
UNIX®, too, has a dialect all its own and, over the past six months, this Speaking UNIX series has provided something of a crash course in command-line locution. This month, learn several helpful phrases that will have you blending with the locals in no time. Grab your toothbrush, pack some comfortable shoes, and update your inoculations. You're off for sun, sand, and shells. (For the sun and sand, scoop up your laptop, head to the beach, plop down near the water's edge, and read this column. And don't forget your sunscreen.)
Take the walking tour
Many of the previous Speaking UNIX columns (see Resources) have presented the
find command -- an invaluable utility for scanning and processing files or even the entire UNIX file system. For instance, I often use
find in combination with
grep or Perl to process a large number of files. Need to know where a variable or constant is defined in a large body of code? Try this:
$ find /path/to/src -type f | xargs grep -H -I -i -n string
The output of the command is a list of file names that contain
string , including the line number and the specific text that matched. The
-n options preface each match with the file name and line number of each match, respectively. The
-i option ignores case.
-I (capital "I") skips binary files.
If you haven't seen
xargs before, it runs the command you specify -- here,
grep with all the listed options -- once for each argument provided through standard input. Assuming that the /path/to/src directory contains files a, b, and c, using
find in combination with
xargs is the equivalent of:
grep -H -I -i -n string a grep -H -I -i -n string b grep -H -I -i -n string c
In fact, searching a collection of files is so common that
grep has its own option to recurse a file system hierarchy. Use
-d recurse or its synonyms
-r. For example, use:
$ grep -H -I -i -n -R string/path/to/src
This command achieves the same thing as
find/xargs. (You'll find that many file-related UNIX utilities have options for recursion.
ls -R recursively lists the contents of a hierarchy.
-R to apply mode, group, and ownership changes to an entire file system hierarchy. Be careful using
chmod -R. You can inadvertently render a directory unusable if you remove its execute bits, say with
chmod -R a-x. To be more selective, try
find . -type f | xargs chmod a-x.)
So, when should you use
xargs, and when should you use
grep? When you need to be selective, use
find command has many conditionals that allow you to choose files that meet specific requirements, such as the conceptual "all regular files modified after midnight and owned by Joe." Otherwise,
grep -R suffices nicely.
One other utility can be more convenient and faster to use than
find. If you're trying to find a file by name, try
locate instead of
find -name. The
locate command periodically (once a day or so, tuned by your system administrator) catalogs every file on your system and builds
a database of path and file names. When you run
locate, it scans its private database, trying to make matches.
For example, running the query
locate '*.1' yields all files and directories whose names end with
.1. (The leading asterisk means match anything.) For convenience, running the
locate fish command is the same as
The currency exchange
Many UNIX utilities modify files. In most cases, altered contents are emitted to standard output where you can use redirection operators to further process (using pipes -- |) or capture the results (using the
Other utilities, typically those that can process many files at a time, can retain the original file for safekeeping and generate a new file for the altered contents. For example, you can use Perl directly from the command line to process a file. The command:
$ perl -i.bak -pe 's/\bdollar(s?)/buck\1/g' file.txt
replaces "dollar" with "buck" and "dollars" with "bucks." The
perl -i command changes file.txt in-place;
perl -i.bak makes a copy of the original file and appends .bak to distinguish it from its new, modified version. Therefore, a command such as:
perl -i.bak -pe 's/\bdollar(s?)/buck\1/g' *
would create a backup for every file in the current directory. Assuming you had files file1.txt, file2.txt, and file3.txt, you would then have file1.txt.bak, file2.txt.bak, and file3.txt.bak. Mistakes happen, so making a backup is smart.
If you make an error and must recover an original, you might simply type:
mv file1.txt.bak file1.txt
But what happens if you have hundreds of files to rename? Certainly, you do not want to type hundreds of individual
mv commands. Instead, you could type this:
foreach file in (*.txt) do mv $file.bak $file done
And that does work well in simple cases such as this one. However, this sort of task is so common that a special utility makes the job even faster. The command:
$ rename 's/\.bak$//' *.bak
performs the same task. The regular expression
s/\.bak$// strips .bak from each file name listed on the command line -- here,
*, or every file -- and uses the shortened name as the target file name.
rename command works especially well when file names are particularly irregular. For instance, consider the contents of this directory, which looks something like a collection of letters from a college freshman.
$ ls RenT.txt bEErMoNey.txt gASmoNey.TXt
foreach script shown above can't handle this problem, because the file names are so irregular. Yet
rename can process it in a few keystrokes:
$ rename 'y/A-Z/a-z/' *
y operator in the regular expression
y/A-Z/a-z/ is used for translation. Translation requires two lists: a list of original characters and a list of replacement characters. If the two lists are the same size, every instance of the first character of the original list in the text is replaced with the first letter of the replacement list. In other words, in the example, every instance of capital "A" is replaced with lowercase "a," "B" with "b," and so on. Lowercase letters in the text are left unchanged.
