Bash Pitfalls

This page shows common errors that Bash programmers make. The following examples are all flawed in some way:

1. for i in $(ls *.mp3)

One of the most common mistakes BASH programmers make is to write a loop like this:

•  for i in $(ls *.mp3); do    # Wrong!
      some command $i          # Wrong!
   done
  
   for i in $(ls)              # Wrong!
   for i in `ls`               # Wrong!
  
   for i in $(find . -type f)  # Wrong!
   for i in `find . -type f`   # Wrong!

Never use a CommandSubstitution -- of EITHER kind! -- around something that writes out filenames.

Why? This breaks when a file has a space in its name. Why? Because the output of the $(ls *.mp3) command substitution undergoes WordSplitting. Assuming we have a file named 01 - Don't Eat the Yellow Snow.mp3 in the current directory, the for loop will iterate over each word in the resulting file name:

•  some command 01
   some command -
   some command Don't
   some command Eat
   ...

You can't double-quote the substitution either:

•  for i in "$(ls *.mp3)"; do # Wrong!

This causes the entire output of the ls command to be treated as a single word. Instead of iterating once for each file name, the loop will only execute once, with all the filenames rammed together.

In addition to this, the use of ls is just plain unnecessary. It's an external command, which simply isn't needed to do the job. So, what's the right way to do it?

•  for i in *.mp3; do  # Better! and...
     some command "$i" # ...see Pitfall #2 for more info.
   done

Let Bash expand the list of filenames for you. The expansion will not be subject to word splitting. Each filename that's matched by the *.mp3 glob will be treated as a separate word, and the loop will iterate once per filename.

Question: What to do if there are no files *.mp3-files in the current directory? Then the for loop is executed once, with i="*.mp3", which is not the expected behaviour!

• Check the loop variable inside the loop:

   for i in *.mp3; do
     [[ -f "$i" ]] || continue
     some command "$i"
   done

Reading lines of a file with a for loop is also wrong. Doubly (or possibly triply) so if those lines are filenames.

Note the quotes around $i in the loop body. This leads to our second pitfall:

2. cp $file $target

What's wrong with the command shown above? Well, nothing, if you happen to know in advance that $file and $target have no white space or wildcards in them.

But if you don't know that in advance, or if you're paranoid, or if you're just trying to develop good habits, then you should quote your variable references to avoid having them undergo WordSplitting.

•  cp "$file" "$target"

Without the double quotes, you'll get a command like cp 01 - Don't Eat the Yellow Snow.mp3 /mnt/usb and then you'll get errors like cp: cannot stat `01': No such file or directory. If $file has wildcards in it (* or ? or [...]), they will be expanded if there are files that match them. With the double quotes, all's well, unless "$file" happens to start with a -, in which case cp thinks you're trying to feed it command line options....

3. Filenames with leading dashes

Filenames with leading dashes can cause many problems. Globs like *.mp3 are sorted into an expanded list (using your locale), and - sorts before letters in most locales. The list is then passed to some command, which incorrectly interprets the -filename as an option. There are two major solutions to this.

One solution is to insert -- between the command (like cp) and its arguments. That tells it to stop scanning for options, and all is well:

•  cp -- "$file" "$target"

The problem with this approach is that you have to insert this disabling for every command - which is easy to forget - and that not all commands support --. For example, echo doesn't support --.

Another solution is to ensure that your filenames always begin with a directory (including . for the current directory, if appropriate). For example, if we're in some sort of loop:

•  for i in ./*.mp3; do
     cp "$i" /target
     ...

In this case, even if we have a file whose name begins with -, the glob will ensure that the variable always contains something like ./-foo.mp3, which is perfectly safe as far as cp is concerned.

4. [ $foo = "bar" ]

This is very similar to the issue in pitfall #2, but I repeat it because it's so important. In the example above, the quotes are in the wrong place. You do not need to quote a string literal in bash (unless it contains metacharacters). But you should quote your variables if you aren't sure whether they could contain white space or wildcards.

This breaks for two reasons:

• If a variable referenced in [ does not exist, or is blank, then the [ command would see the line:

  •   [ $foo = "bar" ]

• .. as:

  •   [ = "bar" ]

• .. and throw the error unary operator expected. (The = operator is binary, not unary, so the [ command is rather shocked to see it there.)

• If the variable contains internal whitespace, then it's split into separate words, before the [ command sees it. Thus:

  •   [ multiple words here = "bar" ]

  While that may look OK to you, it's a syntax error as far as [ is concerned.

