はじめに
help testコマンドによると、「ファイルが存在するか」を判定するオプションは、以下の 2 種類がありました。
今回は、この 2 つのオプションの挙動の違いを調べてみました。
-a FILE True if file exists.
-e FILE True if file exists.
`help test` 出力結果全文
help test
# test: test [expr]
# Evaluate conditional expression.
# Exits with a status of 0 (true) or 1 (false) depending on
# the evaluation of EXPR. Expressions may be unary or binary. Unary
# expressions are often used to examine the status of a file. There
# are string operators and numeric comparison operators as well.
# The behavior of test depends on the number of arguments. Read the
# bash manual page for the complete specification.
# File operators:
# -a FILE True if file exists.
# -b FILE True if file is block special.
# -c FILE True if file is character special.
# -d FILE True if file is a directory.
# -e FILE True if file exists.
# -f FILE True if file exists and is a regular file.
# -g FILE True if file is set-group-id.
# -h FILE True if file is a symbolic link.
# -L FILE True if file is a symbolic link.
# -k FILE True if file has its `sticky' bit set.
# -p FILE True if file is a named pipe.
# -r FILE True if file is readable by you.
# -s FILE True if file exists and is not empty.
# -S FILE True if file is a socket.
# -t FD True if FD is opened on a terminal.
# -u FILE True if the file is set-user-id.
# -w FILE True if the file is writable by you.
# -x FILE True if the file is executable by you.
# -O FILE True if the file is effectively owned by you.
# -G FILE True if the file is effectively owned by your group.
# -N FILE True if the file has been modified since it was last read.
# FILE1 -nt FILE2 True if file1 is newer than file2 (according to
# modification date).
# FILE1 -ot FILE2 True if file1 is older than file2.
# FILE1 -ef FILE2 True if file1 is a hard link to file2.
# All file operators except -h and -L are acting on the target of a symbolic
# link, not on the symlink itself, if FILE is a symbolic link.
# String operators:
# -z STRING True if string is empty.
# -n STRING
# STRING True if string is not empty.
# STRING1 = STRING2
# True if the strings are equal.
# STRING1 != STRING2
# True if the strings are not equal.
# STRING1 < STRING2
# True if STRING1 sorts before STRING2 lexicographically.
# STRING1 > STRING2
# True if STRING1 sorts after STRING2 lexicographically.
# Other operators:
# -o OPTION True if the shell option OPTION is enabled.
# -v VAR True if the shell variable VAR is set
# -R VAR True if the shell variable VAR is set and is a name reference.
# ! EXPR True if expr is false.
# EXPR1 -a EXPR2 True if both expr1 AND expr2 are true.
# EXPR1 -o EXPR2 True if either expr1 OR expr2 is true.
# arg1 OP arg2 Arithmetic tests. OP is one of -eq, -ne,
# -lt, -le, -gt, or -ge.
# Arithmetic binary operators return true if ARG1 is equal, not-equal,
# less-than, less-than-or-equal, greater-than, or greater-than-or-equal
# than ARG2.
# See the bash manual page bash(1) for the handling of parameters (i.e.
# missing parameters).
# Exit Status:
# Returns success if EXPR evaluates to true; fails if EXPR evaluates to
# false or an invalid argument is given.
結論
File test operatorsによると、ファイル演算子の-aは非推奨(deprecated)なため、-eを使った方が良さそうです。
(man test, info testの結果にはファイル演算子として-aはありませんでした)
実際、-aオプションは二項演算子としても使われるため、混同すると非常にややこしくなります。
挙動の違い
# 準備
touch foo.txt
通常
これには違い無し。
[ -a foo.txt ] && echo 'True' || echo 'False'
# True
[ -e foo.txt ] && echo 'True' || echo 'False'
# True
反転
!で反転させると、違いが分かります。
[ ! -a foo.txt ] && echo 'True' || echo 'False'
# True
[ ! -e foo.txt ] && echo 'True' || echo 'False'
# False
-aの場合、Falseにならず、Trueになります。
[ ! -a foo.txt ] の-aが、ファイル演算子ではなく二項演算子として使われているためです。
※! AND foo.txt という意味。
※test コマンドはexit 0を返せばTrueなので、!とfoo.txtはTrue扱い。