If you want to preview what
rename does, add the
-n option. This option shows the actions of the command without making the actual changes:
$ rename -n 'y/A-Z/a-z/' * RenT.txt renamed as rent.txt bEErMoNey.txt renamed as beermoney.txt gASmoNey.TXt renamed as gasmoney.txt $ rename 'y/A-Z/a-z/' * $ ls beermoney.txt gasmoney.txt rent.txt
One gotcha to avoid: On UNIX systems, file names are case sensitive. A directory can contain Aa.Txt and aA.txT. It's possible, as shown above, to write a rename rule that converts case-sensitive files into lowercase file names, thereby causing clashes among previously unique file names. What does
rename do in that case? Let's see:
$ rename -n 'y/A-Z/a-z/' * Aa.Txt renamed as aa.txt aA.txT renamed as aa.txt $ rename 'y/A-Z/a-z/' * aA.txT not renamed: aa.txt already exists $ ls aA.txT aa.txt
If you want to "clobber" existing files as the rename progresses, add the
-f flag. In this example, the result would be one file named aa.txt. And which file was that originally? Because
rename works in alphabetical order, the latter file aA.txT is now aa.txt. Why use
-f? If two files are identical other than name,
rename -f would remove duplicates.
It's not clobberin' time
File management is certainly a large part of using UNIX. The system has innumerable configuration files. You likely have a plethora of data files and personal files. From time to time, you might accidentally remove or overwrite a valuable file. The shell and several file management utilities can help you avoid calamity.
Type the following commands at your shell prompt. These commands work in
bash, but similar options exist in
zsh and other shells.
$ alias mv=mv -i $ alias rm=rm -i $ set -o noclobber
The first two commands replace
mv -i and
rm -i, respectively, at the command line. Interactive mode forces you to confirm your action.
The third command provides a modicum of safety in the shell itself. With
noclobber on, you can't accidentally overwrite a file using the
> redirection operator:
$ ls secret.txt $ cat > secret.txt bash: secret.txt: cannot overwrite existing file
set +o noclobber
You can also force an overwrite at any time using the
>| (a greater-than symbol followed by a vertical bar) redirection operator.
$ cat secret.txt I love green eggs and ham. $ echo "No more secrets" >| secret.txt $ cat secret.txt No more secrets
Secrets of the locals
If you really want to discover a city, you have to visit the local watering holes. Here's a collection of command-line combinations worthy of Zagat.
mkdir -p makes quick work of hierarchy creation. With
mkdir creates each directory and subdirectory found in the named path:
$ mkdir -p make/many/directories/at/once $ ls -R ./make: many ./make/many: directories ./make/many/directories: at ./make/many/directories/at: once ./make/many/directories/at/once:
If you ever need to know when your next pay day is, just type
cal. With no arguments,
cal prints the calendar for the current month. The
cal -3 command shows the previous, current, and next month, and
cal 06 2009 shows you June 2009. (My birthday is on a Monday that year!)
$ cal November 2006 Su Mo Tu We Th Fr Sa 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 $ cal 06 2009 June 2009 Su Mo Tu We Th Fr Sa 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30
Because UNIX has so many commands, it's virtually impossible to remember all the options for all the utilities. Indeed, sometimes you can't even remember the name of the utility.
When stumped, turn to
man man, for example, to see how to use
man itself. You can also see explanations for
man rm and
man mv. And, if you know what topic you're looking for, you can use
make -k to find a list of man pages that relate to that topic.
$ man -k cron cron (8) - daemon to execute scheduled commands (Vixie Cron) crontab (1) - maintain crontab files for individual users (V3) crontab (5) - tables for driving cron dh_installcron (1) - install cron scripts into etc/cron.*
man discovered the man pages of those utilities whose one-line descriptions contained the word cron. Chances are, these man pages explain how to use
cron, the system job-scheduling daemon.
And what are the little numbers for? Each number refers to a section of the online UNIX manual. Section 1 is reserved for all the commands a UNIX user can run in the shell. Section 5 describes file formats. Section 8 catalogs system administration commands. Other sections describe system calls (2), library calls (3), and more.
As you've seen, most commands emit output of one kind or another. Most command-line commands use standard output for results. But others use standard out and standard error to show progress and error messages, in that order. If you want to ignore that sort of output -- which is useful, because it often interferes with working at the command line -- redirect your output to the UNIX bit bucket, /dev/null. Bits check in, but they don't check out.
Here's a simple example:
$ ls secret.txt $ cat secret.txt I am the Walrus. $ cat secret.txt > /dev/null $ cat socrates.txt > /dev/null cat: socrates.txt: No such file or directory $ cat socrates.txt >& /dev/null $ echo Done. Done.
If you redirect the standard output of
cat to /dev/null, nothing is displayed, because all of the bits have been thrown into the virtual "permanent vertical file." However, if you make a mistake, error messages, which are emitted to standard error, are displayed. If you want to ignore all output, use the
>& operator to send
stderr to the bit bucket.
You can also use /dev/null as a zero-length file to empty existing files or create new, empty files:
$ cat secret.txt Anakin Skywalker is Darth Vader. $ cp /dev/null secret.txt $ cat secret.txt $ echo "The moon is made of cheese!" > secret.txt $ cat secret.txt The moon is made of cheese! $ cat /dev/null > secret.txt $ cat secret.txt $ cp /dev/null newsecret.txt $ cat newsecret.txt $ echo Done. Done.
By the way, if you happen to use UNIX on the Macintosh, try the
open command in a Terminal window. For instance, if the current working directory has a file named poodle.jpg, the command
open poodle.jpg launches Preview, the built-in Mac OS X image viewer, with the picture of your poodle. Mac OS X
open is a great link between the command line and the windowing environment of the Macintosh -- and it is often far faster than resorting to the Finder.
Have to visit again!
Whew! That was a fast-paced class, but now you're prepared to travel farther into UNIX. You even know where the bit bucket is when Mother Nature calls.
As always, there's more to cover. In coming months, the Speaking UNIX series delves into job control, regular expressions (a strange dialect, to be sure, but not impossible to master), how to build new utilities you download from the Internet, and more.
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