A more correct way to write this would be:

•  [ "$foo" = bar ] # Pretty close!

This works fine in POSIX-conformant systems even if $foo begins with a -, since POSIX [ determines its action depending on the number of arguments passed to it. On ancient shells, however, it might still cause an error.

In bash, the [[ keyword, which embraces and extends the old test command (also known as [), can also be used to solve the problem:

•  [[ $foo = bar ]] # Right!

You don't need to quote variable references on the left-hand side of = in [[ ]] because they don't undergo word splitting, and even blank variables will be handled correctly. On the other hand, quoting them won't hurt anything either.

You may have seen code like this:

•   [ x"$foo" = xbar ] # Also right!

The x"$foo" hack is required for code that must run on ancient shells which lack [[, and have a more primitive [, which gets confused if $foo begins with a -. But you'll get really tired of having to explain that to everyone else.

If one side is a constant, you could just do it this way:

•   [ bar = "$foo" ] # Also right!

Even on said older systems, [ doesn't care whether the token on the right hand side of the = begins with a -. It just uses it literally. It's just the left-hand side that needs extra caution.

5. cd $(dirname "$f")

This is mostly the same issue we've been discussing. As with a variable expansion, the result of a CommandSubstitution undergoes WordSplitting and pathname expansion. So you should quote it:

•  cd "$(dirname "$f")"

What's not obvious here is how the quotes nest. A C programmer reading this would expect the first and second double-quotes to be grouped together; and then the third and fourth. But that's not the case in Bash. Bash treats the double-quotes inside the command substitution as one pair, and the double-quotes outside the substitution as another pair.

Another way of writing this: the parser treats the command substitution as a "nesting level", and the quotes inside it are separate from the quotes outside it.

6. [ "$foo" = bar && "$bar" = foo ]

You can't use && inside the old test (or [) command. The Bash parser sees && outside of [[ ]] or (( )) and breaks your command into two commands, before and after the &&. Use one of these instead:

•  [ bar = "$foo" ] && [ foo = "$bar" ] # Right!
   [[ $foo = bar && $bar = foo ]]       # Also right!

(Note that we reversed the constant and the variable inside [ for the reasons discussed in pitfall #4.) The same thing applies to ||. Use [[, or use two [ commands.

Avoid this:

•  [ bar = "$foo" -a foo = "$bar" ]     # Not portable.

The problem with [ A = B -a C = D ] (or -o) is that POSIX does not specify the results of a test or [ command with more than 4 arguments. It probably works in most shells, but you can't count on it. You should use two test or [ commands with && between them instead, if you have to write for POSIX shells. If you have to write for Bourne, always use test instead of [.

7. [[ $foo > 7 ]]

The [[ command should not be used for an ArithmeticExpression. It should be used for strings only. If you want to do a numeric comparison, you should use (( )) instead:

•  ((foo > 7))  # Right!

If you use the > operator inside [[ ]], it's treated as a string comparison, not an integer comparison. This may work sometimes, but it will fail when you least expect it. If you use > inside [ ], it's even worse: it's an output redirection. You'll get a file named 7 in your directory, and the test will succeed as long as $foo is not empty.

If you're developing for a BourneShell instead of bash, this is the historically correct version:

•  test $foo -gt 7  # Also right!

Note that the test ... -gt command will fail in interesting ways if $foo is not an integer. Therefore, there's not much point in quoting it properly -- if it's got white space, or is empty, or is anything other than an integer, we're probably going to crash anyway. You'll need to validate your input in advance.

The double brackets support this syntax too:

•  [[ $foo -gt 7 ]] # Also right!

But why use that when you could use ((...)) instead?

8. grep foo bar | while read -r; do ((count++)); done

The code above looks OK at first glance, doesn't it? Sure, it's just a poor implementation of grep -c, but it's intended as a simplistic example. So why doesn't it work? The variable count will be unchanged after the loop terminates (except in Korn shell). This surprises almost every Bash developer at some point.

The reason this code does not work as expected is because each command in a pipeline is executed in a separate SubShell. The changes to the count variable within the loop's subshell aren't reflected within the parent shell (the script).

For workarounds for this, please see Bash FAQ #24. It's a bit too long to fit here.

9. if [grep foo myfile]

Many people are confused by the common practice of using the [ command after an if. They see this and convince themselves that the [ is part of the if statement's syntax, just like parentheses are used in C's if statement.

