Mastering Bash Logical Operators: File Tests, Comparisons, and Performance Tips

Logical Test Operators

-f

– checks if a file exists (e.g., if [ -f filename ]) -d – checks if a directory exists -b – checks for a block device -c – checks for a character device -S – checks for a socket -L – checks for a symbolic link -e – checks if a path exists (file, directory, etc.) -G – true if the file’s group ID matches the effective GID -O – true if the file’s owner ID matches the effective UID -p – checks for a named pipe (FIFO) -r – readable -w – writable -x – executable -s – non‑empty file -u – set‑uid bit set -g – set‑gid bit set -k – sticky bit set

File Comparison Operators

-nt

– first file is newer than the second -ot – first file is older than the second -ef – both names refer to the same inode (hard link)

Logical Connectors

&&

– logical AND || – logical OR -a – AND inside a single [ ] test expression -o – OR inside a single [ ] test expression -z – string is empty -n – string is non‑empty

Using the test Command

Syntax: test EXPRESSION. Examples:

# Simple equality
[ root@localhost ~ ]# test 1 = 1 && echo 'ok'
ok

# Directory existence
[ root@localhost ~ ]# test -d /etc && echo 'ok'
ok

# Numeric comparison
[ root@localhost ~ ]# test 1 -eq 1 && echo 'ok'
ok

When using if:

if test 1 = 1; then echo 'ok'; fi
ok

All tokens and operators must be separated by spaces; they cannot be concatenated.

Bracket Expressions

Single brackets [ ] are the traditional test syntax. Comparison operators such as < and > must be escaped because they are interpreted as redirection symbols. Logical operators || and && are not recognized; use -a and -o instead.

Double brackets [[ ]] extend [ ] capabilities: they allow unescaped < and > for string comparison and support || and && directly.

# Using []
[ root@localhost ~ ]# [ 2 -gt 1 -a 3 -lt 4 ] && echo 'ok'
ok

# Using [[ ]]
[ root@localhost ~ ]$ [[ 2 < 3 && 4 > 5 ]] && echo 'ok'
ok

Performance Comparison

Three forms— test, [ ], and [[ ]] —were timed in a loop of 100 000 iterations checking the current directory.

# test
real 0m0.658s  user 0m0.558s  sys 0m0.100s

# [ ]
real 0m0.609s  user 0m0.524s  sys 0m0.085s

# [[ ]]
real 0m0.311s  user 0m0.275s  sys 0m0.036s

When compatibility with older Bash or POSIX sh is not required, [[ ]] offers the best performance and broader feature set for conditional expressions.