However, that is not the case! [ is a command, not a syntax marker for the if statement. It's equivalent to the test command, except that the final argument must be a ]. For example:

•  if [ false ]; then echo "HELP"; fi
   if test "false"; then echo "HELP"; fi

Are equivalent, checking that the string "false" is non-empty. In both cases HELP will be printed, to the surprise of programmers from other languages.

The syntax of an if statement is:

•  if COMMANDS
   then
     COMMANDS
   elif COMMANDS   # optional
   then
     COMMANDS
   else            # optional
     COMMANDS
   fi              # required

There may be zero or more optional elif sections, and one optional else section. Note: there is no [ in the syntax!

Once again, [ is a command. It takes arguments, and it produces an exit code. It may produce error messages. It does not, however, produce any standard output.

The if statement evaluates the first set of COMMANDS that are given to it (up until then, as the first word of a new command). The exit code of the last command from that set determines whether the if statement will execute the COMMANDS that are in the then section, or move on.

If you want to make a decision based on the output of a grep command, you do not need to enclose it in parentheses, brackets, backticks, or any other syntax mark-up! Just use grep as the COMMANDS after the if, like this:

•  if grep foo myfile >/dev/null; then
   ...
   fi

Note that we discard the standard output of the grep (which would normally include the matching line, if any), because we don't want to see it -- we just want to know whether it's there. If the grep matches a line from myfile, then the exit code will be 0 (true), and the then part will be executed. Otherwise, if there is no matching line, the grep should return a non-zero exit code.

In recent versions of grep you can use -q (quiet) option to suppress stdout.

10. if [bar="$foo"]

•  if [bar="$foo"]      # Wrong!
   if [ bar="$foo" ]    # Still wrong!

As we explained in the previous example, [ is a command. Just like with any other command, Bash expects the command to be followed by a space, then the first argument, then another space, etc. You can't just run things all together without putting the spaces in! Here is the correct way:

•  if [ bar = "$foo" ]

Each of bar, =, the value of $foo (after substitution, but without WordSplitting) and ] is a separate argument to the [ command. There must be whitespace between each pair of arguments, so the shell knows where each argument begins and ends.

11. if [ [ a = b ] && [ c = d ] ]

Here we go again. [ is a command. It is not a syntactic marker that sits between if and some sort of C-like "condition". Nor is it used for grouping. You cannot take C-like if commands and translate them into Bash commands just by replacing parentheses with square brackets!

If you want to express a compound conditional, do this:

•  if [ a = b ] && [ c = d ]

Note that here we have two commands after the if, joined by an && (logical AND, shortcut evaluation) operator. It's precisely the same as:

•  if test a = b && test c = d

If the first test command returns false, the body of the if statement is not entered. If it returns true, then the second test command is run; and if that also one returns true, then the body of the if statement will be entered. (C programmers are already familiar with &&. Bash uses the same short-circuit evaluation. Likewise || does short-circuit evaluation for the OR operation.)

The [[ keyword does permit the use of &&, so it could also be written this way:

•  if [[ a = b && c = d ]]

12. read $foo

You don't use a $ before the variable name in a read command. If you want to put data into the variable named foo, you do it like this:

•  read foo

Or more safely:

•  IFS= read -r foo

read $foo would read a line of input and put it in the variable(s) whose name(s) are in $foo. This might be useful if you actually intended foo to be a reference to some other variable; but in the majority of cases, this is simply a bug.

13. cat file | sed s/foo/bar/ > file

You cannot read from a file and write to it in the same pipeline. Depending on what your pipeline does, the file may be clobbered (to 0 bytes, or possibly to a number of bytes equal to the size of your operating system's pipeline buffer), or it may grow until it fills the available disk space, or reaches your operating system's file size limitation, or your quota, etc.

If you want to make a change to a file safely, other than appending to the end of it, there must be a temporary file created at some point(*). For example, the following is completely portable:

•  sed 's/foo/bar/g' file > tmpfile && mv tmpfile file

The following will only work on GNU sed 4.x:

•  sed -i 's/foo/bar/g' file(s)

Note that this also creates a temporary file, and does the same sort of renaming trickery -- it just handles it transparently.

And the following equivalent command requires perl 5.x (which is probably more widely available than GNU sed 4.x):

•  perl -pi -e 's/foo/bar/g' file(s)

For more details on replacing contents of files, please see Bash FAQ #21.

(*) sponge from moreutils uses this example in its manual:

•  sed '...' file | grep '...' | sponge file

Rather than using a temporary file plus an atomic mv, this version "soaks up" (the actual description in the manual!) all the data, before opening and writing to the file. This version will cause data loss if the system crashes during the write operation, because there's no copy of the original file on disk at that point. Using a temporary file + mv ensures that there is always at least one copy of the data on disk at all times.

14. echo $foo

This relatively innocent-looking command causes massive confusion. Because the $foo isn't quoted, it will not only be subject to WordSplitting, but also file globbing. This misleads Bash programmers into thinking their variables contain the wrong values, when in fact the variables are OK -- it's just the word splitting or filename expansion that's messing up their view of what's happening.

•  msg="Please enter a file name of the form *.zip"
   echo $msg

This message is split into words and any globs are expanded, such as the *.zip. What will your users think when they see this message:

•  Please enter a file name of the form freenfss.zip lw35nfss.zip

To demonstrate:

•  var="*.zip"   # var contains an asterisk, a period, and the word "zip"
   echo "$var"   # writes *.zip
   echo $var     # writes the list of files which end with .zip

In fact, the echo command cannot be used with absolute safety here. If the variable contains -n for example, echo will consider that an option, rather than data to be printed. The only absolutely sure way to print the value of a variable is using printf:

•  printf "%s\n" "$foo"

15. $foo=bar

No, you don't assign a variable by putting a $ in front of the variable name. This isn't perl.

16. foo = bar

No, you can't put spaces around the = when assigning to a variable. This isn't C. When you write foo = bar the shell splits it into three words. The first word, foo, is taken as the command name. The second and third become the arguments to that command.

Likewise, the following are also wrong:

•  foo= bar    # WRONG!
   foo =bar    # WRONG!
   $foo = bar; # COMPLETELY WRONG!
  
   foo=bar     # Right.
   foo="bar"   # More Right.

17. echo <<EOF

A here document is a useful tool for embedding large blocks of textual data in a script. It causes a redirection of the lines of text in the script to the standard input of a command. Unfortunately, echo is not a command which reads from stdin.

•   # This is wrong:
    echo <<EOF
    Hello world
    How's it going?
    EOF
  
    # This is what you were trying to do:
    cat <<EOF
    Hello world
    How's it going?
    EOF
  
    # Or, use quotes which can span multiple lines (efficient, echo is built-in):
    echo "Hello world
    How's it going?"

Using quotes like that is fine -- it works great, in all shells -- but it doesn't let you just drop a block of lines into the script. There's syntactic markup on the first and last line. If you want to have your lines untouched by shell syntax, and don't want to spawn a cat command, here's another alternative:

•   # Or use printf (also efficient, printf is built-in):
    printf %s "\
    Hello world
    How's it going?
    "

In the printf example, the \ on the first line prevents an extra newline at the beginning of the text block. There's a literal newline at the end (because the final quote is on a new line). The lack of \n in the printf format argument prevents printf adding an extra newline at the end. The \ trick won't work in single quotes. If you need/want single quotes around the block of text, you have two choices, both of which necessitate shell syntax "contaminating" your data:

•   printf %s \
    'Hello world
    '
  
    printf %s 'Hello world
    '

18. su -c 'some command'

This syntax is almost correct. The problem is, on many platforms, su takes a -c argument, but it's not the one you want. For example, on OpenBSD:

•  $ su -c 'echo hello'
   su: only the superuser may specify a login class

You want to pass -c 'some command' to a shell, which means you need a username before the -c.

•  su root -c 'some command' # Now it's right.

su assumes a username of root when you omit one, but this falls on its face when you want to pass a command to the shell afterward. You must supply the username in this case.

19. cd /foo; bar

If you don't check for errors from the cd command, you might end up executing bar in the wrong place. This could be a major disaster, if for example bar happens to be rm *.

You must always check for errors from a cd command. The simplest way to do that is:

•  cd /foo && bar

If there's more than just one command after the cd, you might prefer this:

•  cd /foo || exit 1
   bar
   baz
   bat ... # Lots of commands.

cd will report the failure to change directories, with a stderr message such as "bash: cd: /foo: No such file or directory". If you want to add your own message in stdout, however, you could use command grouping:

•  cd /net || { echo "Can't read /net. Make sure you've logged in to the Samba network, and try again."; exit 1; }
   do_stuff
   more_stuff

Note there's a required space between { and echo, and a required ; before the closing }.

Some people also like to enable set -e to make their scripts abort on any command that returns non-zero, but this can be rather tricky to use correctly (since many common commands may return a non-zero for a warning condition, which you may not want to treat as fatal).

By the way, if you're changing directories a lot in a Bash script, be sure to read the Bash help on pushd, popd, and dirs. Perhaps all that code you wrote to manage cd's and pwd's is completely unnecessary.

Speaking of which, compare this:

•  find ... -type d -print0 | while IFS= read -r -d '' subdir; do
     here=$PWD
     cd "$subdir" && whatever && ...
     cd "$here"
   done

With this:

•  find ... -type d -print0 | while IFS= read -r -d '' subdir; do
     (cd "$subdir" || exit; whatever; ...)
   done

Forcing a SubShell here causes the cd to occur only in the subshell; for the next iteration of the loop, we're back to our normal location, regardless of whether the cd succeeded or failed. We don't have to change back manually, and we aren't stuck in a neverending string of ... && ... logic preventing the use of other conditionals. The subshell version is simpler and cleaner (albeit a tiny bit slower).

20. [ bar == "$foo" ]

The == operator is not valid for the [ command. Use = or the [[ keyword instead.

•  [ bar = "$foo" ] && echo yes
   [[ bar == $foo ]] && echo yes

21. for i in {1..10}; do ./something &; done

You cannot put a ; immediately after an &. Just remove the extraneous ; entirely.

•  for i in {1..10}; do ./something & done

Or:

•  for i in {1..10}; do
     ./something &
   done

& already functions as a command terminator, just like ; does. And you cannot mix the two.

In general, a ; can be replaced by a newline, but not all newlines can be replaced by ;.

22. cmd1 && cmd2 || cmd3

Some people like to use && and || as a shortcut syntax for if ... then ... else ... fi. In many cases, this is perfectly safe:

•  [[ -s $errorlog ]] && echo "Uh oh, there were some errors." || echo "Successful."

However, this construct is not completely equivalent to if ... fi in the general case, because the command that comes after the && also generates an exit status. And if that exit status isn't "true" (0), then the command that comes after the || will also be invoked. For example:

•  i=0
   true && ((i++)) || ((i--))
   echo $i # Prints 0

What happened here? It looks like i should be 1, but it ends up 0. Why? Because both the i++ and the i-- were executed. The ((i++)) command has an exit status, and that exit status is derived from a C-like evaluation of the expression inside the parentheses. That expression's value happens to be 0 (the initial value of i), and in C, an expression with an integer value of 0 is considered false. So ((i++)) (when i is 0) has an exit status of 1 (false), and therefore the ((i--)) command is executed as well.

This does not occur if we use the pre-increment operator, since the exit status from ++i is true:

•  i=0
   true && (( ++i )) || (( --i ))
   echo $i # Prints 1

But that's missing the point of the example. It just happens to work by coincidence, and you cannot rely on x && y || z if y has any chance of failure! (This example fails if we initialize i to -1 instead of 0.)

If you need safety, or if you simply aren't sure how this works, or if anything in the preceding paragraphs wasn't completely clear, please just use the simple if ... fi syntax in your programs.

•  i=0
   if true; then
     ((i++))
   else
     ((i--))
   fi
   echo $i # Prints 1

This section also applies to Bourne shell, here is the code that illustrates it:

•  true && { echo true; false; } || { echo false; true; }

Output is two lines "true" and "false", instead the single line "true".

23. echo "Hello World!"

The problem here is that, in an interactive Bash shell, you'll see an error like:

•  bash: !": event not found

This is because, in the default settings for an interactive shell, Bash performs csh-style history expansion using the exclamation point. This is not a problem in shell scripts; only in interactive shells.

Unfortunately, the obvious attempt to "fix" this won't work:

•  $ echo "hi\!"
   hi\!

The easiest solution is unsetting the histexpand option: this can be done with set +H or set +o histexpand

• Question: Why is playing with histexpand more apropriate than single quotes?

  • I personally ran into this issue when I was manipulating song files, using commands like

    mp3info -t "Don't Let It Show" ...
    mp3info -t "Ah! Leah!" ...

    Using single quotes is extremely inconvenient because of all the songs with apostrophes in their titles. Using double quotes ran into the history expansion issue. (And imagine a file that has both in its name. The quoting would be atrocious.) Since I never actually use history expansion, my personal preference was to turn it off in ~/.bashrc. -- GreyCat

These solutions will work:

•  echo 'Hello World!'

or

•  set +H
   echo "Hello World!"

or

•  histchars=

Many people simply choose to put set +H or set +o histexpand in their ~/.bashrc to deactivate history expansion permanently. This is a personal preference, though, and you should choose whatever works best for you.

24. for arg in $*

Bash (like all Bourne shells) has a special syntax for referring to the list of positional parameters one at a time, and $* isn't it. Neither is $@. Both of those expand to the list of words in your script's parameters, not to each parameter as a separate word.

The correct syntax is:

•  for arg in "$@"
  
   # Or simply:
   for arg

Since looping over the positional parameters is such a common thing to do in scripts, for arg defaults to for arg in "$@". The double-quoted "$@" is special magic that causes each parameter to be used as a single word (or a single loop iteration). It's what you should be using at least 99% of the time.

Here's an example:

•  # Incorrect version
   for x in $*; do
     echo "parameter: '$x'"
   done
  
   $ ./myscript 'arg 1' arg2 arg3
   parameter: 'arg'
   parameter: '1'
   parameter: 'arg2'
   parameter: 'arg3'

It should have been written:

•  # Correct version
   for x in "$@"; do
     echo "parameter: '$x'"
   done
  
   $ ./myscript 'arg 1' arg2 arg3
   parameter: 'arg 1'
   parameter: 'arg2'
   parameter: 'arg3'

25. function foo()

This works in some shells, but not in others. You should never combine the keyword function with the parentheses () when defining a function.

Bash (at least some versions) will allow you to mix the two. Most of the shells won't accept that (zsh 4.x and perhaps above will - for example). Some shells will accept function foo, but for maximum portability, you should always use:

•  foo() {
    ...
   }

26. echo "~"

Tilde expansion only applies when '~' is unquoted. In this example echo writes '~' to stdout, rather than the path of the user's home directory.

Quoting path parameters that are expressed relative to a user's home directory should be done using $HOME rather than '~'. For instance consider the situation where $HOME is "/home/my photos".

•  "~/dir with spaces" # expands to "~/dir with spaces"
   ~"/dir with spaces" # expands to "~/dir with spaces"
   ~/"dir with spaces" # expands to "/home/my photos/dir with spaces"
   "$HOME/dir with spaces" # expands to "/home/my photos/dir with spaces"

27. local varname=$(command)

When declaring a local variable in a function, the local acts as a command in its own right. This can sometimes interact oddly with the rest of the line -- for example, if you wanted to capture the exit status ($?) of the CommandSubstitution, you can't do it. local's exit status masks it.

It's best to use separate commands for this:

•  local varname
   varname=$(command)
   rc=$?

The next pitfall describes another issue with this syntax:

28. export foo=~/bar

Tilde expansion (with or without a username) is only guaranteed to occur when the tilde appears at the beginning of a word, either by itself or followed by a slash. It is also guaranteed to occur when the tilde appears immediately after the = in an assignment.

However, the export and local commands do not constitute an assignment. So, in some shells (like Bash), export foo=~/bar will undergo tilde expansion; in others (like dash), it will not.

•  foo=~/bar; export foo    # Right!
   export foo="$HOME/bar"   # Right!

29. sed 's/$foo/good bye/'

In single quotes, bash parameter expansions like $foo do not get expanded. That is the purpose of single quotes, to protect characters like $ from the shell.

Change the quotes to double quotes:

•  foo="hello"; sed "s/$foo/good bye/"

But keep in mind, if you use double quotes you might need to use more escapes. See the Quotes page.

30. tr [A-Z] [a-z]

There are (at least) three things wrong here. The first problem is that [A-Z] and [a-z] are seen as globs by the shell. If you don't have any single-lettered filenames in your current directory, it'll seem like the command is correct; but if you do, things will go wrong. Probably at 0300 hours on a weekend.

The second problem is that this is not really the correct notation for tr. What this actually does is translate '[' into '['; anything in the range A-Z into a-z; and ']' into ']'. So you don't even need those brackets, and the first problem goes away.

The third problem is that depending on the locale, A-Z or a-z may not give you the 26 ASCII characters you were expecting. In fact, in some locales z is in the middle of the alphabet! The solution to this depends on what you want to happen:

•  # Use this if you want to change the case of the 26 latin letters
   LC_COLLATE=C tr A-Z a-z
  
   # Use this if you want the case conversion to depend upon the locale, which might be more like what a user is expecting
   tr '[:upper:]' '[:lower:]'

The quotes are required on the second command, to avoid globbing.

31. ps ax | grep gedit

The fundamental problem here is that the name of a running process is inherently unreliable. There could be more than one legitimate gedit process. There could be something else disguising itself as gedit (changing the reported name of an executed command is trivial). For real answers to this, see ProcessManagement.

The following is the quick and dirty stuff.

Searching for the PID of (for example) gedit, many people start with

$ ps ax | grep gedit
10530 ?        S      6:23 gedit
32118 pts/0    R+     0:00 grep gedit

which, depending on a RaceCondition, often yields grep itself as a result. To filter grep out:

ps ax | grep -v grep | grep gedit   # will work, but ugly

An alternative to this is to use:

ps ax | grep [g]edit

This will ignore the grep itself in the process table as that is [g]edit and grep is looking for gedit once evaluated.

On GNU/Linux, the parameter -C can be used instead to filter by commandname:

$ ps -C gedit
  PID TTY          TIME CMD
10530 ?        00:06:23 gedit

But why bother when you could just use pgrep instead?

$ pgrep gedit
10530

Now in a second step the PID is often extracted by awk or cut:

$ ps -C gedit | awk '{print $1}' | tail -n1

but even that can be handled by some of the trillions of parameters for ps:

$ ps -C gedit -opid=
10530

If you're stuck in 1992 and aren't using pgrep, you could use the ancient, obsolete, deprecated pidof (GNU/Linux only) instead:

$ pidof gedit
10530

and if you need the PID to kill the process, pkill might be interesting for you. Note however that, for example, pgrep/pkill ssh would also find processes named sshd, and you wouldn't want to kill those.

Unfortunately some programs aren't started with their name, for example firefox is often started as firefox-bin, which you would need to find out with - well - ps ax | grep firefox.

Please read ProcessManagement. Seriously.

32. printf "$foo"

This isn't wrong because of quotes, but because of a format string exploit. If $foo is not strictly under your control, then any \ or % characters in the variable may cause undesired behavior.

Always supply your own format string:

printf %s "$foo"
printf '%s\n' "$foo"

33. for i in {1..$n}

The BashParser performs BraceExpansion before any other expansions or substitutions. So the brace expansion code sees the literal $n, which is not numeric, and therefore it doesn't expand the curly braces into a list of numbers. This makes it nearly impossible to use brace expansion to create lists whose size is only known at run-time.

Do this instead:

for ((i=1; i<=n; i++)); do
...
done

In the case of simple iteration over integers, an arithmetic for loop should almost always be preferred over brace expansion to begin with, because brace expansion pre-expands every argument which can be slower and unnecessarily consumes memory.

34. if [[ $foo = $bar ]] (depending on intent)

When the right-hand side of an = operator inside [[ is not quoted, bash does pattern matching against it, instead of treating it as a string. So, in the code above, if bar contains *, the result will always be true. If you want to check for equality of strings, the right-hand side should be quoted:

if [[ $foo = "$bar" ]]

If you want to do pattern matching, it might be wise to choose variable names that indicate the right-hand side contains a pattern. Or use comments.

It's also worth pointing out that if you quote the right-hand side of =~ it also forces a simple string comparison, rather than a regular expression matching. This leads us to:

35. if [[ $foo =~ 'some RE' ]]

The quotes around the right-hand side of the =~ operator cause it to become a string, rather than a RegularExpression. If you want to use a long or complicated regular expression and avoid lots of backslash escaping, put it in a variable:

re='some RE'
if [[ $foo =~ $re ]]

This also works around the difference in how =~ works across different versions of bash. Using a variable avoids some nasty and subtle problems.

36. [ -n $foo ] or [ -z $foo ]

When using the [ command, you must quote each substitution that you give it. Otherwise, $foo could expand to 0 words, or 42 words, or any number of words that isn't 1, which breaks the syntax.

[ -n "$foo" ]
[ -z "$foo" ]
[ -n "$(some command with a "$file" in it)" ]

# [[ doesn't perform word-splitting or glob expansion, so you could also use:
[[ -n $foo ]]
[[ -z $foo ]]

37. [[ -e "$broken_symlink" ]] returns 1 even though $broken_symlink exists

Test follows symlinks, therefore if a symlink is broken, i.e. it points to a file that doesn't exists, test -e returns 1 for it even though it exists.

In order to work around it (and prepare against it) you should use:

[[ -e "$broken_symlink" || -L "$broken_symlink" ]]

38. ed file <<<"g/d\{0,3\}/s//e/g" fails

The problem caused because ed doesn't accept 0 for \{0,3\}.

You can check that the following do work:

ed file <<<"g/d\{1,3\}/s//e/g"

Note that this happens even though POSIX states that BRE (which is the Regular Expression flavor used by ed) should accept 0 as the minimum number of occurrences (see section 5).

39. expr sub-string fails for "match"

This works reasonably well

---

most of the time

word=abcde
expr "$word" : ".\(.*\)"
bcde

But WILL fail for the word "match"

word=match
expr "$word" : ".\(.*\)"

The problem is "match" is a keyword. Solution (GNU only) is prefix with a '+'

word=match
expr + "$word" : ".\(.*\)"
atch

Or, y'know, stop using expr. You can do everything expr does by using Parameter Expansion. What's that thing up there trying to do? Remove the first letter of a word? That can be done in POSIX shells using PE or Substring Expansion:

$ word=match
$ echo "${word#?}"    # PE
atch
$ echo "${word:1}"    # SE
atch

Seriously, there's no excuse for using expr unless you're on Solaris with its non-POSIX-conforming /bin/sh. It's an external process, so it's much slower than in-process string manipulation. And since nobody uses it, nobody understands what it's doing, so your code is obfuscated and hard to maintain.

40. On UTF-8 and Byte-Order Marks (BOM)

In general: Unix UTF-8 text does not use BOM. The encoding of plain text is determined by the locale or by mime types or other metadata. While the presence of a BOM would not normally damage a UTF-8 document meant only for reading by humans, it is problematic (often syntactically illegal) in any text file meant to be interpreted by automated processes such as scripts, source code, configuration files, and so on. Files starting with BOM should be considered equally foreign as those with MS-DOS linebreaks.

In shell scripting: 'Where UTF-8 is used transparently in 8-bit environments, the use of a BOM will interfere with any protocol or file format that expects specific ASCII characters at the beginning, such as the use of "#!" of at the beginning of Unix shell scripts.' http://unicode.org/faq/utf_bom.html#bom5

41. content=$(<file)

Command substitutions (both `` and $() forms) remove all trailing newlines from the command inside them; this includes the Bash $(<file) shortcut. This can result in nasty surprises, especially since it's difficult to know whether the newline in the output is from echo or part of the data. An easy workaround is to add a postfix inside the command substitution and remove it on the outside:

absolute_dir_path_x="$(readlink -fn -- "$dir_path"; echo x)"
absolute_dir_path="${absolute_dir_path_x%x}"

42. for file in ./* ; do if [[ $file != *.* ]]

It's a good practice to prefix your * with a ./ because it avoids problems with badly (maliciously) named files such as -rf. These files will get treated as options instead of file names if not prefixed by something (such as ./).

In this case, however, problems arise because the pattern *.* won't match the pathname in the file variable. Say your file is called todo, the file variable will now hold: ./todo. This will cause a bad match against the pattern, because of the prefixed ./.

What you need to do is fix your pattern to take the prefixed ./ into account: [[ $file != ./*.* ]].

Alternatively, if you want to work with just the filename, you could get the filename out of the pathname:

    for path in ./*; do
        file=${path##*/}

        [[ $file != *.* ]] && rm "$file"
    done

Another alternative could be to stop using the ./ prefix and solve the risk of dash-prefixed filenames by putting a -- on your commands that use the file. Putting a -- before expanding filenames on certain commands will tell the command to stop looking for options. The filename can now no longer be mis-parsed as an option. The downside of this is that not all commands that take options necessarily support -- and you make your code more fragile by requiring that you not forget a -- anywhere you use the file.

   for file in *; do
       [[ $file != *.* ]] && rm -- "$file" # works with most rm's, but is not guaranteed to work with any command.
   done

43. somecmd 2>&1 >logfile

This is by far the most common mistake involving redirections, typically performed by someone wanting to direct both stdout and stderr to a file or pipe will try this and not understand why stderr is still showing up on their terminal. If you're perplexed by this, you probably don't understand how redirections or possibly file descriptors work to begin with. Redirections are evaluated left-to-right before the command is executed. This semantically incorrect code essentially means: "first redirect standard error to where standard out is currently pointing (the tty), then redirect standard out to logfile". This is backwards. Standard error is already going to the tty. Use: somecmd >logfile 2>&1 instead. See a more in-depth explanation, Copy descriptor explained, and BashGuide - redirection.

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CategoryShell